Choose a flashing material that is at least as durable as the roof covering. For example, it would not make sense to flash a 100-year-old slate roof with thin aluminum. Once the aluminum begins to leak, the slates would have to be torn off to replace the flashing. Copper flashing would cost more initially, but because it could last as long as the roof, its life cycle costs would be lower.

Flashing Shed Dormers

Shed dormers require a continuous strip of flashing along the top of the lower roof plane where it meets the steeper roof plane.

Also, they require sidewall flashings. A critical juncture that often fails is where these two flashings meet. Use taller pieces of step flashing towards the top, so the pieces can be bent over the dormer roof to help keep out snow and wind-blown rain (Shed Dormer Flashing, below).

Figure: Shed Dormer Flashing

Side-wall flashing for shed dormers. The top corners of a shed dormer are particularly susceptible to leaks. Use tall pieces of sidewall flashing, combined with a lead flashing patch (as shown here) or a patch using a peel-and-stick flashing membrane. — **Side-wall flashing for shed dormers.** The top corners of a shed dormer are particularly susceptible to leaks. Use tall pieces of sidewall flashing, combined with a lead flashing patch (as shown here) or a patch using a peel-and-stick flashing membrane.

Flashing Chimneys

Chimneys should be surrounded by base, step, and counter-flashings, as shown in Chimney Flashing, below.

Figure: Chimney Flashing

Never attach a flashing to both the chimney and the roof. The base flashing is attached to the roof only, the counterflashing to the chimney only. This allows the house to move independently of the chimney.

Chimney Cricket

Where a chimney breaks through the field of a roof, best practice calls for using a cricket to divert water away from the upper edge on the chimney. Crickets can be covered with the same material as the rest of the roof, using metal valley flashing where it meets the main roof. Or better still, fashion a metal cricket cover with 20-gauge metal as shown in Cricket Construction, below.

Figure: Cricket Construction

A cricket should be built of plywood, then covered by a single piece of metal. To permit expansion and contraction of the large piece of metal, attach it to the roof deck with cleats.

Flashing Skylights

Most skylight flashings come as a kit from the manufacturer. Skylights without flashing kits should be mounted on a curb and flashed as shown in Skylight Opening, below.

Figure: Skylight Opening

Unless it comes with a premanufactured flashing assembly, a skylight should rest on a curb or box that protrudes at least 6 in. above the roof surface.

Flashing Vents

Vent pipes on wood and asphalt roofs are usually flashed using a preassembled metal pan with a rubber boot. Order the unit according to the outside diameter of the vent pipe and install it as shown in Vent Pipe Flashing, below. For slate and tile roofs, a malleable lead jacket will last longer.