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How to properly install flashing

November 01, 2013

Asphalt, wood shakes, slate and other overlapped roofings are fine until they meet an obstruction. Up against a dormer, a vent pipe, a chimney, they need flashing — a patchwork of metal sheets, and sometimes rubber, that bridges the gaps and keeps out water. Here are five common installations, done right.
Step flashing. This seals seams between a shingled roof and a vertical wall; for example, along the edge of a second-story dormer. Small rectangles of metal, usually aluminum, are folded in a right angle so that one side weaves into courses of shingles and the other rests against the wall. The overlaps, or steps (generally three or four inches), shed water like overlapped shingles. A roofing nail high on the wall-side fold of each piece will be covered by siding. The roof side floats in the shingles without nails. You can make your own or buy them precut and prefolded. For backup protection consider a layer of modified bitumen installed on the sheathing. The self-sticking, rubbery sheets also protect valleys and eaves as a backup against ice dams.
Flashing masonry. Covering the edges of flashing between roofing and masonry, like a chimney, is more difficult. On stucco, for instance, one approach is to cut a shallow groove using a circular saw with a masonry-cutting blade, bend the top edge of flashing into the groove, and cement it in place. On bricks and block, chisel or cut out a slot in the nearest mortar joist (about 1/2-inch deep), bend in the top edge, and fill with fresh mortar. On chimneys, the top edge of that primary layer is covered with a second, also folded into a seam, called counterflasing.
Vent-pipe flashing. On older homes, pipes that protrude through the roof are flashed with a metal collar. Most of these installations eventually leak and have to be sealed with tar, again and again. Modern vent-pipe flashing is easier to install and longer lasting. It has an aluminum base sheet like standard flashing that is set in roof tar under shingles on the high side of the pipe, and on top of shingles on the low side. The cool part is a flexible rubber gasket attached to the base that simply slides over the pipe to provide a neat, tight, waterproof seal.
Window and door flashing. Caulking can seal the sides but you need flashing across the top, usually a molded strip of vinyl provided with the unit. On new construction it's easy to tack the flashing to the wall and then cover its upper edge with siding. On renovation work like replacing windows it's more difficult. Then you have to pry up shakes or clapboards without breaking them to slip the flashing underneath. In some cases you have to remove a course for access and then piece in new siding — all without crimping or tearing the flashing. It should extend across the top of the window or door trim, and tuck over its front edge.
Deck flashing. Most decks connect to the house with a long board called a ledger. Deck joists of the same size attach to it and transfer about half the total deck load through the ledger to the house foundation. It's a critical connection — and the one most often responsible for accidents where a deck full of people drops from the house. To protect the wood, flashing has to prevent water streaming down the siding from seeping between the ledger and the house. If it does, both sides of the connection will start to rot. To prevent that, the long strip of flashing should tuck under the siding, then fold out and down over the top of the ledger.


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