Shingle Commercial Roofs

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Background

Asphalt shingles first were produced around 1901 and came into general use about 1910. Around 1915, machinery was developed for roller-die cutting of granular roll roofing materials into irregular shapes on continuous production lines. These products were more fire-resistant than wood shingles; offered more colors, shapes and patterns; and were less expensive. 

Asphalt shingles generally are composed of asphalt, fillers, a reinforcing mat and granule surfacing. They are available in a variety of colors, shapes and thicknesses. Some asphalt shingle products are available with enhanced physical properties. For example, some asphalt shingle products are manufactured to be impact-resistant and their use may be desired in regions susceptible to hail.

Asphalt shingles are installed in courses resulting in a multilayered, overlapping, water-shedding roof covering. Asphalt shingles are installed over an underlayment, which is laid continuously over a roof deck substrate. The asphalt shingles, underlayment and related components, including fasteners and flashings, compose what is referred to as an “asphalt shingle roof system."

An asphalt shingle roof system is applied to a nailable roof deck typically consisting of wood panels (plywood or oriented strand board), wood planks or wood boards that are fastened to the underlying structural framing. The combination of an asphalt shingle roof system and a roof deck make up what is referred to as an "asphalt shingle roof assembly.“

Underlayments 

The use of underlayments is recommended with asphalt shingle roof systems. The use of underlayment requires a continuous substrate to support the underlayment material. An underlayment is applied over an entire roof deck before or during the application of asphalt shingles.

Underlayment should be vapor-permeable unless it is intentionally designed to perform as a vapor retarder. An underlayment performs several functions; it provides:

  • Weather protection for a limited time until a roof covering is installed
  • A secondary weatherproofing barrier should moisture infiltrate the asphalt shingles
  • Separation between a roof covering and a substrate

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

  • To comply with the applicable building code
  • To maintain a Class A, B or C fire rating for a roof assembly
  • To meet the requirements of the manufacturer for a material or system warranty

There are different underlayment configurations that can be used for asphalt roof systems. These configurations can be categorized as follows:

  • Single layer of underlayment
  • Single layer of self-adhering underlayment
  • Double layer of underlayment

UNDERLAYMENT MATERIALS

Materials used as underlayment for asphalt shingle roof systems generally consist of asphalt felts, synthetic sheets, polymer-modified bitumen sheets, and water and ice-dam protection membranes.

Asphalt-saturated and asphalt-impregnated felt underlayments are the most common underlayments used in steep-slope roof systems and use an organic reinforcing mat, an inorganic reinforcing mat or a combination organic mat reinforced with inorganic fiber.

Underlayments are reinforced with mats that are designed to support the asphalt. Reinforcing mats of different thicknesses are used to produce underlayments of different weights. These reinforcements, sometimes referred to as carriers, are made of organic fibers, inorganic fibers, or a combination of organic and inorganic fibers.

Synthetic sheets are produced with polyalefin-based resins (e.&» polyethylene and polypropylene), recycled rubber and/or similar components that do not fit into the organic or inorganic felt underlayment classifications previously discussed. Synthetic underlayments generally are characterized 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 synthetic sheet underlayments vary. The polyolefin-based sheets are either polyethylene or polypropylene or a combination of both. Polyethylene has a lower softening point and melting point, and polypropylene has a higher softening point and melting point, which increases stability. Some synthetic sheet underlayments have relatively low permeance ratings--lower than those for organic and inorganic asphalt felt underlayments.

A water and ice-dam protection membrane is a distinctive type of underlayment. This type of underlayment provides additional protection from moisture intrusion along the eaves, at penetrations, at elevation changes, and in valleys where excessive water runoff or ice dams can occur.

 

Water and ice-dam protection membranes generally consit of a single layer of self-adhering polymer-modified bitumen underlayment.

