Landmark Roofing - Commercial & Residential

There are five generic classifications of low slope roof membranes or systems. Click on a roof system for more information.

Weatherproofing layer or layers — the weatherproofing component is the most important element because it keeps water from entering a roof assembly.
Reinforcement — reinforcement adds strength, puncture resistance and dimensional stability to a membrane.
Surfacing — surfacing is the component that protects the weatherproofing and reinforcement from sunlight and weather. Some surfacings provide other benefits such as increased fire resistance, improved traffic and hail resistance, and increased solar reflectivity.

Steep-Slope Roofing Systems

There are six generic classifications of steep slope roof coverings. Click on a roof covering type for more information.

Asphalt Shingles
Clay Tile and Concrete Tile
Metal Roof Systems (Steep-Slope)
Wood Shakes and Wood Shingles
Synthetic

Steep-slope roof systems typically are composed of individual pieces or components installed in shingle fashion. Steep-slope roof assemblies typically consist of three primary parts:

Roof deck — a roof deck is the structural substrate and usually is a wood-based material such as plywood or oriented strand board (OSB).
Underlayment — underlayment provides temporary protection until a roof covering is installed and provides a secondary weatherproofing barrier. Sometimes underlayment is referred to as "felt" or "paper."
Roof covering — the roof covering is the external watershedding material.

Built up roof membranes, referred to by the acronym BUR, have been in use in the U.S. for more than 100 years. These roof systems are commonly referred to as "tar and gravel" roofs. BUR systems generally are composed of alternating layers of bitumen and reinforcing fabrics that create a finished membrane. The number of plies in a cross section is the number of plies on a roof: The term "four plies" denotes a four ply roof membrane construction. Sometimes, a base sheet, used as the bottommost ply, is mechanically fastened. Built up roofs generally are considered to be fully adhered if applied directly to roof decks or insulation.

The reinforcing fabrics also are called roofing felts or ply sheets. Roofing felts are reinforced with either glass-fiber mats or organic mats. Felts are produced in a standard width of 36 inches and metric width of about one meter.

The bitumen typically used in BUR roof systems is asphalt, coal tar or cold-applied adhesive. The asphalt or coal tar is heated in a kettle or tanker and then applied by mop or mechanical spreader. Asphalt is a petroleum product refined from crude oil; coal tar is derived from the distillation of coal. Cold-applied adhesives typically are solvent-based asphalts that don't have to be heated in a kettle or tanker.

Surfacings for built up roof systems include aggregate (such as gravel, slag or mineral granules), glass-fiber or mineral surfaced cap sheets, hot asphalt mopped over the entire surface, aluminum coatings or elastomeric coatings.

There is only one category of metal roof systems used in low-slope applications — structural metal panel. Structural metal panel roof systems can be used for low slope roofs because of their hydrostatic, or water barrier, characteristics. It is important to note structural metal panel roof systems can be used for steep slope roof assemblies, too.

Most structural metal panel roof systems are designed to resist the passage of water at laps and other joints, as sealant or anti capillary designs can be used in the seams. Structural metal panel roof systems possess strength characteristics that allow them to span supporting members.

There are three components to consider when working iwth a Metal Panel Roof System.

Roof deck

Structural metal panel roof systems are installed over a large variety of substrates. There are two general categories of substrates: one is continuous or closely spaced decking that provides solid support for the metal roof panel, and the other is composed of spaced structural supports (such as purlins) where the metal panels must span between supports. Most structural metal panels are used over spaced structural supports without being supported by a solid roof deck.

Underlayment

Underlayment (or "felt paper" as it is frequently called) is installed over the roof deck before the application of a metal panel roof system. An underlayment performs two primary functions: it provides temporary weather protection until the metal panel roof is installed, and it provides a secondary weatherproofing barrier if moisture infiltrates the metal roof panels.

Underlayments typically are not used with structural metal panel roof systems when intermittent supports are used to carry the roof systems. However, if there is a continuous or closely spaced roof deck, NRCA recommends an underlayment be installed.

Asphalt saturated, nonperforated organic felts are among the most common underlayments used for metal roof systems; they commonly are designated as Type 15 and Type 30 or referred to as No. 15 and No. 30, which are reflective of a once used pound per square weight designation.

Vapor retarders, insulation and ventilation

Condensation should be expected to develop on the undersides of metal roof panels. Careful consideration should be paid to vapor retarder, insulation and ventilation issues. Because every building is in some way unique, building owners and designers may need to consult moisture-control specialists.

Polymer-modified bitumen or modified bitumen (MB) sheet membranes were developed in Europe in the early 1960s and have been in use in the U.S. since the mid 1970s. Polymer-modified roof membranes are composed of reinforcing fabrics that serve as carriers for the hot polymer-modified bitumen as it is manufactured into a roll material. MB roof system membranes are composed of multiple layers, much like BUR membranes. MB roof systems typically are installed as a two-ply system and almost always are fully adhered.

SBS polymer-modified bitumen membranes commonly are installed in hot moppings of asphalt (similar to BUR systems) or cold adhesive. Some SBS modified membranes are self adhering; that is, they contain an adhesive backing.

