How to Install Roof Flashing on a Shed Where it Meets the House

Peel back the first six inches of roofing layers from both the shed and the house on an existing roof to expose the sheathing.

Cut rolled sheet metal, aluminum or copper, into 12- by 12-inch squares with tin snips or metal shears. Cut enough sheet metal to run the length of the shed.

Place the cut sheet metal between two pieces of wood in a vice with six inches extending beyond the wood and six inches clamped.

Use a rubber mallet to tap the pieces bending them over to fit in the valley equally between the shed and the house.

Place a bent piece of sheet metal on the valley--beginning at the edge of the roof, called the eave--with six inches of flashing on each roof. Allow the first piece to hang over the eave by one inch.

Nail the flashing into the house sheathing and into the shed sheathing with aluminum nails or copper nails, depending on the type of metal flashing. Use copper nails for copper flashing and aluminum nails for aluminum flashing. Using dissimilar metal nails will cause a galvanic reaction and erode the integrity of the flashing.

Load a caulking gun with roofing cement and run a thick bead around the edges of the flashing and over nail heads.

Overlap the second piece of flashing onto the first by one inch, nail in place on both roofs and run a thick bead of roofing cement around the edge of the flashing and over nail heads. Continue to place flashing overlapping by one inch onto the previous piece, nailing in and applying roofing cement until flashing covers the valley between the two roofs.

Put the shingles back in place and nail in beginning at the eave and working your way up the roof on both the house and shed.