X-E2of3_NRCARoofingManualSlopeRoofSlate.pdf

The NRCA Roofing Manual: Steep-slope Roof Systems—2013

Slate Roof Systems

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6, 5.

CONTENTS

Introduction

Chapter 1—Roof Decks

1.1 Wood Panels

1.2 Wood Planks and Wood Boards

1.3 Other Deck Types

Chapter 2—Underlayments

2.1 Underlayment Materials

2.2 Underlayment Design and Installation

Chapter 3—Slate

3.1 Slate Materials

3.2 Slate Roof System Design and Installation

Chapter 4—Roof System Configurations

Chapter 5—Roof Re-covering

Chapter 6—Construction Details

6.1 Information Applicable to All Construction Details

6.2 Index of Drawings

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6, This section of The NRCA Roofing Manual: Steep-slope Roof Systems—2013 contains information specific to slate roof assemblies. Chapter 1—Roof Decks provides information specific to roof decks appropriate for slate roof systems.

Chapter 2—Underlayments provides information regard-ing the materials, design and installation of underlayment used in slate roof systems. Chapter 3—Slate provides information about slate, including material, design and installation considerations. Chapter 4—Roof System Con-figurations provides detailed technical information regard-ing combining roof deck and slate roof system components into slate roof assemblies. Chapter 5—Roof Re-covering addresses re-covering existing roof systems with new slate roof systems. Chapter 6—Construction Details provides information about and representative graphic depictions of typical construction detail conditions specific to slate roof systems.

In addition to the information contained herein, other por-tions of The NRCA Roofing Manual contain information that may be useful when considering slate roof systems.

For example, information regarding ventilating slate roof systems is contained in the Condensation Control sec-tion of The NRCA Roofing Manual: Architectural Metal Flashing, Condensation Control and Reroofing.

Types of Slate Roof Systems

Slate roof systems may be classified into three general categories:

Standard Slate Roof Systems: Standard slate roof sys-tems are those roof systems composed of standard com-mercial slate that is about 3⁄16 of an inch to ¼ of an inch thick. The most common types of slate have uniform lengths and widths with square tails or butts. Standard

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INTRODUCTION

SLATE ROOF SYSTEMS

The NRCA Roofing Manual: Steep-slope Roof Systems—2013

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6, commercial slate may be used to form a variety of designs on a roof and are suitable for many buildings where a steep-slope roof covering with a long service life is desired.

If desired, butts or corners of each slate may be trimmed to a diamond, hexagonal or other pattern for specific por-tions of or for areas of a roof system. Variety in the pat-tern of standard slate roof systems is sometimes attained by using random widths. A staggered butt line can be achieved by using longer length slates randomly.

Graduated Slate Roof Systems:

systems feature variations in thickness, size and exposure.

In graduated slate roof systems, the thickest, widest and longest slates are placed along eaves. As slate courses prog-ress upslope to ridges, slates of gradually diminishing size and/or thickness are used, creating a “graduated” effect.

This effect is intended to make the eave-to-ridge distance look longer and give a roof a more massive appearance.

Slate for roof systems of this type can be obtained in any combination of thicknesses from 3⁄16 inch to 1½ inches.

roof systems may be installed in a wide variety of patterns.

Textural Slate Roof Systems: Because slate shingles come in a variety of sizes, thicknesses, textures and col-ors, a wide range of appearances can be created. When a range of thicknesses are mixed throughout a roof system, the roof system is sometimes referred to as a textured or textural roof system. Textural slate roof systems are com-posed of slate that has a rougher texture than standard slate. This slate type also has uneven tails or butts.

Active Slate Quarries: Active roofing slate quarries in North America exist in New York, Pennsylvania, Ver-mont, Virginia, and the Canadian provinces of Quebec and Newfoundland. The main area of slate production in Pennsylvania is the Lehigh district. The active Vermont quarry district lies in Bennington and Rutland counties and extends into Washington County in New York. Vir-ginia slate operations are now conducted only in Buck-ingham County at Arvonia.

Imported slate is available from Brazil, China, South Africa, Spain and Wales.

There is concern with the durability of roofing slate from some quarries. Slate can be tested to recognized standards if the source of a slate at a quarry provides a consistent quality. Tests include those in ASTM C406, “Standard Specification for Roofing Slate,” especially the modulus of rupture test. British Standard 680 has an acid-resistance test for carbonate impurities, especially calcite. A new European standard for petrographic examinations can de-scribe the mineralogy and fabric of the stone and identify most problematic features of building stones.

Slate Tools: Specialized tools for slate roofing include the slate hammer, stake, ripper, roof scaffold bracket and roof ridge, or hook ladder.

A slate hammer has many uses. The hammer head is used for driving nails, the claw for pulling nails, the point for punching slate and the knife edge on the shaft for cutting slate.

