Roofing System 9
PROS AND CONS FOR SINGLE-PLY ROOFING
|Highly productive installation.||Thinner sheets are easier to puncture, May not be good choice for roofs with extensive mechanical equipment requiring maintenance.|
|Easily repaired and modified.||Due to high price-point pressures, manufacturers may offer materials and details that are not as reliable as owners may require.|
|Better appearance compared to BUR and MB||Cold fluid adhesives may have high VOC content.|
|Available with a variety of cool coatings.|
MESH REINFORCED ELASTOMERIC COATINGS (MREC)
Mesh Reinforced Elastomeric Coatings are gaining market share in today’s roofing world. Composed typically of acrylic elastomeric and polyester reinforcing mat applied in multiple layers for a final dry film thickness of 52 mils, these systems have changed the traditional roof repair and replacement paradigm. Eliminating the environmental aspects of removing an old 4 ply BUR, the MREC allows cleaning of aggregate, and application of 52 mil system with predictable 15–۲۰ year life at a fraction of traditional replacement cost.
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. NRCA recommends SPF intended for use as a roof system to have a minimum density of 2.8 pounds per cubic foot and a minimum compressive strength of 40 pounds per square inchigure 10. This SPF roof is being installed with a robotic sprayer.The substrate can be 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 with robotic sprayers. Each pass (or lift) of foam is typically between 1/2 to 1-1/2 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.
A protective surfacing is required for long-term performance of an SPF roof system. A protective coating must serve multiple functions in protecting the underlying SPF and should be selected from coatings that have been specifically designed for SPF and have a proven history of performance when used over SPF. Protective surfacings are a part of SPF roof systems to provide weatherproofing, ultraviolet (UV) protection, mechanical damage protection, and fire resistance. This is typically accomplished by using one of the following coatings.
COATINGS FOR SPRAYED POLYURETHANE FOAM
Acrylic Coating: This is the least expensive of the coatings, and generally offers the shortest service life (although the best acrylics can last longer than some of the polyurethane coatings). Acrylic coatings should be used as part of an SPF roof system in order to comply with ASTM D6083. With acrylics, re-coating is required about every 10 to 15 years, depending upon the quality of the coating material, application, and climate. They are typically white.
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. Both one- and two-part coatings are available. One-part coatings are typically gray, although white is available. Two-part coatings are typically white. Single-component polyurethane coatings should be used as part of an SPFP roof system complying with ASTM D6947.
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). More than other coatings, silicone coatings are prone to being pecked by birds. To avoid the pecking, granules are commonly broadcast into the coating while it is wet. Silicone coatings should be used as part of an SPF roof system complying with ASTM D6694.
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. Course sand can also be used for these purposes. Granules or sand are broadcast into a coating while it is wet. If granules are used, they should be selected and installed according to the coating manufacturer’s recommendations.
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. At parapets and equipment curbs, one of the previously described coatings is applied on the vertical surfaces and out several inches onto the field of the roof. Because water vapor can migrate through the foam, the aggregate surfacing option should not be specified in situations where the annual net vapor flow is downwards. As with aggregate-surfaced BUR, consideration should be given to aggregate blow off.
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. Installation of SPF roofing is especially sensitive to temperature, relative humidity, wind speed, and other environmental factors.
SPF systems have several important attributes. Besides readily lending itself to complex roof shapes, SPF roofs are exceptionally thermally efficient, since they do not have mechanical fasteners or insulation board joints, which create thermal bridges. Also, field research has demonstrated that they have exceptionally good wind resistance. Notably, an SPF roof is not in imminent danger of leaking if the coating is weathered away or ruptured or the aggregate surface is displaced, provided that the penetration does not extend all of the way through the foam (which is generally unlikely). Damaged areas should be promptly repaired however to prevent further damage to the underlying foam due to UV exposure. This attribute is in stark contrast with the other low-slope system options, in which leakage typically occurs if the membrane is punctured.
Standing-Seam Metal Roofing (SSMR)
Standing-seam metal roofs are often used for their appearance. However, it is extremely difficult to make all of the metal-to-metal joints permanently waterproof.
SSMR systems are either hydrostatic that are designed and constructed to be totally water resistive (like a roof membrane) or hydrokinetic that is not totally resistive to water intrusion and rely on slope to shed water.
There are two primary approaches to low-slope metal roofs:
۱٫ Hydrostatic: With hydrostatic systems the panels have standing seams, which raise the joint between the panels above the water line. The seam is sealed with sealant tape or sealant in case it becomes inundated with water backed up by an ice dam or driven by wind.
Most hydrostatic systems are structural systems (e.g., the roof panel has sufficient strength to span between purlins or nailers). A hydrostatic structural panel (which cannot span between supports) may be specified if a solid deck is provided.
۲٫ Hydrokinetic: Most standing seam metal roofing panels are hydrokinetic, or water shedding, and therefore require a slope greater than 3:12 (25 percent).