Prior to the mid-to-late 1970s, almost all low-slope roofs were asphalt or coal tar built-up roofs. However, during the last two decades of the 20th century, a variety of other types of low-slope roof systems began to compete with traditional built-up roofs (BUR). These newer systems included modified bitumens, single-plies, sprayed polyurethane foam, and metal panels. While the modified bitumen systems are related to BUR, the other low-slope alternatives are radically different. Along with new choices of membrane materials, plastic foam roof insulations also emerged in the 1970s. The abundance of materials from which to choose has greatly complicated roof system design.

Roof Decks
Office buildings typically have steel or concrete decks, although plywood or OSB decks are also used on smaller buildings. The deck can have significant influence on the roof system.

Vapor Retarders
Vapor retarders are typically constructed with sheet materials such as 6 mil polyethylene, a two-ply built-up membrane or a one-ply modified bitumen sheet.

Insulation
There are two categories of roof insulation, rigid and non-rigid. Rigid boards are typically used in low-slope assemblies. Non-rigid insulations are typically used in attic spaces and in pre-engineered buildings.

Rigid Insulation Boards
Board-stock insulation has sufficient compressive resistance to support the roof membrane. The following common types of rigid insulation boards are available :

• Perlite : This is a low R-value insulation. It is commonly used as a cover board. It has good fire resistance, but when exposed to water, it loses compressive resistance, turns to mush and can be easily compressed. When hot asphalt is applied over ½" boards, the potential for development of blisters in built-up and hot-applied modified bitumen membranes is increased.

• Polyisocyanurate : This is a high R-value insulation. This is one of the plastic foam insulations. It is widely used in low-slope roof systems.

• Polystyrene : There are two types of polystyrene insulation, molded expanded and extruded expanded. The two types have distinctly different properties. Polystyrene is one of the plastic foam insulations. Polystyrene boards should not be in direct contact with PVC membranes, otherwise the polystyrene will leach plasticizers out of the PVC. A suitable separator needs to occur between polystyrene and PVC.

• Molded Expanded Polystyrene (MEPS or EPS) : This is moderate R-value insulation. The low-density product is relatively inexpensive. Solvent-based adhesive and hot asphalt disintegrate MEPS. Hence, if either of these are used, a suitable cover board needs to be installed over the MEPS. MEPS can also be decomposed at high temperature. Therefore, MEPS should not be used underneath a black membrane unless a suitable cover board is installed between the MEPS and the membrane. MEPS cells are filled with air. Therefore, unlike the other plastic foam insulations, MEPS does not thermally age (i.e., loose R-value over time). MEPS is not very resistant to water vapor—when exposed to water vapor drive, MEPS can absorb a considerable amount of moisture.

• Extruded Expanded Polystyrene (XEPS) : This is a high R-value insulation. XEPS is very resistant to water vapor drive. However, as with MEPS, XEPS should not be exposed to solvent, hot asphalt or very high temperature. But unlike MEPS, in order to avoid membrane splitting, XEPS should not be used below a built-up or modified bitumen membrane (even if a cover board is installed over the XEPS). XEPS is the only insulation suitable for use above the roof membrane in protected membrane roof (PMR) systems. However, boards intended for PMRs need to be specifically manufactured for this application. Some minor water absorption may occur in boards located above the membrane during the roof's service life. To account for the R-value reduction due to the water absorption, it is recommended that the roof designer reduce the board's initial R-value by 10%. XEPS boards with extremely high compressive resistance are available for use in plaza decks where high compressive loads occur.

• Wood fiberboard : This is a low R-value insulation. It is commonly used as a cover board. This board has good compressive resistance. However, when exposed to water, it looses compressive resistance and can be easily compressed.

