Overview of Low-Slope Roof Coverings, Part I

Anuj Srivastava
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  • Overview of Low-Slope Roof Coverings, Part I

    Flat roofs and low-slope roofs use special membranes to prevent water from leaking into the building, while providing roof insulation. Membrane systems are commonly used in commercial buildings, which often have flat roofs. However, this roofing method has gained popularity in the residential sector as well.

    This article is the first of two, providing an overview of the membrane types that are normally used on low-slope roofs. They can also be used on steep roofs, but fire resistance is reduced and special precautions are needed.

    Planning to install a new roof? Make sure you meet Local Laws 92 and 94.


    Built-up Roofs (BUR)

    Built-up roof membranes are made of alternating bitumen layers. They can use asphalt or coal tar, and felts as reinforcements sheets. The bitumen provides waterproofing while the felts enhance physical characteristics. For an adequate performance of the system, it is necessary to ensure complete embedment of the felts into the bitumen. 

    Fiberglass felts are the most commonly used type for asphalt BURs. Typically, polyisocyanurate insulation is used under built-up roof assemblies, but in some cases mineral wool will be used. 


    • Robust
    • Can be easily repaired
    • Durable when properly installed


    • Installation safety issues must be considered
    • Noxious fumes
    • Asphaltic materials tend to get brittle over time, and protective coatings must be used to extend their service life
    • Requires qualified and experienced professionals for installation

    Modified Bitumen (MB)

    Asphalt is a common roofing material, but its physical properties are not optimal: it is not UV stable and can get brittle with age and weather, present cracks and other types of degradation. To make asphalt suitable for roofing applications, it must be modified with polymers and other chemicals, and that is where the term “modified bitumen” appears (MB).


    MB membranes are tough and resistant: they are made from prefabricated and polymer-modified asphalt sheets, with a reinforcement layer. The quality of MB products depends on the quality and compatibility of bitumen and polymers, their manufacturing process, and the reinforcement within the sheet. The main types of modified bitumen sheets are:

    • Atactic polypropylene (APP): APP polymer is blended with asphalt and fillers, and normally supplied as one-meter rolls from factory. APP sheets are reinforced with fiberglass and polyester, individually or combined. APP bitumen roofing consists of a base sheet, an interply sheet, and a cap sheet.
    • Styrene-Butadiene-Styrene (SBS): SBS polymers are also blended with asphalt and formed into rolls, like APP bitumen sheets. In terms of performance, SBS sheets have better flexibility and weather resistance compared with APP.
    • Styrene-Isoprene-Styrene (SIS): SIS sheets are made from a mixture of SIS polymer, asphalts and fillers, which are then rolled into one-meter rolls. The top sheet has embedded mineral granules, or a laminated UV-protective surface like aluminum foil.
    • Styrene-Ethylene-Butylene-Styrene (SEBS): SEBS polymers are blended with asphalt in a factory, then transported to the job site where they are reheated in special tankers or kettles. Their application is very similar to that of BURs, but SEBS bitumen is expensive, and not commonly used for that reason.


    • Robust and durable
    • Easily modified and repaired
    • Versatile, and available in a variety of coatings


    • Cold fluid adhesives may contain high levels of VOCs (volatile organic compounds)
    • Safety issues, especially with torch down installations that require open flames
    • Manufacturing procedures are key: some suppliers develop unreliable products, so always check multiple options and look for quality certifications.


    Single-Ply Roof Membranes

    Single-ply roof membranes are made of thermoplastics and thermosetting products: they are formed into sheets in factories, and installed with a single thickness. Compared with BUR or MB membranes, single-ply sheets are very lightweight, except for ballated systems. There are six methods for securing single-ply roofing systems:

    • Fully Adhered: The membrane is adhered to the substrate in a continuous layer of adhesive, ideally an insulation cover board. This is considered the best installation method for single ply roofs, due to its wind uplift resistance and physical performance.
    • Ballasted: The membrane is placed over the substrate and covered with ballast to resist wind uplift. The ballast can be large aggregate or concrete pavers.
    • Mechanically Attached: The membrane is laid over the substrate, and then adhered with discrete fastener rows. There are multiple ways of fastening and fabricating seams with this method. 
    • Other less common methods include electromagnetic induction welding, loose-laid air-pressure equalization system, and protected membrane roof.

    As mentioned before, there are two types of single-plies:

    • Thermoplastic single-plies: These materials do not cure during fabrication, hence their name. Field-fabricated seams can be welded, and they provide high reliability when properly installed.
    • Thermoset single-plies: Contrary to thermoplastics, these materials cure during fabrication. Once cured, they can only bond together with special adhesives and tapes.


    • Better appearance than BUR and MB membranes
    • Easily installed, repaired and modified


    • Cold fluid adhesives may contain high levels of VOCs
    • Sheets are subjected to punctures, and not recommended for high-traffic roofs (e.g. roofs with HVAC equipment that requires frequent maintenance)

    Additional types of membrane roofing systems will be covered in the next article. For buildings in New York City, consider that most roof installations are now subject to Local Laws 92 and 94 of 2019, which require either a solar panel system or a green roofing system.

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    Tags : roofing roof installation membrane roof roof materials

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