APP polymer-modified bitumen membranes typically are heat-welded or torch-applied. Consumers should be cautioned that NRCA does not recommend torch-applying a modified bitumen membrane sheet directly to a wood deck.

Generally, APP modifiers impart a "plasticized" quality to asphalt, and SBS modifiers impart a "rubberized" quality to asphalt. MB membranes and EPDM, a thermoset membrane, often are confused by consumers because of colloquialisms used by roofing contractors. MB and EPDM membranes are sometimes called "rubber roofs."

Surfacings for MB membranes include aggregate surfacing, mineral surfacing, metal foil-laminate surfacing and smooth liquid-applied surfacing.

Single-ply membranes are factory-manufactured sheet membranes. They generally are categorized as either thermoplastic or thermoset. Thermoplastic materials can be repeatedly softened when heated and hardened when cooled. Thermoset materials solidify, or "set," irreversibly after heating. Single ply membranes commonly are referred to by their chemical acronyms, such as ethylene propylene diene monomer (EPDM).

Single-ply sheet membranes are produced using one of three manufacturing processes: calendering, extruding or spread coating. The membranes may contain reinforcement layers. Common reinforcements for single ply membranes include polyester fabrics or scrims, glass fiber, or a felt or fleece backing.

A finished sheet's thickness typically is referred to as mil thickness; 1 mil equals 0.001 inch. Common mil thicknesses for these sheet membranes range from 30 mils to 60 mils.

Single-ply membranes can be installed fully adhered, mechanically attached or held down with ballast. Most single-ply roof systems do not receive surfacings.

In many instances, a combination of attachment methods are used to secure a roof system. For instance, an insulation may be mechanically attached to the substrate with the roof membrane fully adhered to the insulation.

Thermoplastic materials are distinguished from thermoset materials in that there is no chemical crosslinking. These membranes can be repeatedly softened by heating or hardened when cooled. Because of the materials' chemical nature, thermoplastic membranes typically are seamed by heat welding with hot air or solvent welding.

There are five common subcategories of thermoplastic roof membranes.

Polyvinyl Chloride (PVC)
PVC Alloys or Compounded Thermoplastics
- Copolymer Alloy (CPA)
- Ethylene Interpolymer (EIP)
- Nitrile Alloys (NBP)
- Tripolymer Alloy (TPA)
Thermoplastic Olefin (TPO)
Chlorinated Polyethylene (CPE)

The most common thermoplastic roof membranes are PVC and TPO. The following provides general descriptions of these two systems.

Polyvinyl Chloride (PVC)

PVC sheets are produced by calendaring, spread coating or extruding, and typically are reinforced with polyester or glass-fiber mats or scrim. PVC sheets contain plasticizers and stabilizers, as well as other additives to impart flexibility and achieve other desired physical properties. Some membranes are available with nonwoven fleece backing adhered to the underside of a sheet.

Sheet widths range from 6 feet to 12 feet wide.

Sheets are typically 45 mils to 90 mils thick.

Seams are sealed by heat or chemical welding.

PVC membranes are produced in numerous colors, though gray and white are the most common.

Thermoplastic Olefin (TPO)

TPO membranes are produced by calendering with lamination, extrusion with lamination, or extrusion-coating techniques. TPO sheets are a blend of polypropylene and ethylene propylene polymers and usually are reinforced with polyester. TPO sheets contain colorant, flame retardants, UV absorbers and other proprietary substances to achieve desired physical properties.

Sheet widths range from 6 feet to 12 feet wide
Sheets are typically 40 mils to 100 mils thick
Seams are sealed by heat welded with hot air
TPO membranes commonly are white or black

PVC and TPO roof membranes can be installed fully adhered, mechanically attached or ballasted. Most PVC and TPO membranes do not receive surfacings.

Thermoset membranes incorporate principal polymers that are chemically cross linked or vulcanized. Membranes that are vulcanized also may be referred to as "cured." One characteristic of true thermoset polymers is once they are cured, they only can be bonded to simliar materials with adhesives.

Ethylene Propylene Diene Monomer (EPDM)
Chlorosulfonated Polyethylene (CSPE)
Epichlorohydrin (ECH)
Neoprene (CR)
Polyisobutylene (PIB)

The most common thermoset roof membrane is EPDM. EPDM principally is composed of two compounds, ethylene and propylene, that are derived from oil and natural gas. The following are some characteristics of EPDM roof membranes:

Sheet widths range from 7.5 feet to 50 feet wide.
Sheets are typically 45 mils and 60 mils thick.
Seams are sealed using liquid adhesives or special formulated tape.
The membranes commonly are black, but white is available.

EPDM roof membranes can be installed fully adhered, mechanically attached (using batten bars) or ballasted. Most EPDM membranes do not receive surfacings.

Asphalt shingles are composed of
1) a base material, either organic felt or glass-fiber mat, that provides support for the weather-resistant components and gives a shingle strength
2) asphalt and fillers
3) surfacing material, generally in the form of mineral granules, that provides protection from impact and UV degradation, also improves fire resistance.