A slate stake is used with a slate hammer to cut slate in the field. It is a T-shaped tool that can be stuck into a roof deck or scaffold plank and provides a solid support at the underside of slate along the line where a cut or hole punch is to be made. It is also used as a straight edge to mark slate before cutting or trimming slate at valleys and other flashing areas.

A slate ripper is used to take out broken slate by cutting or pulling the concealed nails. The upper portion of a ripper is designed to hook around nails and loosen or cut them by force. The handle is designed to be struck with a hammer to assist in cutting or loosening a slate nail after the ripper has been positioned around or against the nail.

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6, A roof deck is the structural substrate to which a slate roof system is applied. A roof deck must be capable of safely supporting dead loads, including the weight of a slate roof system, and the design live loads, including any additional loads that may be required by the applicable building code. A roof deck also must be able to provide adequate withdrawal resistance for fasteners used to at-tach a roof system.

ence the type of roof deck, rafter spacing, deck slope and other structural characteristics that are appropriate for a roof deck.

Because of the typical longevity of slate roof systems, a roof deck material of comparable long-term expected service life is important. Slate roof systems require a struc-ture capable of supporting heavier loads than most other steep-slope roof systems. The thickness and structural characteristics of wood sheathing or decking should be increased with thicker layers or heavier weight slates. The structural framing should be designed to accommodate the expected loads.

Slate may be applied directly over the following substrates:

The proper thickness and species of wood panel, wood plank or wood board roof deck required for a specific roof assembly should be determined by the design loads, including wind uplift, anticipated for the roof assembly

End joints of each piece of decking should be staggered.

The end joints should also be centered over the support-

In some instances, roof deck types other than wood pan-els, wood planks or wood boards may be encountered.

For example, in some situations, metal, concrete, gypsum, cementitious wood fiber, or other nonwood or fabricated wood materials are used as structural decks for steep-slope roof systems. In these instances, NRCA recommends a nailable substrate consisting of appropriately designed

CHAPTER 1

ROOF DECKS

The NRCA Roofing Manual: Steep-slope Roof Systems—2013

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6, wood panels, wood planks or wood boards be installed over a structural deck to provide an adequate substrate for a slate roof system.

Slate roof systems should be applied only over continu-ous wood panels or closely spaced wood planks or wood boards. Spaced, or “skipped,” wood plank or wood board decks are not considered an adequate substrate for a slate roof system.

The bottom, leading or eave edge of a roof deck should be flush or extended out from the outer edge of the fascia board or crown molding without gaps to provide for a continuous surface to properly terminate a slate roof system.

NRCA recommends designers specify roof deck slopes intended for the application of slate roof systems at 4:12 or greater.

1.1 Wood Panels

Wood-panel roof decks can be subdivided into two gen-eral types: plywood roof decks and oriented strand board

Plywood panels are composed of thin wood layers called veneers that are peeled from logs. The veneers are laid at right angles to one another then glued together under heat and pressure. This cross-lamination orientation adds strength and stability to the panels. Panels consist of a number of cross-laminated layers that vary in number according to a panel’s thickness or design.

OSB panels are composed of layers of compressed, glued wood strands. These strand layers are oriented at right angles to one another before being glued under heat and pressure and formed into panels. OSB panel performance

All plywood and wood-based panels used to support slate roof systems should be rated for structural use as roof sheathing. Most building codes require a label on plywood or wood-based panels, ensuring the plywood or wood-based panel complies with the criteria set forth in one of the following standards:

and Industrial Plywood,” for all-veneer plywood

Structural-Use Panels” dards and Policies for Structural-Use Panels,”

Performance standards PS 1 and PS 2, which were initi-ated by APA, have been developed under the “Procedures for the Development of Voluntary Product Standards” of the U.S. Department of Commerce. Performance stan-dard PRP-108 was developed by APA.

Adherence to PS 1, PS 2 and PRP-108 standards is vol-untary by wood panel producers; they are not required to meet these standards to sell their products.

NRCA recommends plywood panels intended for use as roof sheathing in slate roof assemblies meet or exceed the requirements set forth in PS 1. Because of the longevity of slate roof systems and concerns about the long-term performance of OSB panels, NRCA does not recommend OSB panels be used as a substrate for slate roof systems.

When plywood is used as a roof deck material, NRCA recommends the use of a minimum of four-ply, ⅝-inch-thick plywood for 16-inch rafter spacings, and five-ply, ¾-inch-thick plywood for 24-inch rafter spacings. These minimum thicknesses are intended to provide adequate support for the slate roof system and adequate pull-out resistance for the fasteners used to attach the slates to ply-wood panels. In some instances, thicker plywood panels may be necessary to accommodate specific design loads anticipated for the roof assembly and the distance between the supporting members.