 

Self-adhering polymer-modified bitumen membranes are reinforced with glass fiber or contain a thin layer of polyethylene on the top side. Most of these membranes should not be exposed for extended periods of time before the application of asphalt shingles unless expressly stated on the packaging. Some self-adhering polymer modified bitumen membrane products incorporate a granule surfacing to provide a more slip-resistant surface for workers. A release paper covers the bottom side of such a membrane to prevent it from sticking to itself when it is wound into a roll. This release paper should be removed during application.

 

Self-adhering polymer-modified bitumen sheets used as water and ice-dam protection membranes typically range in thickness from 20 mils up to 60 mils.

UNDERLAYMENT DESIGN AND INSTALLATION

Asphalt shingles are designed for use as multilayered, water-shedding 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 an asphalt shingle roof system, different underlayment materials and configurations will be appropriate.

Roof Slope: 

For roof substrates having slopes of 3:12 up to 4:12, a single layer of self-adhering polymer-modified bitumen underlayment or a minimum double-layer underlayment should be specified. When a double layer underlayment is specified, it should be applied horizontally in shingle fashion, with half the roll width minus 1-inch exposure and half the roll width plus 1-inch underlap.

Underlayment Configurations: 

There are different underlayment configurations that can be used for asphalt shingle roof systems. Generally, these configurations can be categorized as follows:

  • Single layer of underlayment
  • Single layer of self-adhering underlayment
  • Double layer of underlayment

Drip Edge Metal: 

The use of drip edge metal at eaves and rakes provides a means of terminating the underlayment and asphalt shingles and provides for efficient water shedding, most building codes require the use of drip edge metal for asphalt shingle roof systems at eaves and rakes.

Asphalt Shingles

Asphalt shingles are designed for use as multilayered, water-shedding roof components that rely on the slope of a roof substrate to effectively shed water. Asphalt shingles are categorized by their reinforcements and shapes. The shapes or styles of asphalt shingles are identified as follows:

  • Strip shingles:These shingles are in a strip form and generally longer in width than height.

Common dimensions for standard shingles are 12 inches by 36 inches. For metric shingles, the common dimensions are about 13 inches by 39 inches. These dimensions can vary depending on the manufacturer, and some newer products are available in larger dimensions than traditional standard- or metric-sized asphalt strip shingles. Products referred to as "three-tab" and laminated shingles all are categorized as strip shingles. Strip shingles are “self-sealing"; that is, they contain adhesive seal strips that bond to overlying or underlying courses of shingles for wind resistance.

  • Laminated strip shingles: Sometimes called "dimensional" or "architectural" shingles, these shingles have additional material laminated to strips to create random thicknesses, thereby giving the shingles a more dimensional appearance. Laminated shingles are "self-sealing", that is, they contain adhesive seal strips that bond to overlying or underlying courses of shingles for wind resistance. Dimensions and weights vary depending on the manufacturer. Laminated shingles can also take the form of full double laminates, cut-out laminates (sculptured) and triple laminates.

ASPHALT SHINGLE INSTALLATION

Starter courses for asphalt shingles are consistent for each shape or style of shingle. Before the first course of shingles is installed, a starter course is applied directly over the underlayment or water and ice dam protection membrane along the eave of a roof system. Starter courses shed water that may migrate through the joints and cutouts of the shingles in the overlying first course and hold the leading edge of the first course.

When attaching asphalt shingles, the intended location of fasteners depends on the particular shingle type. Fastener location information is provided for the following asphalt shingle types

  • Three-tab strip shingles
  • Laminated strip shingles

For standard size (36-inch-wide) three-tab strip shingles and metric-size (39⅜-inch-wide) strip shingles, manufacturers generally specify full-width shingles be fastened with four nails. When using standard-size shingles with an exposure of 5 inches or metric-size shingles with an exposure of 5 5/8 inches, the nail locations should be in a horizontal nail line about ¾-inch above the top of the shingle's cutouts, about 1 inch in from each edge and centered over each cutout.