The most common form of asphalt shingles are strip shingles. They are rectangular, the most prevalent size being 12 inches wide by 36 inches long. Strip shingles most frequently have three tabs that are exposed along the length of the shingle for visual effect and are called 3-tab strip shingles.

Shingles may be produced in a single layer or two or more layers. The latter generally are known as laminated strip shingles, or architectural shingles, and they have a three dimensional appearance.

Both 3-tab asphalt shingles and laminated asphalt shingles contain a strip of factory applied adhesive that is activated by the sun's heat after installation and seals each shingle to the next course. The seal strip also provides much of a shingle's resistance to wind uplift.

Clay tile is produced by baking molded clay into tile. The density of the clay is determined by the length of time and temperature at which it is heated. Tiles may be glazed and also may have surface texture treatments applied. As a result, there are a wide variety of tile profiles, styles, finishes and colors available. In addition, each tile may have separate field, ridge, hip, gable and terminal tiles of various shapes. Installation methods depend on the nature of the tile being installed; that is, whether it is two piece, one piece, interlocking or flat.

Concrete tiles are made of portland cement. The material is mixed and extruded on molds under high pressure. The exposed surface of a tile may be finished with cementitious material colored with synthetic oxide additives. The tiles are cured to reach the required strength. They generally have lugs on their undersides for anchoring to batten strips. There are additional waterlocks or interlocking ribs on the longitudinal edges that impede movement and prevent water infiltration.

As with clay tile, there are a wide variety of profiles, styles, finishes and colors available. Color may be added to the surface of a tile or dispersed throughout (color through). Special texture may be added in surface treatment. In addition, each tile type may have separate field, ridge, hip, gable and terminal tiles that are various shapes.

There are three general categories of metal roof systems used for steep-slope roofing applications: architectural metal panel, structural metal panel and metal shingle/shingle panels. Generally, architectural metal panel roof systems are watershedding and are intended for use on steep slope roofs. Structural metal panel roof systems are used on low and steep slope roofs. Structural metal panel roof systems can be used on low slope roofs because of their hydrostatic, or water barrier, characteristics.

Because architectural metal panel roof systems typically are designed to be used on steep slopes that will shed water rapidly over the metal panels' surface, the seams typically are not watertight. Many architectural metal roof systems are well suited for use on roof slopes of 3 inches per foot (14 degrees) or greater. One exception to the general slope guidelines for architectural metal panel roof systems is the traditional flat seamed, soldered or welded metal roof system, such as copper. It may be specified on slopes less than 3 inches per foot (14 degrees). Solid roof sheathing, or decking, is required for architectural metal panel roof systems, and NRCA recommends using underlayment.

Most structural metal panel roof systems are designed to resist the passage of water at laps and other joints, as sealant or anti capillary designs can be used in the seams. Structural metal panel roof systems possess strength characteristics that allow them to span supporting members.
Metal shingles and shingle panels are available in numerous varieties for use as steep-slope roof coverings. Most of the metal shingles are press-formed during the manufacturing process to provide a variety of shapes. These products can take the shape of individual or multiple asphalt, tile, slate or wood shingle configurations.

Wood shakes and wood shingles are manufactured from western red cedar, cypress, pine and redwood trees. Shakes are split from logs and reshaped by manufacturers for commercial use. They are thicker at the butt end than shingles; generally one or both surfaces are split to obtain a textured effect. A split and resawn shake has a split face and sawn back. A taper sawn shake has a natural taper and is sawn on both sides. Wood shingles are sawn on both sides and have an even taper and uniform thickness. When applied to shingles, the industry terms "Perfection" and "Royal" mean 18 inch and 24 inch lengths, respectively.

Cedar shakes and cedar shingles are available pressure treated with fire retardants and chemical preservatives for increased fire resistance and to prevent premature rot and decay in some climates.

Pine shakes are made from southern yellow pine and are taper sawn. They also are available pressure treated with preservatives to protect against decay and insects. Interlayment felts are required for pine shakes.

"Synthetic" as it pertains to steep-slope roofing materials refers to manufactured products that replicate asphalt shingles, concrete tile, clay tile, metal panels, slate, wood shakes and wood shingles. Synthetic roof coverings contain recycled plastic and/or rubber as a key ingredient. These products have been available since 1993.

There are some advantages to using synthetic roof coverings when compared to their traditional counterparts. Synthetic slate, or "fake slate," for example, weighs substantially less than natural slate. The reduction in weight allows synthetic slate to be installed over conventional roof decks. Some synthetic products purport to be hail-, mold- and algae-resistant. Several synthetic cedar shake and cedar shingle manufacturers claim a labor savings, because fire-retardants or anti-algae coatings do not have to be applied to the product.

Despite the benefits, there are some significant drawbacks. Synthetic roof coverings are relatively new and there isn't a proven track record about their performance. Most synthetic products are manufactured with dyes or coloring agents and it is unknown whether these products will fade because of ultra-violet exposure and weathering. It also is unknown whether these products will become more brittle or less flexible over time. And most important, model building codes do not recognize any synthetic roof coverings. You need to check with your local building department before installing these products. Because of these reasons, caution should be exercised when using synthetic roof covering products.

Low Slope Roofing Systems