When OSB is used as a roof deck material, a minimum of ⅝-inch-thick OSB is recommended for 16-inch rafter spacings and ¾-inch-thick OSB for 24-inch rafter spac-ings. These minimum thicknesses are intended to provide adequate support for a slate roof system and adequate pull-out resistance for the fasteners used to attach slates to OSB panels. In some instances, thicker OSB panels may be necessary to accommodate specific design loads antici-pated for the roof assembly and the distance between the supporting members.

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6, NRCA has concerns about the long-term performance of OSB panels, including those addressed by PS 2 and PRP- 108, used as substrates for slate roof systems. Although NRCA acknowledges the widespread use of OSB panels for constructing roof deck substrates, experience has shown that OSB panels are subject to dimensional chang-es, ridging and fastener backout resulting from changing moisture conditions that roof deck substrates normally encounter. As a result, and because of the long-term life expectancy of slate roof systems, NRCA does not recom-mend that OSB panels be used as a substrate for slate roof systems.

Local codes may allow wood panels of ⅜-inch minimum thickness for use as roof deck material. NRCA is con-cerned with deflection when thin wood panels are used as roof deck sheathing. Adequate fastener withdrawal resis-tance also is of concern in these scenarios.

Plywood and OSB sheathing panels should be installed with about ⅛-inch minimum gaps at panel edges to allow for expansion of the panels. Panel edge clips, often called H-clips, are sometimes used to provide this gap and also provide additional support of panel edges between widely spaced framing members.

End joints of wood sheathing panels that do not occur over supporting members should be supported to provide adequate bearing for the sheathing panel ends.

Preservative-treated Panels: Caution should be taken when roof decks are constructed of wood that has been treated with an oil-borne preservative. Many roofing material manufacturers recommend wood roof decks be constructed with wood that has been treated with a non-oil preservative pressure treatment or with nontreated air-or kiln-dried lumber. When preservative-treated wood panels are used, the roofing material manufacturer can be consulted for its specific recommendations for roofing material application on preservative-treated wood panel roof decks.

For additional information regarding preservative wood treatment, the American Wood Protection Association can be consulted.

Fire-retardant-treated Panels: Because of the panels caused by premature, heat-induced activation of the intended chemical reaction, special care should be taken to analyze and investigate the complete building envelope when considering the use of FRT wood-panel decks in the design of a steep-slope roof assembly. NRCA does not recommend using FRT wood as a roof deck material.

1.2 Wood Planks and Wood Boards

Wood plank and wood board roof decks are composed of solid-sawn dimensional lumber. The terms wood “plank” and wood “board” generally are differentiated by thick-ness and width.

Wood planks are long, relatively wide pieces of lumber.

Specifications sometimes vary in thickness from 2 inches up to 5 inches with the width dimensions in the plane of the roof deck. Wood planks may be single or double tongue-and-groove, straight-edge, ship-lapped or grooved for splines on longitudinal edges.

Wood boards are pieces of lumber that are less than 2 inches thick with square edges. Board widths are typically between 4 inches and 12 inches wide and are laid with their width dimensions in the horizontal plane of a roof deck. Boards less than 4 inches wide are sometimes clas-sified as strips. Use of nominal 6-inch-wide wood boards is suggested for roof decks to prevent excessive movement and splitting. Boards that are thinner than nominal 1 inch are not considered strong enough to support roof loads.

The proper thickness and species of wood plank and wood board deck required for a specific roof assembly should be determined by the design loads, including wind uplift, anticipated for the roof system and the distance each adjacent piece of decking should be staggered. The end joints should also be centered over the supporting

Preservative-treated Wood: Since Jan. 1, 2004, preservative-treated lumber produced for consumer use is

Some commercial uses of CCA are still available.

Second-generation treatments initially introduced to

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6, treated with these formulations contains high levels of free copper and consequently has a strong tendency to corrode metal. Fasteners, anchors, connectors and flash-ings that come into contact with it need to be of specific composition to withstand its corrosive character.

Third-generation wood preservative formulations intro-ticles of water-insoluble copper carbonate in suspension, instead of, or in combination with dissolved copper of the second-generation treatments. According to preservative wood treatment manufacturers, wood can be treated with “micronized” copper carbonate to total copper levels that are less than those required for second-generation pre-servatives and retain adequate long-term concentrations, because micronized treatments are less prone to leaching.

Manufacturers claim third-generation treatments are less corrosive to metal because of their lower copper concentrations.

The International Building Code, 2012 Edition and Interna-tional Residential Code, 2012 Edition require preservative-treated wood to comply with American Wood Protection treatments are included in the AWPA Standard U1, “Use Category System: User Specification for Treated Wood.”

Labels indicating wood treated with preservatives that comply with AWPA Standard U1 requirements contain the “AWPA U1” designation, producer identification, preservative treatment designation, preservative retention level and the AWPA Use Category. AWPA Use Catego-ries appropriate for wood used in exterior construction, not in contact with the ground, are UC3A for coated wood and UC3B for uncoated wood.

not been standardized by AWPA and therefore should be considered proprietary to each manufacturer. Manu-facturers possess evaluation reports by ICC Evaluation code compliance to the code authority having jurisdic-wood contain the ICC-ES evaluation report number and a general description of the intended application such as also include an AWPA Use Category alongside a general use description, but such use only is considered proper for standardized treatments.