• Composite boards : Composite boards typically consist of two layers of different types of insulation that are laminated together in a factory. The primary insulation is typically polyisocyanurate or MEPS. The secondary layer is typically perlite, wood fiberboard, oriented strand board (OSB), plywood or gypsum board. Composite boards made with OSB or plywood are commonly referred to as "nail base." Some nail base products have a small ventilation cavity between the primary insulation and the OSB or plywood. With some composite boards, the secondary layer (which is typically the top surface) is superficially adhered to the primary layer. With these boards, it is important to mechanically attach the composite board rather than adhere it. Otherwise the secondary layer could easily detach.

Low-Slope Roof Coverings
The following membranes are typically used on low-slope roofs, but may also be used on steep-slope roofs. When used on steep-slopes, the system's fire resistance may be reduced and/or special precautions may be needed when used on steep-slopes.
Note: Liquid-applied membranes are available, but are not commonly used. They should only be considered when unique circumstances occur.
 

• Built-up Roofs (BUR) : Built-up membranes are composed of alternating layers of bitumen (either asphalt or coal tar) and reinforcement sheets (felts). Fiberglass felts are typically used for asphalt BURs, however, polyester felts are available. The asphalt is typically hot-applied, however, cold-applied asphalt is available (cold-applied asphalt incorporates solvent). The membrane is either adhered to the substrate in bitumen, or a base sheet (i.e., a heavy felt) is mechanically attached. Exposed asphalt is susceptible to relatively rapid weathering. Therefore, BURs are surfaced with aggregate, a field-applied coating or a mineral surface cap sheet. If aggregate is specified, wind blow-off should be considered. Coatings include aluminum-pigmented asphalt, asphalt emulsion (reflective or non-reflective), and acrylic. Coatings can enhance fire resistance. However, if coatings are specified, periodic recoating will be required. Because of future maintenance demands, coatings are not recommended. Base flashings are typically constructed with modified bitumen sheets. Although coal tar is still available, the vast majority of BURs are constructed with asphalt.

• Modified Bitumen (MB) : MB membranes exhibit general toughness and resistance to abuse. They are typically composed of pre-fabricated polymer-modified asphalt sheets. Polymers are added to bitumen to enhance various properties of the bitumen. The quality of MB products is highly dependent on the quality and compatibility of the bitumen and polymers, and the recipe used during the blending process. There are three primary types of MB sheets, as well as field-applied modified mopping asphalt :

• Atactic polypropylene (APP) : APP polymer is blended with asphalt and fillers. The mixture is then factory-fabricated into rolls that are typically one meter wide. The prefabricated sheet, commonly referred to as a cap sheet, is typically reinforced with fiberglass, polyester or a combination of both. The sheets are available smooth (i.e., unsurfaced); embedded with mineral granules of a variety of colors; or factory-surfaced with metal foil such as aluminum, copper or stainless steel. The aluminum foil is available in colored finishes. APP MB membranes are generally resistant to high-temperature flow.

• Styrene-butadiene-styrene (SBS) : SBS polymer is blended with asphalt and fillers. The mixture is then factory-fabricated into rolls with reinforcement and surfacing similar to APP MB sheets. SBS sheets generally have good low-temperature flexibility. SBS MB is susceptible to premature deterioration when exposed to UV radiation. Although the sheets could be coated in the field, factory-surfacing is recommended. SBS MB membranes are typically composed of a base sheet and an SBS cap sheet. The cap sheet is either heat-welded (i.e., torched) to the base sheet, or it is adhered in cold adhesive or hot asphalt. Mechanically attached systems are also available. Hot asphalt is not recommended for attachment of cap sheets because of the great potential for development of blisters.

• Styrene-isoprene-styrene (SIS) : These self-adhering sheets are blended with SIS polymer, asphalt and fillers. The mixture is then factory fabricated into either 3 feet or 1 meter wide rolls. The top of the prefabricated sheet is available with embedded mineral granules or a factory-laminated UV-protective surfacing, such as aluminum foil. The bottom surface has a release paper to keep the sheet from bonding to itself while rolled. A similar product is commonly used under steep-slope roof coverings to provide ice-dam protection. However, the steep-slope underlayments do not have a UV-protective surfacing. SIS MB roof membranes currently capture a very small share of the low-slope market.

