Understanding Differences Between Low-Slope Roof Membranes

A low-slope roof is any roof that slopes 14 degrees or less. Selecting the ideal kind of roofing material for your low-slope or flat roof is critical to protecting your facility and maximizing the value of your investment. To make an informed decision on an optimal roofing solution, you need to begin by understanding the fundamental differences among all of your options. Let’s get started by discussing the five fundamental types of low-slope roof systems and membranes.

Built-Up Roof (BUR) Membranes

Built-up roof membranes have been in use for more than a century in the United States. Also known as a tar and gravel roof, BUR membranes are made of alternating layers of bitumen and reinforced fabric to create a membrane. The bitumen is typically composed of asphalt, tar, or adhesive. The membrane is then covered with aggregate, glass fiber, cap sheets, or another coating to provide long-term protection.

Single-Ply Membranes

A single-ply membrane is made up of manufactured sheets that are either categorized as thermoplastic or thermoset. Thermoplastic membranes are softened when heated and harden as they cool. This process can be repeated without damage the materials. Thermoset materials, on the other hand, permanently solidify after they’ve been heated. The most common thermoplastic materials are polyvinyl chloride (PVC) and thermoplastic olefin (TPO). The most common thermoset membrane is ethylene propylene diene terpolymer (EPDM).

For single-ply membranes, durability is measured by a sheet’s thickness, which is generally referred to as mil thickness: 1 mil equals 0.001 of an inch. Common mil thicknesses range from 30 mils to 60 mils, although thicker membranes are available.

Polymer-Modified Bitumen Sheet Membranes

Polymer-modified bitumen or modified bitumen (MB) sheet membranes have been in use in the U.S. since the mid-1970s. Polymer-modified membranes are made of reinforced fabric covered by hot polymer-modified bitumen. MB membranes are made up of multiple layers, similar to BUR membranes.

There are two types of MB membranes: SBS polymer-modified bitumen and APP polymer-modified bitumen. SBS membranes are typically installed by hot mopping asphalt or a cold adhesive. APP membranes are generally heat welded or applied with a torch. APP tends to impart a more plastic quality to the asphalt, whereas SBS usually rubberizes the asphalt.

Spray Polyurethane Foam-Based (SPF) Systems

SPF roof systems are installed by mixing and spraying a liquid that forms the base of an adhered roof system. A protective coating is applied to protect the foam from the elements.

Metal Panel Systems

This one is fairly self-explanatory. A structural metal panel system is installed over a substrate and a protective underlayment.

While this list is designed to get you started by understanding the basic differences among low-slope roof membranes, the team at C.I. Services is happy to answer any questions you may have. And we’d love to help with any of your roofing needs. Contact us today.

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