NRCA suggests the following guidelines when encounter-ing currently available preservative-treated wood:

vanized steel fasteners and connectors should not be used in contact with treated wood. Hot-dipped galvanized fasteners complying with ASTM A153 and connectors complying with ceptable. Type 304 or Type 316 stainless-steel fasteners and connectors are recommended for maximum corrosion resistance.

ings may be acceptable for use with treated wood. When considering the use of such propri-etary coated fasteners and connectors, fastener manufacturers can be consulted for specific information regarding the performance of their products in treated wood and any precautions or special instructions that may be applicable.

products should not be used in direct contact with any treated wood. ACQ-treated wood is not compatible with aluminum.

accessories, except for 300-series stainless steel, should not be used in direct contact with treated wood. Metal products, except stainless steel, may be used if separated from treated wood by a spacer or barrier, such as a single-ply membrane or self-adhering polymer-modified bitumen membrane.

NRCA is of the opinion that the corrosion-related con-cerns regarding the use of currently available treated wood possibly outweigh the benefits that treated wood provides as a component in roof assemblies. In many in-stances, nontreated, construction-grade wood is suitable for use in roof assemblies as blocking or nailers provided reasonable measures are taken to ensure the nontreated wood remains reasonably dry when in service. Where a

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6, specific design provides for a secondary means of water-proofing, NRCA now considers the use of nontreated, construction-grade wood to be an acceptable substitute for treated wood.

1.3 Other Deck Types

NRCA recognizes other types of structural decks are sometimes used in constructing slate roof assemblies, particularly in nonresidential construction. NRCA rec-ommends that slate be attached only to structural wood panel decks or to wood plank or wood board decks. Bat-ten systems are rarely used in North America. NRCA does not recommend the direct attachment of slate to gypsum, concrete plank, cementitious wood fiber, or similar nonwood or fabricated wood materials.

In some situations, metal, concrete, gypsum, cementi-tious wood fiber, other nonwood or fabricated wood materials are used as structural decks for steep-slope roof systems. In these instances, NRCA recommends an ap-propriately designed nailable substrate consisting of wood panels, wood planks or wood boards be installed over a structural deck to provide an adequate substrate for a slate roof system.

For ventilation purposes, if necessary, and to allow ac-ceptable clearance for proper fastener penetration of a wood nailing substrate, NRCA suggests using wood bat-tens or metal channels attached over a structural deck to raise and separate the wood nailing substrate from the deck surface below. The design, placement, spacing, height and attachment of wood battens or metal channels are the responsibility of the designer. A complete roof as-sembly, including the structural deck, wood nailing sub-strate and roof covering, should be designed to meet the applicable building code. Key factors to consider during the design phase include fire resistance, structural loading and wind-resistance requirements of the applicable build-ing code.

Nail-base Insulation: Nail-base insulation is com-posed of a layer of insulation that is factory-adhered or laminated directly to a nailable wood panel, such as ply-wood or oriented strand board.

Some steep-slope roof assemblies requiring an appropri-ate nailable substrate are constructed using nail-base insulation. In these assemblies, the wood panel serves as the nailing surface for the roof system. When nail-base insulation is installed in this configuration, its uplift resis-tance should match the uplift resistance of the underlying structural roof deck and the wood panel thickness should be as recommended in Section 1.1—Wood Panels.

When nail-base insulation is used, the insulation adhered directly to the wood panel prevents any ventilation from occurring between the roof system and insulation. A lack of ventilation and the heat-resistant properties of insula-tion increase the temperature to which a roof system is subjected during high-temperature periods. Increased temperature may adversely affect the roof system’s service life.

Vented nail-base insulations provide a narrow ventilation space, varying in depth, between the insulation and wood panel by using spacers to separate the two components.

This space can be one-directional, running parallel to the roof slope, or cross-directional, allowing airflow in all directions. The amount of ventilation provided by these products is limited but may prevent heat buildup to some extent.

Problems have occurred with vented and unvented nail-base insulation products caused by moisture vapor transport through the joints between panels. The types of problems that have been reported are condensation within the ventilation space and dimensional changes in wood panels leading to underlayment buckling and roof covering buckling.

Designers should carefully consider the moisture trans-port direction and amount expected for the roof assembly when specifying nail-base insulation systems.

Nail-base insulation should possess adequate compressive strength and shear strength to withstand the loads expect-ed for the roof system. For vented nail-base insulation, spacer material and distance between spacer blocks should provide adequate compressive strength and the bearing surface of spacers should provide adequate shear strength.

NRCA does not recommend using vented nail-base insu-lation in slate roof assemblies unless the nail base manu-facturer can provide data that can verify the board can support the anticipated loads.