For areas considered to be high-wind regions, six-nail attachment of asphalt strip shingles may be required by the applicable building code. For these situations, manufacturers generally specify full-width shingles be fastened with six nails. When using conventional-size shingles with an exposure of 5 inches or metric-size shingles with an exposure of 5 5/8 inches, manufacturers typically specify the nail locations should be in a horizontal nail line about 5/8-inch above the top of the shingle's cutouts, about 1 inch in from each end, and about 1 inch to the left and right of each cutout.

For standard size (36- inch- wide) laminated strip shingles and metric-size (39 inch-wide) laminated strip shingles, manufacturers generally specify full-width shingles be fastened with four nails. For standard-size shingles applied with an exposure of 5 inches, the nail locations should be in a horizontal nail line about 5½ inches above the shingle's butt edge. For metric-size shingles applied with an exposure of 5 5/8 inches, the nail locations should be in a horizontal nail line about 6-1/8 inches above the shingle's butt edge. Nails should be located about 1 inch in from both ends, with two additional nails spaced approximately evenly between.

For areas considered to be high-wind regions, six-nail attachment of laminated shingles may be required by some building codes. For these situations, manufacturers specify full-width shingles be fastened with six nails. For conventional-sized shingles applied with an exposure of 5 inches, the nail locations should be in a horizontal nail line about 5½ inches above the shingle's butt edge. For metric-sized shingles applied with an exposure of 5-5/8 inches, the nail locations should be in a horizontal nail line about 6-1/8 inches above the shingle's butt edge. Nails should be located about 1 inch in from both ends, with four additional nails spaced approximately evenly between. 

Asphalt shingles may be butted and nailed as work progresses up either side of a hip or ridge.

Most manufacturers produce factory-made hip and ridge shingles for use as hip and ridge coverings with laminated strip shingles. Factory-made hip and ridge shingles are generally not provided for three-tab strip shingles and individual shingles. Hip and ridge coverings from full three-tab strip shingles are prepared by cutting the individual tabs.

Application of hip and ridge shingles should begin at the lower end of a hip or at a ridge at the side opposite the most common prevailing wind. The pieces are applied shingle fashion with each lapping over the previous cover and fastened with a concealed nail on each side of the hip or ridge line. The shingles should be kept warm enough to allow for bending over the hip or ridge line without cracking the shingles. Hip and ridge tab exposure should match the exposure of the field shingles.

Exposed fasteners should be sealed with sealant or asphalt roof cement on the last ridge piece of a run at the intersection of hips to a ridge and at the intersection of a ridge to another plane.

A valley is created at the downslope intersection of two sloping roof planes. Water runoff from the portions of roof areas sloping into a valley flows toward and along the valley. Because of the volume of water and the lower slope along a valley line, such an area is especially vulnerable to leakage. A clear, unobstructed drainage path is desired in valleys so the valley can carry water away roof system. With asphalt shingle roof systems, there are three basic types of valleys:

  • Open valley
  • Closed-cut valleys
  • Woven valleys

These three general types of valleys are constructed only after the necessary layer(s) of underlayment and any valley lining material specified have been applied to a deck.

Valley underlayment construction consists of an additional full-width sheet of ASTM D4869, Type I or Type II (No.15) or ASTM D4869, Type III or Type IV (No. 30) asphalt-saturated underlayment felt or polymer-modified bitumen underlayment, base sheet, or water and ice-dam protection membrane. This valley underlayment is centered in a valley. Valley underlayment sheets are secured with only enough roofing nails to hold them in place until the balance of valley materials is applied. The courses of underlayment from the fields of two adjoining roof areas are extended so each course overlaps the valley underlayment by at least 12 inches. A valley is then lined with the balance of the valley flashing. Another recognized installation method is weaving intersecting underlayment courses through a valley in addition to the sheet centered in the valley on top of the underlayment. All layers of underlayment in and through a valley should be tight with no bridging. 

To prevent leakage, it is important with all types of valley construction to avoid placing fasteners near the center of a valley: Generally, fasteners should be kept back from the center of the valley a minimum of 8 inches. However, on low-slope roof valleys or in climates where freeze-thaw cycling or intense rainfall may be regularly anticipated, holding nails back farther from the center of the valley is not uncommon.

