Waterproofing with Flashing
You may not think about it while you’re sitting with the family around the fireplace on a cold night, but the flashing that surrounds the other end of your chimney does its part to keep the insides of your home warm and dry as much as a cozy fire. Done differently for different styles of chimneys, properly installed flashing is the unsung hero of household waterproofing, and is perhaps the most versatile of all waterproofing products.
Flashing comes in a variety of different materials from rubber, roofing felt, and plastic, but the most popular and common materials are galvanized steel, aluminum, and copper. Aluminum flashing can be purchased from most home improvement and building supply stores.
For The Chimney
A chimney that is constructed along a rake must be flashed along three sides where it meets the roof. If you’re reroofing at the same time, trim the shingles so they will fit up against the flashing, then apply a generous amount of roof patch. If you’ve torn off old shingles, flash and then counter flash around the chimney. The idea here is to bend up the old flashing so it can be used again for waterproofing purposes.
The general idea for all types of chimneys is to allow a two-inch overlap with each piece. At the bottom and top corners of the portion of the chimney adjacent to the rake, which is the point where the shingled roof ends and hangs over the ground, you must form tight boxes of flashing that will be used for waterproofing purposes.
The process is similar for a chimney that sits in the middle of a roof, but you’ll need to work around all four sides instead of just three.
But flashing isn’t just for chimneys. “Step” flashing is used along walls and can be installed against a vertical wall to ensure a sturdy waterproofing seal. It protects vulnerable areas like the roof valleys and eaves.Common around dormers, flashing sits under siding and shingles and resembles a series of stairs going up the side of a wall. In fact, that’s where the name “step” flashing comes from.
Drip caps and Z-flashing are essential for flashing walls. The latter is used to seal the horizontal folds between pieces of plywood and drip caps seal the frames over doors and windows.
And Skylights Too
One of the most important uses for flashing is in waterproofing skylights. If a homeowner has any problems with leaking skylights, it's usually because the shingles join the flashing incorrectly. At the top of the skylight, the first run of shingles must fit snugly up against, and rest on top of, the aluminum or galvanized sheet metal collar of the flashing. Because of this need for attention to detail, most experts agree the best time to install skylights is when the roofing is being done.