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6, NRCA recommends the use of underlayments with slate roof systems. The use of underlayment requires a continu-ous substrate to support the underlayment material. An underlayment is applied over an entire roof deck before or during the application of roof slate.

When designing a slate roof system, the selection of an appropriate underlayment material should carefully be considered. Roof slate service life length typically exceeds service life lengths of many other steep-slope roof cover-ing materials. It is important for the long-term service-ability of a slate roof system the selected underlayment material is characterized by durability that is comparable to the roof slate durability.

An underlayment performs several functions; it provides:

roof covering is installed substrate

In addition, underlayments generally are necessary with slate roof systems for the following reasons:

roof assembly for a material or system warranty imperfections impermeable. When vapor impermeable underlayment materials are installed over an entire roof deck area, the underlayment functions as a vapor retarder. Designers should carefully consider moisture vapor transport in a roof assembly when specifying vapor-impermeable under-layments. Additional information about design consider-ations affecting vapor retarder placement in roof assem-blies can be found in the Condensation Control section

CHAPTER 2

UNDERLAYMENTS

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6, tions are addressed in this manual:

underlayment

2.2—Underlayment Design and Installation.

2.1 Underlayment Materials

Asphalt Felts: Asphalt-saturated and asphalt-impregnated felt underlayments traditionally have been the most commonly used steep-slope roof underlayments.

An asphalt felt underlayment may be reinforced with an organic mat, an inorganic mat or a combination organic

NRCA has concerns regarding the use of asphalt felt materials as underlayments for slate roof systems because rials’ service lives typically fall short of roof slates’ service lives.

Synthetic Sheets:

propylene), recycled rubber and/or similar components tilayer laminates composed of woven or nonwoven fabrics acterized by high tensile strength, light weight, long-term ultraviolet resistance, low or no moisture absorption, and low shrinkage and wrinkling.

Thickness, tensile strength and abrasion resistance for sheets are either polyethylene or polypropylene or a com-bination of both.

used in steep-slope roof applications where traditionally asphalt felts have been used. The use of synthetic under-layments raises questions about vapor retarder placement in a roof assembly because many synthetic sheet under-layments have relatively low permeance ratings—lower than those for asphalt felt underlayments and may per-form as vapor retarders when installed over an entire roof permeance ratings higher than those of asphalt felt un-consulted for synthetic sheet underlayment permeance properties.

uct manufacturers possess evaluation reports by ICC stantiate code compliance to the code authority having jurisdiction. Evaluation protocols that establish guidelines for code acceptance of synthetic sheet underlayments are for Polypropylene Roof Underlayments.”

Polymer-modified Bitumen Sheets: Polymer-bitumen base sheets are sometimes used as underlayments used in low-slope membrane roof systems.

underlayments are installed in a manner similar to asphalt felt underlayments.

common polymers currently used are atactic polypropyl-ene and styrene butadiene styrene. These polymers alter the basic physical characteristics of asphalt and provide enhanced weathering, aging and sealing characteristics.

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6, 5.

propylene) products typically range in thickness from about 35 mils up to 90 mils.

meance values and may perform as vapor retarders when installed as underlayments over an entire roof deck area.

Self-adhering Polymer-modified Bitumen Sheets:

sheet is a distinctive type of underlayment. This type of underlayment typically is used to construct water and ice-dam protection membranes which provide additional protection from moisture intrusion along the eaves and at various flashing locations, including penetrations and elevation changes, and in valleys where excessive water runoff or ice dams can occur.

Water and ice-dam protection membranes generally con-bitumen sheet.

not be exposed for extended periods of time before the application of slate unless expressly stated on the packag-covers the bottom side of the sheet to prevent it from sticking to itself when it is wound into a roll. This release water and ice-dam protection membranes typically range in thickness from 20 mils up to 60 mils.

and ice-dam protection underlayments. This standard addresses thickness, tear resistance, adhesion properties, low-temperature flexibility and thermal stability, as well as other physical properties.

NRCA recommends designers specify self-adhering slate roof systems.

bitumen sheets produce underlayment products for bitumen water and ice-dam protection underlayments does not address this issue. For slate roof systems, the bitumen sheet underlayment products is generally not considered necessary.

permeance values and may perform as vapor retarders when installed as underlayments over an entire roof deck area.

2.2 Underlayment Design and

Installation roof components that rely on the slope of a roof substrate to effectively shed water. Depending on a roof substrate’s slope and exposure conditions of a slate roof system, dif-appropriate.

As a general rule, the underlayment is a critical system component in slate roof systems. In many instances, slate will outlive the underlayment over which the slate was installed. Therefore, if the long-term water-shedding characteristics of an underlayment are necessary to pro-designers should carefully consider the type and quality given to good local practices, which draw upon experi-ence with various underlayment products and methods that have been developed to address special conditions.

Roof Slope: NRCA recommends designers specify or more.

