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Statistics from the National Roofing Contractors Association indicate that about two-thirds of our low slope commercial roofing activity is conducted on existing buildings. If you have buildings in this category, this column presents a sequence that might be helpful as you deal with re-roofing and re-cover issues.
The Roofing File
The best time to establish a roofing file is during the design phase. The file should contain the areas/buildings covered, the specifications and plans, and minutes of any pre-job conferences. Historical information, such as previous surveys and inspections, should be available, as well as previous repairs.
Roof warranty documents should indicate whether the warranties or guaranties are still in effect or have expired. If you have experienced leakage, did you notify the guarantor as required? That would also include notification before installation of new equipment on the existing roof.
Re-Cover vs. Replacement
The term “re-cover” generally implies the roof membrane is in trouble, but the underlying roof system is salvageable. For bituminous roof systems, this might mean scarifying the gravel surface and flood coat, followed by patching or repair, and then applying a new surfacing insulation layer or membrane, or both. Since building codes are calling for upgraded thermal insulation, you may be forced to add more insulation including re-cover boards and a brand new roof membrane. (Codes limit a structure to two roof membranes, so if you already have two membranes in place, you may need to tear off the existing upper roof membrane to comply with this restriction.
How many roof membranes are presently on the building? Take roof cuts all the way down to the deck to determine this. Is the existing thermal insulation soaked or disintegrated? Moisture surveys can be very helpful to determine the condition of the insulation. Infrared, nuclear and capacitance surveys detect symptoms of wet insulation. Electronic leak detection can detect leakage, even under vegetated roofs.
For bituminous roof systems, dissolving a small specimen of the bitumen in mineral spirits can determine if the roof is asphaltic or coal tar pitch based. Asphalt will rapidly dissolve, while pitch is hardly affected.
Hopefully, the new insulation and membrane will last as long as the original roof and will qualify for a long-term warranty on both materials and workmanship.
Repair is different. It could mean that you are planning limited permanent patching on a routine basis, perhaps performed by the occupant’s own work force. With the introduction of torch-applied modified bituminous roof systems, this is a good substitution for the traditional asphalt mastic and mesh to rehabilitate flashings and pitch pockets.
If open flame torches are used, the applicator should attend a certificated torch-applicator training program (CERTA). A fire watch should remain on the roof for at least 30 minutes after the last torch is extinguished to check for hot spots. Use an infrared non-contact thermometer to assure that no hidden, smoldering hot spots remain undetected, even after the 30-minute period.
If the existing roof system is one of the newer single-ply roof systems, it is still repairable by scrubbing the membrane with detergent, applying primer, and then applying a repair patch on the cleaned and primed membrane.
Thermoplastic and thermoset: What’s the difference? A thermoplastic material is one that softens upon heating, returning to its original physical properties when the heat source is removed. If the laps on an existing thermoplastic single ply membrane are failing or you need to restore some punctures, a heat gun can be used to reseal the defects. You need 120 volt electric outlets on the roof or a portable electric generator to provide the needed heat flux. The repair material needs to be compatible with the existing membrane and may not be available at the local big-box home center.
Thermoset materials, on the other hand, are cross-linked (vulcanized) in the factory, and are not thereafter heat-weldable. The most popular thermoset membrane for roofing is EPDM (ethylene propylene diene monomer/terpolymer). Solvent-based polymeric adhesives of butyl or neoprene are gradually being replaced by pressure-sensitive tapes.
Additional concerns include proper water drainage. If the existing system ponds water, additional drains may be needed. Proper slope can be accomplished by using tapered insulation during the re-covering process. Consider investing in the Repair Manual for Low-slope Membrane Roof Systems by NRCA, SPRI and ARMA – it can be very helpful to your in-house maintenance staff.
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