A metal valley is constructed by installing typically 8-foot or 10-foot lengths of corrosion-resistant metal from the low point to the high point in the valley. Asphalt shingles are lapped onto both sides of the valley metal, leaving a clear space between the roofing material to channel water runoff down the valley. 

A minimum 36-inch-wide layer of heavyweight felt, polymer-modified bitumen membrane or self-adhering underlayment is centered in the valley under the field underlayment. If valley underlayment is lapped, the ends should overlap at least 12 inches and be adhered.

Valley metal should be installed so the downslope end is flush with the eave shingle starter strips. When heavy laminated shingles are used, the valley flanges can be hemmed and each section fastened with metal clips spaced 8 inches to 24 inches. In many regions, metal valleys are fastened by nailing along outer flanges and the flanges are not hemmed.

The type and minimum thickness of the metal used in an open valley should be commensurate with the expected

service life of the asphalt shingle roof system. Suggested valley metal for asphalt shingle roof systems be fabricated from one of the following metal types and minimum thicknesses:

  • 26-gauge galvanized steel
  • 26-gauge refinished galvanized steel
  • 26-gauge stainless steel
  • 26-gauge Galvalume*
  • 0.032-inch-thick aluminum
  • 0.032-inch-thick prefinished aluminum
  • 16-ounce copper
  • 16-ounce lead-coated copper

Valley metal should also be formed into a W-shape with a splash diverter or rib in the center. A center rib can be especially beneficial in valleys where adjoining roof areas are of unequal slope because the rib helps prevent wash over of runoff. A center rib should not be less than 1 inch high. For easier installation and for controlling thermal expansion and contraction, valley metal pieces used with asphalt shingle roofing should be no longer than 10 feet.

Valley metal for use with asphalt shingles should be a minimum of 24 inches wide. This means the flanges on each side of a metal valley centerline are about 11 inches wide. This allows asphalt shingles to lap onto a flange at least 4 inches.

Open valleys permit clear, unobstructed drainage and are advantageous in locations where fallout from surrounding foliage settles on a roof surface and tends to accumulate in valleys, where slopes are relatively shallow and where high-definition laminate shingles are used.

In climates prone to heavy accumulations of snow and ice or regular freeze-thaw cycling, open valley construction can be enhanced by the following procedures:

  • Using clips to secure flanges where heavy shingles are used
  • Lining a valley with a self-adhering polymer modified bitumen underlayment material before application of the metal valley
  • Stripping in the flanges on each side of a metal valley with a 9- to 12-inch strip of self-adhering polymer-modified bitumen underlayment material. The self-adhering polymer-material is adhered onto the valley metal flange and the underlying width of similar self-adhering polymer-membrane material. Adding a closure at the eave end of the W-shaped valley metal to minimize water and ice infiltration
  • Tapering the valley so it is wider at its low point than it is at its high point

Tapering a valley has the following advantages:

  • It allows for increase in water runoff volume to be received at the downslope end.
  • It allows any ice that may form within a valley to free itself when melting and slide down and exit the valley rather than lodging somewhere along the length of the valley.

A valley's width, or the amount of space between the intersecting asphalt shingles, should increase uniformly so the valley widens as it continues downslope. The difference in the width of the upper end of a valley and lower end is referred to as the taper. In most climates, the amount of valley taper is suggested to be about 1/8 inch for 12 inches. For example, in a valley 16 feet long, the distance between asphalt shingles should be 2 inches greater at the bottom of the valley than at the top. The length of a valley may necessitate a wider metal profile to allow for taper from top to eave.

In closed-cut valleys, shingles on one side of the valley are installed across the valley and shingles from the other side are cut about 2 inches short of the centerline of the valley. The closed-cut valley method can be used with most types of strip shingles and light weight laminates. Single-layer, individual-type shingles cannot be used because nails may be required at or near the center of the valley.

