Double flashing vs. single flashing on tile roofing
Q. My tile roof is leaking in an unusual way. No moisture gets into the house, but at the edges of the roof I've seen water running from beneath the tiles. When I bought the house, my home inspector said this might happen when it rains because of leaking at the vent pipes. He advised an upgrade, but I didn't think it was a big deal at the time. Since then, I've noticed that my neighbor's roof is different. The sheet metal around his vent pipes overlaps the tiles, preventing leakage. On my roof, the metal is installed under the tiles, allowing water intrusion. I asked my roofing contractor if this is a problem. He assured me that it is an accepted roofing method. Does this sound right to you?
A. The sheet metal fittings on the vent pipes are called flashing. Flashing serves as a moisture seal at roof penetrations, to direct water flow over the top surfaces of the roofing.
With tile roofing, there are two ways to flash a vent pipe: single flashing, as on your roof, or double flashing, as on your neighbor's roof. Leakage is less likely to occur when water flows over the tops of the tiles, rather than beneath them. This is a matter of common sense. Unfortunately, the building code, contrary to common sense, does not specify which flashing method to use on tile roofs. Instead, the code requires that tiles be flashed according to the tile manufacturer's specifications. This creates a conflict of interest for those manufacturers. Here's why.
When a contractor bids a roofing job, the objective is to submit a competitive price estimate in order to secure the contract. A high bid lessens the chance of obtaining the job, and the extra cost of double flashing the vent pipes results in a higher bid. If a contractor uses tiles that require double flashing, the competitive edge in the bidding process is lost. Therefore, some contractors are inclined toward tiles for which single flashing is allowed.
Tile manufacturers recognize this dilemma. Accordingly, they specify double flashing as the best method for installing their products, while listing single flashing as an acceptable alternate method. This double standard allows contractors the option to use single flashing if they so choose. Thus they are able to shortcut the quality of their work while maintaining full compliance with manufacturer's specifications and the building code.
Your contractor is correct when he says that single flashing is an accepted method, but this should not be construed to mean that it is a good method. Beneath the tiles, your only leak protection is a waterproof membrane. When your tiles were installed, this membrane was punctured by hundreds of nails. Each one of these penetrations is a potential leak source.
Double flashing is obviously the more competent and reliable method of installation. It may not be a legal requirement, but it is the only sensible way to flash a tile roof.
• To write to Barry Stone, visit him on the web at www.housedetective.com, or write AMG, 1776 Jami Lee Court, Suite 218, San Luis Obispo, CA 94301.
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