Underlayment Configurations: There are dif-dressed in this manual:

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flashing locations, n along the eaves g penetrations and elevation changes, n valleys where excessive water not be exposed for extended periods of time before the application of slate unless expressly stated on the packag-t are necessary to pro-designers should carefully consider the type and quality

Single Layer of Mechanically Attached Underlayment:

attached to a roof deck with fasteners.

For single-layer applications, the underlayment should be applied horizontally in shingle fashion and lapped as rec-a minimum of 2 inches and end laps of a minimum of 4 be offset. Underlayment should be fastened with nails to hold the material in place before and during the installa-tion of a primary roof covering.

and patterns for underlayment in high-wind regions in an attempt to hold the underlayment in place in the event of wind blow-off of the primary roof covering.

Installing nonadhered underlayment in vertical runs, slopes that are more difficult to cover with horizontal runs. If the strapping method is used, NRCA recom-mends the underlayment sheets be offset at a slight angle to promote drainage overlap, and side-lap and end-lap cover with horizontal runs.

Single-layer of Self-adhering Underlayment: This con-

This type of underlayment is designed to be used as a cally, manufacturers recommend side laps of a minimum of 3½ inches and end laps of a minimum of 6 inches for laps and end laps should be sealed and the membrane sheet adhered to the substrate.

When specifying self-adhering underlayment, designers may not be able to be completely removed from the exist-ing deck without damaging the roof deck.

Double Layer of Mechanically Attached Underlayment:

attached to a roof deck with fasteners.

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Figure 2-1: Single-layer underlayment

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P

* 6" MIN.END LAP *

* END- AND SIDE-LAP DIMENSION

VARIES BY PRODUCT/MATERIAL

Figure 2-2: Single-layer self-adhering underlayment

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6, When a double-layer underlayment is required, the ma-terial should be applied horizontally in shingle fashion.

For double-layer applications, all underlayments should be lapped a minimum of one-half the sheet width plus minimum of 4 inches or as recommended by the manu-facturer. Underlayment should be fastened with nails to hold the material in place before and during the primary and patterns for underlayment in high-wind regions in an attempt to hold the underlayment in place in the event of wind blow-off of the primary roof covering.

and underlayment layers are identical materials, they are commonly installed in shingle fashion as described previ-temporarily or underlayments of two different composi-tions are used, each layer may be applied as a single layer.

Two Layers of Underlayment: A two-layer underlay-the second layer may be mechanically fastened or adhered

The same underlayment material may be used for both layers or a different-composition underlayment may be used for each layer. Each underlayment layer is lapped as appropriate for the underlayment product, in shingle

357The NRCA Roofing Manual: Steep-slope Roof Systems—2013 Slate Roof Systems | Chapter 2—Underlayments

Figure 2-4B: Two layers of underlayment—first layer mechanically attached, second layer adhered

Figure 2-3: Double-layer underlayment

Figure 2-4A: Two layers of underlayment—both layers mechanically attached

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6, sheets lapped approximately one-half the roll width in shingle fashion.

be applied consecutively or at two different times, for ex-roof deck as required for wind-uplift resistance and to hold the material in place until primary roof covering applica-tion is completed. The second layer should be fastened to a roof deck as required for wind-uplift resistance and to hold the underlayment in place until primary roof covering ap-and patterns for underlayment in high-wind regions in an attempt to hold the underlayment in place in the event of wind blow-off of the primary roof covering.

Underlayment Application in High-wind Areas: The International Building Code, 2012 Edition

International Residential Code, 2012 Edition slate roof system underlayment application in areas sub-ject to high winds. The two model codes provide similar prescriptive requirements for these situations.

For underlayment installed in areas subject to wind speeds respectively—the two model codes provide the following requirements:

accordance with the manufacturer’s installation instructions exceeding 36 inches on center.

For underlayment installed in areas subject to wind respective applicable wind maps—the two model codes provide the following requirements:

inch spacing in areas between side laps and 6-inch spacing in the side laps er’s installation instructions with the exception that all laps be a minimum of 4 inches shank and sufficient length to penetrate through the roof sheathing or a minimum of ¾ inches into the roof sheathing.

As an alternative to fastened underlayment, self-adhering

Water and Ice-dam Protection Underlayment: Regardless of the type of underlay-ment required or roof slope, in locations where the aver-age temperature for January is 30 F or less, NRCA rec-ommends installation of a water and ice-dam protection protection membranes in areas where the mean January temperature is 25 F or less. Designers are urged to consult the applicable building code.

Water and ice-dam protection membranes generally are single-layer applications of self-adhering polymer-which should be designated on the packaging.

Water and ice-dam protection membranes should be ap-plied starting at a roof system’s eaves and extend upslope to a point corresponding to a minimum of 24 inches— determined in the level plane—inside the exterior wall extend up the adjoining walls, to the top of flashing.