The first course of shingles is laid on the roof section with the lowest slope, or if the slopes are equivalent, on the side with the shortest distance to the ridge (or the side that will receive the lowest runoff volume). To avoid placing a nail in an overlapped shingle too close to the valley center, it may be necessary to cut a strip short and continue from this cut end through the valley with a full-length shingle. The shingle intersecting the valley should continue through the valley and extend at least 12 inches onto the adjoining roof area.

No nails should be located closer than 6 inches to the center of the valley. Two nails should be placed at the end of each terminating shingle. The upper corner of each end shingle should be trimmed to restrict water from migrating back under the courses. It is suggested to set the end of the cut shingles in a bead of roof cement.

In a woven valley situation, shingles from adjoining roof areas are woven to form a closed valley.

Woven valleys are generally limited to three-tab strip shingles and light-weight laminated shingles on roof systems where the roof slope is 5:12 or more, making the valley sloped at 3:12. Where heavy accumulations of debris plant growth may occur between shingle cutouts and joints, a woven valley may hamper runoff. Therefore, as with all valley types, specifying a woven valley should be carefully considered to be sure it is beneficial for a particular project.

It is not recommended the use of woven valleys with heavyweight laminated or individual locking-type shingles. When heavyweight laminated shingles from opposite sides of a valley are woven, they can make for a relatively thick buildup of material, which can make the resultant valley irregularly sloped and slower-draining. Individual locking-type shingles cannot be used with woven valley construction because nails are required for each tab, which would mean placing nails at or near the center of a valley.

When constructing a woven valley, the last shingle in an individual course may not extend fully through the valley. If the shingles were nailed, the last nail would be too close to the valley centerline. Instead, the shingle should be cut and a full-length shingle installed butted to the end of the cut shingle so the full shingle passes through the center of the valley.

The first course of shingles is laid along the eaves (or downslope perimeter) of one roof area, through the valley and onto the adjoining roof area at least 12 inches. The first course from the opposite roof area is similarly laid, ending on top of the first area shingle course. Succeeding courses are then laid alternately.

No nails should be located closer than 6 inches to the center of the valley. The end of each terminating shingle should be nailed.

FLASHINGS

Because roof systems are frequently interrupted by the intersection of adjoining roof sections, adjacent walls or penetrations such as chimneys and vent pipes--all of which create opportunities for leakage special provisions for weather protection must be made at these locations. Careful attention to flashing details is essential to successful long-term roof system performance, regardless of roof system construction. The use of self-adhering underlayment material is suggested at flashing and termination details such as ridges, chimneys, walls, around dormers, dormer tops, rakes, eaves, valleys, pipes, vents, curbs and kick-outs.

Flashings are divided into the following categories:

  • Penetration flashings
  • Vertical surface flashings
  • Skylight flashings
  • Steep- to low-slope transitions

The type and minimum thickness of the metal used for metal flashings should be commensurate with the expected service life of the asphalt shingle roof system.

There are many other small penetrations that need to be flashed into asphalt shingle roof systems, such as vent pipes, exhaust vents, exhaust fans, furnace or water heater flue pipes, electrical standpipes and others. This is typically accomplished with the use of some type of flat flange that extends around a penetration and is installed under shingles on the upslope side of a penetration and over the shingles on the downslope side. Attached and sealed to the flange is a cylinder, rectangular box or neoprene gasket that is used to seal around a penetration. 

These flashing components are sometimes supplied by other trades but may be installed by a roofing contractor.

Flashings at a vertical surface roof plane intersection should have a relatively smooth substrate on the roof plane and up a sufficient height on the vertical to receive the metal flashing. Rough or contoured vertical surfaces, such as cut stone and rough timber, should be provided with a flush substrate above the roofline configured to accept the vertical flashing and/or counterflashing.

There are four types of metal flashings commonly used at locations where an asphalt shingle roof system intersects a vertical surface: apron flashing, step flashing, cricket or backer flashing, and counterflashing. 

Generally, before flashings are applied, a layer of water and ice-dam protection membrane should be applied to a roof deck around any vertical roof penetrations. In addition, a water and ice-dam protection membrane may be installed over the underlayment at the base of walls and around chimneys or curbs.

