Roof system types
Roof systems and materials generally are divided into generic classifications: low slope and steep slope. Low slope roofing includes water impermeable, or weatherproof, types of roof membranes installed on slopes less than or equal to 3:12 (14 degrees). Steep slope roofing includes water-shedding types of roof coverings installed on slopes exceeding 3:12 (14 degrees).
Low-slope roofing
There are five generic classifications of low slope roof membranes or systems:
- 1.1 Built-up roof (BUR) membranes
- 1.2 Metal panel roof systems for low-slope applications
- 1.3 Polymer-modified bitumen sheet membranes
- 1.4 Single-ply membranes
- Thermoplastic membranes (e.g., PVC, TPO)
- Thermoset membranes (e.g., EPDM)
- 1.5 Spray polyurethane foam-based (SPF) roof systems
Most low-slope roof membranes have three principle components:
- 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.
With some roof membranes, a component may perform more than one function.
Steep-slope roofing
There are six generic classifications of steep slope roof coverings:
- 2.1 Asphalt shingles
- 2.2 Clay tile and concrete tile
- 2.3 Metal roof systems for steep-slope applications
- 2.4 Slate
- 2.5 Wood shakes and wood shingles
- 2.6 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 water-shedding material.
Low-slope roofing
1.1 Built-up roof (BUR) membranes
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.
Photo of an aggregate-surfaced BUR
1.2 Metal panel roof systems for low-slope applications
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.
Example of a structural metal panel roof system
1.3 Polymer-modified bitumen sheet membranes
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.
There are two types of MB roofing membranes:
- 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.
Photo of a mineral-surfaced MB roof system
A roof system composed of a built up roof membrane with 2 plies or 3 plies and a polymer-modified bitumen membrane cap sheet is commonly referred to as "hybrid" system. NRCA considers this type to be a polymer modified bitumen membrane system.
1.4 Single-ply membranes
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.
1.5 Spray polyurethane foam-based (SPF) roof systems
SPF-based roof systems are constructed by mixing and spraying a two-component liquid that forms the base of an adhered roof system. SPF can be installed in various thicknesses to provide slope to drain or meet a specified thermal resistance (R-value). A protective surfacing is then applied to the foam to provide protection from the elements.
The first component of an SPF-based roof system is rigid, closed cell, spray polyurethane foam insulation. The foam is composed of two components: isocyanate and polyol. Transfer pumps are used to get the components to a proportioning unit that properly meters the two at a one to one ratio and heats and pumps them through dual hoses. The components are mixed at the spray gun, which is used to apply them to a substrate.
The second component, the protective surfacing, typically is a spray applied elastomeric coating, though hand and power rollers can be used. The protective surfacing also can be a membrane, such as a fleece backed thermoset single ply membrane. The purpose of the surfacing is to provide weatherproofing, protect the foam from UV exposure, provide protection from mechanical damage and assist with the fire-resistant characteristic of the roof system.
The generic types of coatings used on SPF-based roof systems include:
- Acrylic
- Butyl Rubber
- Hypalon™
- Silicone
- Aromatic Polyurethane Elastomer
- Aliphatic Polyurethane Elastomer
- Modified Polyurethane Elastomer
Mineral granules or sand may be broadcast into a coating to provide increased surface durability and aesthetic value.
The chemicals used to produce polyurethane foam compounds are manufactured or blended to perform in various temperature ranges. Manufacturers of spray polyurethane foam based roof systems offer materials in different reaction profiles. Profile change is created by altering the chemical ingredients, usually with a catalyst, to compensate for ambient temperature changes. Conditions such as wind speed, sunlight, surface moisture, humidity and temperature of the substrate can affect the reaction of the polyurethane foam.
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.
Example of an SPF-based roof system
Steep-slope roofing
2.1 Asphalt shingles
Asphalt shingles are composed of: a base material, either organic felt or glass-fiber mat, that provides support for the weather-resistant components and gives a shingle strength; asphalt and fillers; and surfacing material, generally in the form of mineral granules, that provides protection from impact and UV degradation and 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. Metric shingles are 13-1/4 inches by 39-3/8 inches. 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. Some shingles have a strip of cellophane applied to each shingle to prevent the sealing strips from bonding the shingles together when packaged. When the shingles are installed, the self-sealing strips will not align with the cellophane strips and will bond to adjacent shingles. For this reason, the cellophane strips do not have to be removed.
2.2 Clay tile and concrete tile
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.
Example of a clay tile roof
Concrete tiles are made of portland cement, sand and water in varying proportions. 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.
2.3 Metal roof systems for steep-slope applications
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 water-shedding 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.
Example of an architectural metal panel roof system
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.
Seam types
There are many categories of metal panels. The term standing seam often is used as a generic description for a class of metal roof seams. The name standing seam is derived from the fact that the seams are joined together above the panel flats. The term also is used to refer to a panel profile that includes a standing seam: the vertical leg/flat pan and the trapezoidal seam. The trapezoidal standing seam is more commonly associated with structural metal panels. Other panel types are batten, flat, bermuda and shingled.
The original batten seam consisted of vertical leg panels placed between wood batten strips and covered with a cap. Today, many batten seam panels are constructed entirely of metal. Because they are designed to shed water, batten seam panels primarily are used in architectural applications.
Example of standing seam profiles
A flat seam is created with individual panels applied in a shingled application. One panel edge is folded back on top of itself; the other panel is folded under, and the two panels are hooked together. Again, because flat panels are considered watershedding, they are more commonly found in architectural installations. Some flat seam panel systems, such as copper, are commonly soldered, which allow them to perform on less steep slopes.
Example of a flat seam
2.4 Slate
Roofing slate is a dense, durable, naturally occurring material that is essentially nonabsorbent. Two properties of slate are cleavage and fracture. It has natural cleavage, which permits it to be easily split in one direction. Fracture, usually occurring at right angles to the cleavage, is called the grain. Roofing slate commonly is split so the length of the slate runs in the direction of the grain. The surface texture of slate after being split for commercial use derives from the characteristics of the rock from which it was quarried. Some slate splits to a smooth, practically even surface, while other yields a surface that is rough and uneven.
The color of slate is determined by its chemical and mineral composition. Because these factors differ in various regions, roofing slate can be obtained in a variety of colors. In addition, exposure to weather causes slate to change color. The degree of change varies depending on the slate. Slate exhibiting minimal color change is known as "permanent" or "unfading" slate. Slate that shows a more marked color change is known as "weathering" slate. Between unfading slate and weathering is "semi weathering" slate.
Example of a slate roof system
There are several classifications for slate roof systems. The first is standard slate, which refers to slate that generally is from 3/16 inch (5 mm) to 1/4 inch (6 mm) thick with uniform length. The category "standard smooth" refers to standard slate that has a relatively smooth surface, in comparison with "standard rough" or just "rough". Rough slate has a rougher texture and generally is available in thicker pieces. Finally, there is "graduated/textural slate," which is designed with varying lengths and thicknesses and generally is rougher than standard slates.
2.5 Wood shakes and wood shingles
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.
2.6 Synthetic
"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.