Water and ice-dam protection membranes for downslope perimeters and various flashing locations, including tran-sitions, valleys and penetrations, are recommended in cold regions. In climates where ice damming can be severe, consideration should be given to extending the water and ice-dam protection membrane farther up the slope.

As an option in cold regions, underlayment from the

358 The NRCA Roofing Manual: Steep-slope Roof Systems—2013 Slate Roof Systems | Chapter 2—Underlayments

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6, 5.

d ice-dam protection membranes should be ap-plied starting at a roof system’s eaves and extend upslope to a point corresponding to a minimum of 24 inches— determined in the level plane—inside the exterior wall protection membrane and underlayment are considered independent and separate components and provide a measure of redundancy for the roof system. In view of ited service life histories of self-adhering sheet products, derlayment extended over water and ice-dam protection membrane for slate roof systems.

Water and ice-dam protection membranes by them-selves should not be relied on to keep leaks or moisture problems from occurring. Careful consideration of roof tumen underlayments should not be left exposed for long material as soon as practical to prevent degradation. The manufacturer can be consulted for extended exposure

359The NRCA Roofing Manual: Steep-slope Roof Systems—2013 Slate Roof Systems | Chapter 2—Underlayments

Figure 2-5: Areas of the U.S. with an average January temperature below 30 F, composite of 1981-2012 data. Map is based on data provided by the NOAA/ESRL Physical Sciences Division, Boulder, Colo., from its website, www.cdc.noaa.gov. Contours represent U.S. Climate Divisions.

Figure 2-6: Installation of water and ice-dam protection membrane

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6, 5.

Figure 2-6:

t are considered independent and separate components and provide a measure of redundancy for the roof system.

time recommendations.

cool/cold climates may need the application of heated air to assist sealing to the substrate and at side and end laps.

consulted for additional information.

Local codes should be consulted for any additional re-quirements for the use of water and ice-dam protection membranes.

360 The NRCA Roofing Manual: Steep-slope Roof Systems—2013 Slate Roof Systems | Chapter 2—Underlayments

Figure 2-7: Field underlayment extended to eaves over water and ice-dam protection membrane

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6, Slate rock originates from sedimentary particles of clay and silt. Silt and other particles were originally washed down into streams, which transported the particles into ancient seas. As quantities of these particles were deposited over time, successive parallel layers of the material accumulated.

These accumulations of sediment build up over millions of years to varying thicknesses and are called beds.

The pressure of successive overlying layers compressed and consolidated the lower underlying sedimentary lay-ers, which formed shale. Years of these geological forces caused great pressure on the underlying shale layers and, as pressures rose, heat and force loads combined to chem-ically change original clay materials into harder materials such as chlorite, mica and quartz. Many years of these combined geological forces transformed relatively weak shale rock into hard slate.

The original layering of materials gives slate natural cleav-age planes. These natural cleavage planes are what allows slate to be easily split into relatively thin layers. Slate also possesses secondary natural grain planes that usually occur at right angles to cleavage planes. Roofing slate is commonly split so that the length of a slate runs in the direction of the natural grain. See Figure 3-1 on page 362.

The surface texture of slate after a slate is split for com-mercial use depends on the quality and characteristics of the rock from which it was quarried. Many slates split to smooth, practically even, uniform surfaces and others split to surfaces that are rough and uneven. As a result, a wide range of surface textures are available.

Some slate contains narrow “ribbons of rock” that are dif-ferent in chemical composition and color from the princi-pal slate. Slate that has been trimmed so that the ribbons are eliminated is known as clear slate. Slate that contains acceptable ribbons is sold as ribbon stock.

3.1 Slate Materials

Slate is characterized by its color, thickness and weight, size and physical properties.

CHAPTER 3

SLATE

The NRCA Roofing Manual: Steep-slope Roof Systems—2013

361The NRCA Roofing Manual: Steep-slope Roof Systems—2013 Slate Roof Systems | Chapter 3—Slate

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6, Colors: The color of slate is determined by its chemical and mineral composition. Because these factors differ in various regions, it is possible to obtain roofing slate in a variety of colors and shades. Various shades of the same slate may be used to provide interesting color mottling or shaded patterns when applied across a roof or when interspersed randomly. The diversity of color makes slate a sought-after material for the creation of certain architec-tural or aesthetic arrangements.

Exposure to weather causes all slate to change slightly in color and composition. The degree of color change varies. Slate that exhibits minimal color change is classi-fied as unfading. Slate that exhibits a more obvious color change is known as weathering. Weathering slate offers designers another variation in roof aesthetics. Weathering and semi-weathering gray and green slates contain some slates that can weather to buff or tan as the mineral par-ticles in the slate oxidize, providing a varied color pattern throughout a roof system.