• Styrene-ethylene-butylene-styrene (SEBS) : SEBS polymer is blended with asphalt in a factory. The SEBS modified asphalt is then reheated at the job site in specially-designed tankers or kettles. The hot modified asphalt is applied in a manner that is virtually identical to BUR. The membrane is typically surfaced with aggregate. SEBS modified mopping asphalt is extremely expensive, and therefore not commonly used. Modified mopping coal tar was introduced in the mid-1990s, but it has very limited market share.

• Single-Ply : The single-ply family of roof membranes is composed of thermoplastic and thermoset products. Single-ply sheets are factory-fabricated and installed in a single thickness. Single-ply membranes are relatively easy to install on steep or complex roof slopes. In comparison to BUR or MB membranes, they are also very light weight (except for ballasted systems). However, unless used in a PMR configuration, they do not offer nearly as much toughness and resistance to abuse as do BUR and MB membranes.

There are four primary methods for securing the membrane the roof deck or other substrate:

• Fully adhered : The membrane is adhered in a continuous layer of adhesive. Some single-plies are also available with a factory-applied tackifier on the underside of the sheet, which permits them to be self-adhering (several manufacturers introduced these products in the early 2000s).

• Ballasted : The membrane is loose-laid over the substrate and then covered with ballast to resist wind uplift. Ballast can either be large aggregate, concrete pavers, or specially designed lightweight interlocking concrete pavers weighing approximately 10 psf [49 kg/m²]. If crushed aggregate is specified, a stone-protection mat between the membrane and aggregate should be specified to avoid puncturing the membrane. A stone-protection mat is also recommended when smooth aggregate is used because some sharp fragments are often among the smooth aggregates. Also, aggregates sometimes fracture into very sharp pieces after they have been installed. It is also a conservative practice to specify a mat to protect against abrasion and puncture from fragments during paver installation. A somewhat thinner mat is normally sufficient for paver-ballasted jobs.

• Mechanically attached : The membrane is loose-laid except for discrete rows of fasteners. There are a variety of fastening and seam fabrication with this method.

• Protected membrane roof : See the Modified Bitumen (MB) section above.

Thermoplastic Single-plies
Thermoplastic materials do not cross-link, or cure, during manufacturing or during their service life. Field-fabricated seams are typically welded with robotic hot-air welders. Hand-held hot-air welders are used to weld seams at flashings and penetrations. Thermoplastic membrane seams are typically extremely reliable, resulting in a very low incidence of seam failures. These sheets are normally around 5 to 12 feet wide [1.5 to 3.6 m]. Some manufacturers weld the sheets together in the factory to form large sheets that are then welded together on the roof. Primary membrane types in this category are:

• Polyvinyl chloride (PVC) : PVC membranes are among the oldest single-plies still available. If in contact with polystyrene insulation, the polystyrene will cause the plasticizers in the membrane to leach out. To avoid such membrane embrittlement, a separator sheet needs to be installed between the membrane and the polystyrene. To avoid membrane damage, a separator is also needed to isolate PVC from asphalt and coal tar products. The ballasted attachment method is not recommended because fine dust particles from the ballast or particulate fall-out from the atmosphere may leach plasticizers from the membrane. PVC membranes are available in a wide variety of colors. This membrane is often selected for steep-slope roofs where a strong or unique color is desired.

• PVC Alloys or Compounded Thermoplastics (also referred to as PVC blends) : These membranes are related to PVC membranes. They are primarily compounded from PVC, but they have additional polymers that provide somewhat different physical properties. Only a very small number of manufacturers make these products. The primary types of membranes in this category are: copolymer alloy (CPA), ethylene interpolymer (EIP) and nitrile alloy (NBP).

