Low-Slope Roofing: Specifying a Quality, Cost-Effective Roof System While Considering the Skilled Labor Shortage

The skilled labor shortage may impact your choices when specifying low-slope roof systems
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Specification Considerations: Materials and Durability

Roof system specification is generally undertaken with a strong eye toward performance and durability because the roof is one of the most vulnerable parts of a building, especially during hazard events. A failure in the roof system can have devastating effects on all areas of the building, critical equipment, and on occupants, and it can contribute to significant downtime after an event if the building is not occupiable due to roof damage.

During storms, the roof system must be able to withstand all types of threats, including high wind, windborne debris impacts, and hail. In extreme climates, a roof system may need to be durable enough to handle long periods of harsh UV exposure or heavy, consistent snow loads. This type of exposure and the key role played by the roof system in protecting the building, its contents, and its occupants requires that architects have confidence in the materials included in their chosen roof system.

Therefore, it is imperative to have a fundamental understanding of the material options, their performance histories, and relative prices. This knowledge can help architects align specifications to reflect a client’s cost-to-value (durable, watertight protection) balance.


As with other building products, roof systems must meet or exceed building codes and insurance requirements. Low-slope roof systems are primarily rated for fire and wind. Proper attachment is the primary consideration for roof systems, and this course will focus on single- and multi-ply roof membrane systems.

Fire Ratings

ASTM E108 defines testing methods and fire ratings for both resistance to fire from above or from under the roof deck. Products are classified as Class A, B, or C, and local codes determine the class of materials required based on location and occupancy type. Roofing product manufacturers will provide the fire rating details for the system, which typically include the classification of the deck construction type supporting the roof system (combustible or noncombustible), roof incline restrictions, and any other requirements, such as barrier boards, ply sheets, and surfacing.

Wind Ratings

ASCE 7 defines requirements for wind uplift resistance, with ASTM D6630 specifically addressing low-slope roof membrane assembly performance. Proper wind-resistant product specification depends on proper design with accurately determined wind loads and wind-resistance capacities based on building location, height, and configuration. Therefore, product-specific ratings should be used as a guide only after determining the project-specific wind loads and required wind-resistance capacity. In addition, for added assurance of a roof system’s wind-resistance capacity a safety factor is often applied to the roof system’s design wind loads. For low-slope roof systems, a minimum safety factor of 2.0 is usually applied. Once the roofing system’s minimum recommended design wind-resistance values are determined, architects should specify roof systems with wind-resistance capacities equal to or greater than these minimum values.

Single-Ply Systems

There are several types of single-ply roof systems that are relatively new to the roofing market. Roofing contractors increasingly turned to EPDM single-ply roofing membranes during the 1970s in response to the oil crisis. TPO single-ply systems began to appear in the market in the 1990s, with self-adhering products introduced in 2002. Single-ply systems have gained popularity primarily due to competitive material cost, before consideration of installed cost.


Single-ply roofing systems are so named because the waterproofing element of the system is comprised of a single layer of material. Products commonly consist of a synthetic sheet material of either a thermoset or thermoplastic compound. Sheets are laid atop a substrate, commonly gypsum-based coverboards, and are either attached to the substrate only in overlapping joints which are made watertight through adhesive or hot air welding or are fully affixed to the substrate with adhesive. Thermoset materials, typically ethylene propylene diene monomer (EPDM), cannot be heat welded at the seams and must use an applied adhesive. Thermoplastic single-ply membranes, generally either thermoplastic polyolefin (TPO) or polyvinyl chloride (PVC), can be hot-air welded together to form cohesive laps.


The most common commercial single-ply roofing system today is TPO. However, because the current formulations have only been on the market for a few years, data is still being gathered on TPO and this product’s ability to resist damage due to heat, UV, and general weather events. While early formulations experienced failures due to heat or UV exposure, the newer formulations are meant to address these early concerns, and time-in-place will be a reliable indicator of durability.

Aside from material breakdown, TPO roof failures generally occur as a result of improper mechanical attachment, inadequate sealing of seams, or damage to the membrane during installation. Installation by an unskilled TPO roofing crew can significantly increase the risk of these types of failures, leading to increased installation time and costs. In addition, as a single-ply TPO, PVC, and EPDM can be vulnerable to damage from foot traffic, they may not be an ideal choice for roofs loaded with mechanical equipment that may require frequent maintenance.

Multi-Ply Systems

Built-up roofing, or BUR (non-modified asphalt), and modified bitumen (asphalt modified with performance-enhancing polymers and additives) are the two most common types of multi-ply roofing systems. Modified bitumen roofing systems continue to be a popular choice for low-slope commercial projects.


Advances in traditional non-modified asphalt roofing have led to new materials and methods, including modified bitumen, which builds on the durability inherent in a multi-ply system. Multi-ply modified bitumen roof systems are comprised of layers of membranes that have been modified with either plasticizers (APP) or rubberizers (SBS). APP additives enhance a membrane’s resistance to oxidation and UV degradation and overall toughness; SBS additives allow the membranes greater flexibility or stretch, increasing resistance to rooftop strain and durability. Depending on their roll in the roof system, membranes are commonly surfaced with sand, film, or minerals/granules. New products have also led to new and improved installation methods over traditional built-up roofs, set only in hot asphalt.


Bituminous roofing systems have long been in use and have proven to be a safe and durable roofing choice. In fact, bituminous asphalt has been used for centuries as protection against fungal decay. The redundancy of layers provides increased durability over a single-ply roofing system and reduces the risk of leaks. Loaded roofs, such as green, living roofs or roofs that house PV systems or large amounts of mechanical equipment, are especially good candidates for the durability of a modified bitumen assembly. Because of the multiple layers and the resiliency of the materials, modified bitumen systems can withstand high foot traffic, and today’s products provide superb tensile strength, as well as thermal performance.

Specification Considerations: System Application

Given the current skilled labor shortage, in addition to material performance, installation time and complexity should also be a consideration when specifying low-slope roof systems.

Single-Ply Systems

Installation Methods

There are several methods of single-ply roof installation. The fully adhered method requires an application of manufacturer-specified adhesive and is installed by gluing the membrane directly to insulation or the substrate. This method is not considered environmentally friendly as the adhesive can emit unpleasant fumes and adhesive storage buckets may need to be properly disposed of.

A self-adhered single-ply membrane comes with a factory-applied adhesive on the underside of the membrane. A backing material is removed, and the membrane is rolled into place without the use of additional adhesives. Membranes can come with adhesive in the full seam area or can be sealed using a hot-air welder.

The mechanically attached method consists of fastening each row of membrane to the roof deck with screws and barbed plates. Seams are sealed using a hot-air welder.

Skilled Labor Requirements/Time to Install

Mechanically attached TPO membranes require workers skilled in welder use to fuse the overlapping membranes together. For fully adhered systems, installers must understand how to correctly apply the product as recommended by the manufacturer.


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Originally published in Architectural Record
Originally published in November 2018