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In the case of penetrations, such as vent pipes and drains, the weak link is the method used to attach the roof to the penetration. With single-ply membranes, this means a boot or field-formed membrane that is adhered or welded to the roof membrane with a drawband — a metal collar that fits around the protrusion — and sealant at the top. One should watch to see that the drawband is tightly attached but does not cut into the flashing and that the sealant at the top is installed with no voids or unadhered sections. The membrane flashings on single ply roofs at pipes and other penetrations, no matter what the material used, are usually the first items on the roof to deteriorate from age and abuse, so careful attention should be paid to the condition of these flashings.
A modified-bitumen or asphalt roof will use concrete rings with pourable sealers. Alternatives are pitch pans with sealers, or metal penetration flashings with roofing membrane adhered to the metal flanges and the field of the roof. With the concrete rings, the weak spots are between the pourable sealer and the penetration and the concrete ring and the roof membrane.
The pourable sealer will crack over time and any movement in the penetration will cause a separation between the penetration and the sealer, especially if the sealer is too thick. This leaves a direct path for water to enter the building. Watching for signs of cracks in the sealant at the bottom of the concrete ring and around the penetration. Re-sealing these areas as needed will help keep the roof watertight. Pourable sealers should fill the retaining ring to overflowing to keep the ring from holding water.
With pitch pans, there are additional problems. Pourable sealers in a pitch pan have the same drawbacks as the concrete rings. If the sealer used is asphalt or roofing cement, it will shrink over time, causing cracks within the sealer itself or causing it to pull away from the penetration. This leaves a direct path for water to enter the roof. Regular maintenance of the pitch pans should include crowning the sealer to shed water away from the penetration. Because the pitch pans are metal, checking the pan for rust is essential. Wherever the rusting occurs, there will eventually be a hole.
Metal penetration flashings are dependent upon compression tape and elastomeric sealants to keep the penetration watertight. The sealants should be examined on a regular basis and replaced when they begin to harden and crack. With both the pitch pans and the metal flashings, the membrane stripping between the flanges of the metal and the roof should be adhered tightly to both the metal and the roof.
Roof Problems Often Start at the Seams
Roof Penetrations Can Be the Weak Link