• Thermoplastic polyolefin (TPO) : TPO is the latest thermoplastic membrane introduced into the marketplace. It was commercialized in North America in the early 1990s. It is formulated from polypropylene, polyethylene or other olefinic materials. Unlike PVC and PVC blends, TPO membrane do not rely upon plasticizers for flexibility, so embrittlement due to plasticizer loss is of no concern. TPO membranes are typically white, and are available in sheet widths up to 12' [3.6 m].

• Ketone ethylene ester (KEE) : This membrane is also referred to as a tripolymer alloy (TPA), and the polymer is known by the trade name of Elvaloy. KEE sheets are similar to PVC.

Thermoset Single-plies
Thermoset materials normally cross-link during manufacturing. Once cured, these materials can only be bonded together with a bonding adhesive or specially formulated tape. Primary membrane types in this category are :

• Ethylene propylene diene monomer or terpolymer (EPDM) : EPDM is a synthetic rubber sheet. As of 2005, EPDM enjoys the largest market share of the single-plies in service in North America. EPDM membranes are extremely resistant to weathering, and they have very good low-temperature flexibility. However, EPDM is susceptible to swelling when exposed to aromatic, halogenated and aliphatic solvents, and animal and vegetable oils such as those exhausted from kitchens. On portions of roofs where the membrane may be exposed to these materials, an epichlorohydrin membrane can be specified over the EPDM as discussed below. EPDM membranes are suitable at airport buildings, provided liquid fuel is not spilled on the membrane. In fully adhered applications, typically a contact adhesive is applied to the substrate and the sheet. After the adhesive dries, the sheet is mated with the substrate. Another method of application uses fleece-backed EPDM, which is set in a low-rise sprayed polyurethane foam adhesive.

• Epichlorohydrin (ECH) : This sheet is similar in appearance to EPDM. ECH, however, is resistant to hydrocarbons, solvents and many greases and oils, so it can be used in areas of the roof that are exposed to chemical discharges that are harmful to EPDM. Because of its permeability, the ECH manufacturer recommends placing ECH over an EPDM membrane. Because it is so specialized, ECH is seldom used. Only one manufacturer produces it in North America.

Sprayed Polyurethane Foam (SPF)
SPF is a very unique type of roof system. The membrane is constructed by spraying a two-part liquid onto a substrate. The mixture expands and solidifies to form closed-cell polyurethane foam. The substrate can be either the roof deck, an existing roof membrane (provided the existing roof is suitable for re-covering), gypsum board or rigid insulation. The foam is applied with hand-held sprayers or by robotic sprayers. Each pass (or lift) of foam is typically between ½ to 1-½ inches [13 to 38 mm] thick. If a greater total thickness is desired, two or more passes are normally required. The total thickness of the foam can be easily varied to provide slope for drainage. The foam needs to be protected from UV radiation. This is typically accomplished by using one of the following surfacing :

• Acrylic coating: This is the least expensive of the coatings, but generally offers the shortest service life.

• Polyurethane coating: When properly formulated, this coating offers long service life. This can be the toughest coating available in terms of impact and tear resistance, although a wide range of physical properties is available in this product category.

• Silicone coating: Silicone coatings offer exceptionally good weather resistance and long service life. These coatings are typically offered in a gray color, as silicone coatings pick up dirt (if a white silicone is installed, it will soon become gray).

• Mineral granules: Mineral granules (similar to those used to surface asphalt shingles) can increase the durability of a coating and provide greater slip-resistance to persons on the roof.

• Aggregate surfacing: Properly formulated and installed SPF is quite resistant to liquid water. Therefore, aggregate of the size used on BUR systems can be applied directly over the foam.

​​
Other considerations : The worker performing the spraying must be very skilled and knowledgeable. If the qualifications of the contractor and the spray mechanic cannot be reasonably assured, it is prudent to specify an alternative system.