Apron flashings provide a weather proofing transition material where a roof area intersects a head wall. Common locations for apron flashings are the front, or downslope, side of a dormer, chimney or curbed roof penetration, and horizontal-to-vertical transitions.

For asphalt shingle roof systems where a roof area intersects a vertical side wall, individual pieces of metal flashing are installed at the end of each shingle course. This is referred to as step flashing. 

It is recommended to use metal step flashing that is 7 inches long by 8 inches wide for standard-size shingles, so a 2-inch minimum step flashing head lap is achieved and a 4-inch extension is obtained onto each underlying shingle and 4 inches up the vertical surface. 

For nonstandard shingles, the length of step flashing can be determined by using the general guideline:

Exposure + 2 inches = length of step flashing

For example, if a shingle's exposure is 8 inches, the size of the step flashing would be 8 inches wide by 10 inches long.

The following sequential procedures for new construction may be used to install step flashing at vertical walls prior to the application of the wall cladding or siding:

  • Extend the underlayment material or ice-dam protection membrane, if used, about 3 inches to 4 inches up the vertical wall.
  • Install the starter course. Butt the starter course that intersects the wall tightly against the wall, and fasten the shingle or roll starter material in place.
  • Apply the first piece of metal step flashing over the starter course so the step flashing extends about 4 inches up the vertical wall and overlaps the starter course by about 4 inches. Each step flashing should be placed just upslope from the exposed edge of the shingle that will overlap it. Nail the step flashing near the upper corner of the flange.
  • Install the first shingle course, and butt the end of the shingle that intersects the wall firmly against the step flashing. Fasten the shingle in place, but omit the nail in the end of the shingle that would be driven through the step flashing.
  • Install the second step flashing, being sure to overlap the first step flashing a minimum of 2 inches. Fasten this second step flashing in the same manner as the first step flashing.
  • Install the second shingle course, and butt the end of the shingle that intersects the wall firmly against the second step flashing. Fasten this second shingle in the same manner as the first shingle. Install the succeeding step flashing and shingle courses in the same manner as the first and second step flashing and shingle courses.
  • The wall's felt or air-retarder sheet material and the cladding or siding must be brought down over the upper portion of the step flashings' vertical flange a minimum of 2 inches to serve as counterflashing. The siding should be held far enough above the roof surface so the ends of the siding can be painted or maintained as necessary to prevent dampness from degrading the siding and roofing materials.

Special attention needs to be paid to the bottommost step flashing where an eave intersects a continuous vertical surface to ensure water is diverted to the outside of the wall covering.

When a roof area intersects the upslope side of a chimney or curbed roof penetration, a cricket or backer flashing is installed. A cricket diverts water around a penetration, and a backer flashing provides a weatherproofing transition material where a roof intersects the back of a penetration.

Backer flashing is generally limited to penetrations that are 24 inches wide or less. 

Crickets should be installed at the up-slope side of chimneys or curbed roof penetrations when any of the following conditions apply:

  • The chimney or curb is more than 24 inches wide
  • A large volume of water, snow, ice or debris is expected because of climate or large surface area above the penetration
  • The roof slope is 6:12 or greater
  • The average January temperature is 30 F or lower, and significant accumulations of snow and ice are likely to accumulate on the upslope side of the chimney or curb

Apron, step, cricket and backer flashings require some form of counterflashing to cover and protect their top edges from water intrusion. In many instances, the wall covering or cladding material performs the counterflashing function. When this does not occur, a metal counterflashing mounted to a vertical wall should be installed along the top edge of flashing metal. 

The counterflashing material should be compatible with the cladding or substrate (e.g., aluminum and masonry should not be in contact)

Reference: National Roofing Contractors Association, The NRCA Roofing Manual: Steep-slope Roof Systems, National Roofing Contractors Association 10255 W. Higgins Road, Suite 600, Rosemont, IL 60018, 2011

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