4 FM quick reads on Roofing
1. LEED Standard Measures Cool Roof
Today's tip is to use the Solar Roof Index (SRI) to evaluate a cool roof. The U.S. Green Building Council's LEED 2.2 is the first national specification to use a relatively new measure of reporting a cool roof's properties. LEED 2.2 sustainable sites credit 7.2 states that to receive one point, building owners should use a roof with a Solar Reflective Index (SRI) of 78 over at least 75 percent of the roof's surface for roofs with slopes less than 2:12.
SRI, a unit developed by Lawrence Berkeley National Laboratory, incorporates reflectivity and emissivity properties into one standardized measure.
SRI is calculated with a complex formula spelled out in ASTME 1980. It measures a roof's combined thermal properties on a scale of 1 to 100, defined so that a standard black (reflectance 0.05, emittance 0.90) is 0 and a standard white (reflectance 0.80, emittance 0.90) is 100. Some hot roofs can have negative values, and some white thermoplastics and white roof coatings have scored as high as 104 to 110.
SRI as a method for reporting cool roof data will probably take a little while to catch on. Most manufacturers still report separate emissivity and reflectivity data in their literature, but the Cool Roof Rating Council, an organization that verifies and labels cool roofing products, has begun using the measure, while retaining reflectivity and emissivity measurements.
Different roofing technologies have different SRI values. Asphalt coatings, for example, have aluminum pigments added to asphalt cutbacks and emulsions to give coatings SRI values of 21-30 on a scale of 0-100.
Acrylic elastometers, on the other hand, have a highly reflective surface, often with an SRI greater than 100. Most highly reflective acrylic elastomers are white, and workers can install them over existing bituminous or non-bituminous roofing. Acrylic elastomers typically are specified at 12 mils for five-year warranties and at 20 mils for 10-year warranties. Some manufacturers specify up to 40-mil applications.
Identify and fix wet spots on roof
Today's tip is to deal with wet areas on the roof effectively. First, check the roof one of three types of moisture surveys: infrared, nuclear or capacitance.
Infrared surveys measure the heat retained or lost in insulation that has become damp. Ballasted roofs aren't a good candidate for infrared surveys because the rock itself retains a lot of heat, giving potentially false readings. Nuclear moisture surveys measure hydrogen atoms in the roof, meaning that any membrane with a large hydrogen chemical component will send positive readings. Water is a good conductor of electricity, and capacitance surveys measure electricity traveling through the roofing material. This won't work on a roof with wet or ponded areas, and may require modified instruments on EPDM roofs.
If you have 100,000 square feet of roof and four 8-by-10-foot areas are wet, replacing those sections makes sense. But if 30 percent of your roof is wet and it's scattered throughout the roof, the labor to replace all of those sections probably equals the cost of just tearing off the entire roof.
But what if the roof is leaking just after a recent replacement? It's not that farfetched — due to poor design or installation, many roofs experience water leakage soon after construction.
In many instances, water leakage through a roof membrane can go unnoticed because a vapor retarder at the bottom of the roof system captures the water. The captured water absorbs into the insulation, significantly decreasing the thermal value of the insulation and causing premature deterioration of the roof system.
Generally, the membrane does not allow bulk water leakage. Most leaks through a system arise from unreliable detailing.
Roofing system manufacturers provide standard details for perimeter conditions, which typically have the flashing exposed and terminated on the wall surface. They rely on sealants to prevent water infiltration. Manufacturer details typically do not address leaks around the roof system.
For example, in most instances, roof terminations consist of surface-mounted conditions (exposed termination bars or metal flashing) or reglet-set flashing (a small cut in a wall system to insert the metal flashing). In a brick masonry wall, water can bypass the flashing, infiltrate the masonry, and migrate into the insulation. Instead, the design of roof flashing for a masonry wall should incorporate a through-wall flashing that extends through the masonry to capture and divert water out of the wall above the flashing.
How Do Commission Your Roof?
Today's tip, from David Reid and John Wilkins of Gould Evans Architects, is about strategies you should consider when it comes to commissioning your roof.
Most facility managers understand "commissioning" as a strategy that only applies to HVAC systems. But commissioning roof installations is a critical component of a water-tight and energy efficient roof.
Reid and Wilkins suggest three areas to examine when commissioning roofing.
First, look for "weak links" in the integrity of the roofing system, as most of the failures result here. Look at roof terminations, penetrations, flashings at corners, intersections, eaves, curbs and parapets, and drainage systems.
Secondly, especially if you're installing a green roof, but really for any roof, double and triple check that the waterproofing membrane truly is water tight. Perform a leak-detection test appropriate to the type of membrane you've installed. For example, flowing tests flow water continuously over the surface of the waterproofing membrane for a minimum of 24 hours without closing the drains or erecting dams. Electric field vector mapping pinpoints breaches in the roof membrane by tracing the flow of an electric current across the membrane surface.
Thirdly, part of commissioning the roof is creating a plan to protect the membrane until construction is completed and all components - including HVAC, etc. Reid and Wilkins suggest a product called protection board to make sure the roof isn't penetrated or damaged during the rest of the installation or construction process.