Overview of Low-Slope Roof Coverings, Part II

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

    In this article we will continue discussing the coverings available for low-slope roofs. Like in any engineering decision, each option brings advantages and limitations, and the best choice will change depending on the application. 

    In the previous article we covered built-up roofs (BUR), modified bitumen (MB) and single-ply roof membranes. This time, we will compare sprayed polyurethane foam (SPF), metal panels, fluid-applied roofing membranes and mesh reinforced elastomeric coatings (MERC).

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    Sprayed Polyurethane Foam (SPF)

    Sprayed polyurethane foam (SPF) is a unique type of roofing system:

    • The membrane is made by first spraying a two-part liquid onto a substrate, such as the roof deck.
    • The mixture then expands and solidifies to create a closed-cell polyurethane foam.

    The National Roofing Contractors Association recommends that SPF must have a minimum density of 2.8 lb per cubic foot, and a minimum compressive strength of 40 psi. In addition to roof decks, other substrates may include existing roof membranes suitable for recovering, rigid insulation or gypsum board. SPF layers are applied with hand-held or robotic sprayers, where each layer of foam is about ½ to 1-½ in thick. Any desired thickness or slope variation can be achieved by controlling the amount of foam.


    SPF roof systems require a protective layer to ensure long-term performance. Coatings can be used to provide ultraviolet protection, weatherproofing, fire resistance or mechanical protection. Here is a list of the different coating types available:

    • Acrylic Coating: This coating offers a short service life, but is also the least expensive type. When using acrylic, re-coating is necessary every 10 to 15 years, depending on the local climate, material quality and application.
    • Polyurethane Coating: This coating offers a long service life, and is also resistant to impact and tearing when applied properly. This coating is typically white, and is available in one-part and two-part versions.
    • Silicone Coating: This coating offers superior weather resistance and a long service life. It is commonly available in gray, since white silicone tends to pick up dirt.
    • Mineral Granules: Mineral granules are similar to those used when surfacing asphalt shingles. They increase the durability of a coating, while providing slip-resistance for roof traffic. Coarse sand and granules are placed into a coating while it is wet, always following the manufacturer's guidelines.
    • Aggregate Surfacing: SPF coatings are quite resistant to liquids, therefore aggregates like those used on BURs systems can be applied directly over the foam. Since water vapor can enter foam, aggregate surfacing is not recommended in situations where the vapor flow is downward.

    The installation procedure is key when using SPF systems, and therefore workers must be skilled and qualified. The installation of SPF roofing is susceptible to temperature, humidity, wind speed and other environmental aspects. To manage weather risks effectively, always work with qualified contractors.


    • Can be shaped into complex roof systems
    • Thermal efficiency
    • Wind resistance
    • Does not tend to leak when damaged


    • Not UV resistant
    • Requires extra coating to ensure performance
    • Requires highly qualified labor

    Metal Roofs and Panels


    Standing-seam metal roofs provide an aesthetic modern look. Unfortunately, creating metal-to-metal joints that are waterproof can be very difficult. These systems can be hydrostatic or hydrokinetic:

    • Hydrostatic systems have a design and construction that is completely water-resistant.
    • Hydrokinetic systems are not completely resistant to water intrusion, requiring slope variations to shed water.

    Metal panels are not normally considered an option for low-slope roofs, but some of these systems are suitable for the application. The NRCA recommends a minimum slope of ½ in per foot for hydrostatic assemblies, and 3 in per foot for hydrokinetic systems.

    To install low-slope metal panel systems, water resistance must be provided throughout the entire roof surface. These systems should be designed and installed with the purpose of making them membrane-like. To achieve this, the panel joints must be sealed or soldered together. Also, fasteners that puncture the panel at any joint must be sealed with gasketed washers. This ensures all metal joints are watertight, remaining like as the roof experiences thermal expansion and contraction.

    Coatings are used to protect metal panels from corrosion and other types of weather damage. Factory-coated panels are recommended and available in several options, where polyvinylidene fluoride is the most common.

    Hot and Cold Fluid-Applied Roofing Membranes

    Liquid-applied roof membranes are commonly used as waterproofing systems, since they have no reinforcement. They have become a popular option for roofs that are already reinforced.

    Among these roof systems, the most common type consists of a hot-applied rubberized asphalt system with uncured neoprene reinforcement. Another common reinforcement for liquid-applied membrane roof systems is polyester reinforcing fabric or fleece.

    Mesh Reinforced Elastomeric Coatings (MREC)

    Mesh reinforced elastomeric coatings are becoming popular in the roofing industry, and they are composed of acrylic elastomeric and polyester reinforcing mats. These mats are applied in multiple layers until achieving the desired thickness, and they are changing the traditional methods for roof repair and replacement.

    When replacing a traditional BURS system, MREC offers a life expectancy of 15 to 20 years at a fraction of the conventional replacement cost.

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    Tags : roofing roof installation membrane roof roof materials

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