When texture and color are essential considerations in roof system design, architects and building owners should consider the source of slate, its potential for color change

362 The NRCA Roofing Manual: Steep-slope Roof Systems—2013 Slate Roof Systems | Chapter 3—Slate

CLEAVAGE PLANE

1. QUARRIED

BLOCK OF

SLATE ROCK

GRAIN PLANE

3. SCULPTED

BLOCKS

4. EIGHTS

5. ROUGH

ROOFING

SLATES

2. CROSS-CUT

BLOCKS

QUARRIED BLOCKS OF SLATE ROCK ARE CROSS-CUT INTO SECTIONS, THEN BLOCKS ARE

SPLIT (SCULPTED) ALONG THE GRAIN. BLOCKS ARE THEN SPLIT ALONG THE CLEAVAGE

PLANE INTO THINNER BLOCKS OFTEN REFERRED TO AS "EIGHTS." THE EIGHTS ARE SPLIT

ALONG THE CLEAVAGE PLANE TO YIELD EIGHT INDIVIDUAL ROUGH SLATES. THESE ROUGH SLATES

ARE THEN TRIMMED AND THE NAIL HOLES PUNCHED TO PRODUCE A FINISHED ROOFING SLATE.

Figure 3-1: Cutting quarried slate to produce roofing slate

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6, and the effect of weathering on the slate to be used. Slate producers know from experience the probable degree of color change for various slate materials obtained from their quarries. Producers may be consulted regarding se-lection of particular slate for a specific project.

For the purpose of classifying the basic natural colors of roofing slate currently available in large quantities for general use, the following color nomenclature may be used:

In addition, slate distributors use the following names for certain roofing slate:

When selecting a slate color from a distributor, the color should be preceded by the word “unfading” or “weather-ing” to designate the color stability or change that may be expected for a particular type of slate. In addition to the colors previously mentioned, slate distributors use differ-ent combinations of names to identify the colors of slate.

Weight: A square of slate on a roof system set at the standard 3-inch head lap will vary in weight from 700 pounds to 8,000 pounds depending on the thickness of each slate.

Slate varies significantly in weight because of the numerous thicknesses and types available from the different quarries.

Figure 3-2 shows the approximate weights of roofing slates of different thicknesses. The actual weight of slate products may be from 10 percent above to 15 percent below the weights shown.

A quarry or supplier can be consulted for specific slate weight(s), length, width dimensions and thickness. Slate is also available with sawn or chamfered edge, depending on the slate producer.

Quantities Per Square: In the U.S., slate is sold by the square. A square of roofing slate is defined by the U.S.

Department of Commerce, National Bureau of Standards in Simplified Practice Recommendation No. 14 as: “...a sufficient number of slate shingles of any size to cover 100 square feet of plain roofing surface, when laid with the approved or customary standard head lap of 3 inches.

Slates for surfacing flat roofs are usually laid tile-fashion, without lap, in which case a square of slate would cover an area greater than 100 square feet.”

The quantity of slate per square varies from 89 pieces of 26- by 14-inch slate to 686 pieces of 10- by 6-inch slate.

These quantities include an allowance for the 3-inch head lap.

It should also be noted that for roofs of lesser slope, where a 4-inch head lap is used, an additional quantity of slate is required to cover 1 square of roof. For very steep roofs or vertical siding/cladding applications, where a 2-inch overlap is sufficient, fewer slates will be needed.

363The NRCA Roofing Manual: Steep-slope Roof Systems—2013 Slate Roof Systems | Chapter 3—Slate

Approximate Weight of Roofing Slate at Various Thicknesses (Based on 3-inch Head Lap)

Slate Thickness Weight

3/16 inch to ¼ inch 700 lbs/sq. to 1,000 lbs/sq.

⅜ inch 1,500 lbs/sq.

½ inch 2,000 lbs/sq.

¾ inch 3,000 lbs/sq.

1 inch 4,000 lbs/sq.

1¼ inches 5,000 lbs/sq

1½ inches 6,000 lbs/sq

1¾ inches 7,000 lbs/sq.

2 inches 8,000 lbs/sq.

Figure 3-2: Approximate weight of roofing slate at various thicknesses

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6, Figure 3-3 shows common length and width dimensions for slate, the minimum number of slates required per square and respective exposures for the slates listed.

Slate in sizes other than those listed may also be available.

Standards: To satisfy the need for slate classifica-tion, the National Slate Association established various standards for commercial slate during the 1800s. The primary standard was based on visual grading of slate by color, surface texture, straightness, thickness and weight.

Even though grading was visual and varied from region to region, the standard helped ensure slate products (that met the standard) would contain certain physical charac-teristics essential for quality slate roofing. For many years, roofing slates were specified according to the National Slate Association’s standard.

Since 1957, ASTM International has established a con-sensus material standard for slate used for roofing shin-gles. This material standard is designated as ASTM C406, “Standard Specification for Roofing Slate.” ASTM C406 addresses material characteristics, physical properties and sampling procedures appropriate to the selection of slate for use as roof shingles.

ASTM…