4  FM quick reads on Roofing

1. Keys to Ensuring Successful Rooftop PV System

The first step in the successful installation of a rooftop photovoltaic (PV) system is to ensure the underlying roofing system is compatible with the intended PV system or that it can be upgraded for use with a PV system. For a rooftop PV system, the process of integrated design — having all parties involved with a project, including maintenance and engineering, at the design table — requires knowledge of both the roofing and PV industries.

Having the right people at the design table helps to create a proper path from design to installation to maintenance. To ensure a successful design, managers need to:

  • make sure the roofing system will provide at least 20 additional years of useful service
  • specify a cover board as a substrate for the roofing membrane in order to prevent damage and to protect the energy-efficiency properties of the roof insulation
  • match the roof membrane's thickness and proven performance to the required service life of the PV system
  • use construction details that are well established and meet the manufacturer's requirements
  • elevate framing and conduits above the roof surface to promote drainage, which considerably reduces the potential for leaks
  • design penetrations with round framing so flashing installations are more effectively and efficiently installed
  • install sacrificial membranes or walkways at critical traffic locations
  • provide additional membrane layers or coatings at flashings to increase durability
  • engage qualified professionals during the design and planning phase to ensure compliance with all building codes and safety regulations
  • make sure the rooftop PV system installation does not compromise the roofing system's warranty
  • make sure that the roofing system's manufacturer has accepted all PV system details — especially attachments and penetrations — if not during the design stage, at least prior to starting the PV installation on the rooftop.

The manager's overall goal is to make sure the rooftop PV system will do more than just survive. It also should help ensure continuous building operations.

Keep Good Records of Roof Conditions

Recording roof conditions with photographs is important for documentation purposes, particularly for components still under warranty. That way, should concerns arise, visual evidence can support the written inspection report. If roof conditions call for more comprehensive evaluation, retain a design professional, usually an architect or engineer with experience in roof rehabilitation, to conduct a detailed investigation.

Routine roof inspections can serve as a starting point for establishing a storehouse of information on roof systems at your facility. On a regular basis, collect and update information on roof assemblies, such as:

  • Roof system type (e.g., modified bitumen, EPDM, TPO, copper, slate)
  • Manufacturer
  • Warranty period and coverage
  • Approximate area
  • Linear footage of perimeter flashing, coping, or gravel stops
  • Date of installation
  • Installer/contractor (if known)

Use a roof plan to log data for each roof area, along with a plan of the floor below the roof. Keeping a chart of information that corresponds to the roof plan simplifies recordkeeping by providing a visual depiction of the properties of each roof area.

As part of the data collection, make an effort to keep up-to-date records of maintenance and repair efforts. Such information enables accurate assessment of maintenance practices, and it may point to areas in need of rehabilitation or replacement. Such documentation may also be necessary for warranty coverage.

At least twice annually, review and update the roof log as part of the inspection process. In this way, you will maintain an up-to-date, at-a-glance reference that will streamline management of roof systems. For multi-building facilities or for buildings with many roof areas, such recordkeeping is particularly important to keeping tabs on roof conditions.

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.


Roofing , energy efficiency

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