How to Flash Shingles With Horizontal Siding

Determine the type of siding and see how it is fastened. Tongue-and-groove siding will be easiest to deal with, overlapped planks the most difficult. Check whether the junction is sealed with roofing paper or similar base flashing up the wall and down the roof.

Remove the first course of siding board if possible. Use a pry bar to pull up the bottom of the bottom siding board. You may have to pull up the second siding board a bit to allow the first board to move. Remove nails with a nail puller once the heads are exposed and slide the siding board out. With tongue-and-groove siding, loosen the board and pull it away from the groove above. Try to pull an overlapped board out; leave it in place if you cannot remove it but you can lift it enough to slide flashing under it.

Install base flashing of roofing paper or similar membrane if none exists. Slip it up under the siding 8 to 10 inches if possible. Fold it across the roof joint and staple it in place with a construction stapler on both the wall and the roof.

Slip the top edge of bent metal, aluminum or galvanized steel flashing under the first siding board location, at least 4 inches up the wall. Put a bead of roofing cement at the top of the flashing. Secure it with shingle nails driven into the wall sheathing with a hammer, preferably at stud points. Nail the bottom piece of flashing onto the roof decking. Put a bead of cement under that bottom edge as extra protection.

Replace the wall siding, using the old nails and nailing points or putting in new nails. Run a bead of caulking along the top edge of the flashing if possible, as extra sealant. Leave enough space under the siding for the shingles to slide underneath. Shingle the roof, starting from the bottom up to the flashing.

Lay shingles up to the wall line. Cut the last course to fit so the top edge of those shingles goes underneath the bottom of the first siding board. Add a bead of caulking over that junction for final protection.