Keeping water out: layers and flashing

1. A roof sheds water

A pitched roof works because every layer overlaps the one below it, like fish scales. Water runs down the slope, over the edge and into the gutter. Roofs rarely leak across a plain slope. They leak where something interrupts it.

2. The layers, from the bottom

From the bottom: the rafters, the plywood deck, underlayment as a second line of defense, a metal drip edge at the eave, then shingles. Each layer laps over the one below it going up the slope, so water always runs over the top of the next layer, never under it.

3. Drip edge and gutter

At the eave, the drip edge guides water off the roof edge and into the gutter, and keeps it from curling back behind the fascia. Underlayment goes over the drip edge on the rakes and under it at the eave, so water always flows toward the outside.

4. Step flashing at a wall

Where the roof meets a wall, each shingle course gets its own L-shaped piece of step flashing. One leg lies flat under the shingle, the other runs up the wall. Each piece laps over the one below, so water is handed down the wall edge, course by course, never behind it.

5. Counterflashing and the kickout

Counterflashing is fixed to the wall and laps over the step flashing, so water coming down the wall cannot get behind the metal. At the bottom, a kickout flashing steers the water from the wall flashing into the gutter instead of letting it soak the wall behind the siding.

6. A vent pipe and its boot

A pipe through the roof gets a flashing base and a rubber boot. The flashing goes under the shingles on the upslope side and over them on the downslope side, so water runs around the pipe. Rubber boots crack in the sun, so they are a common leak to look for.

7. What happens with no flashing

Take the step flashing away and rain runs into the joint between roof and wall. Here the roof surface is cut away so you can see inside: the water soaks the deck, drips into the attic and stains the ceiling below. Slow leaks like this often go unseen for months, until the framing is rotted and the repair is large.

8. What an inspector looks for

Walk every transition: wall junctions, chimneys, vent pipes, valleys and skylights. Look for flashing that is missing, rusted, lifted or sealed only with caulk, cracked boots, and stains on the ceiling or in the attic. Caulk is a temporary patch, never a substitute for flashing.