You probably already know that duct tape and caulk cannot stop a leaking skylight. Homeowners have tried this remedy unsuccessfully for years. Modern manufacturers developed a product that works much better -- rubbery asphalt with a sticky side that comes in rolls. Fixing that annoying leak becomes a routine procedure that you can do by yourself in less time than it takes to make your husband take out the trash.
Remove the shingles from around the base of the skylight by prying up the nails with a pry bar. Remove enough of them to provide an 18-inch-wide perimeter around the skylight. Start by removing the shingles along the top first and work your way down along each side. Place the shingles to the side, in the order you removed them for easy replacement. To uncover the nails for removal, carefully lift the shingle flap from above.
Scrape off the old tar around the perimeter of the skylight with the chisel. Remove the existing L-shaped flashing from around the edges of the skylight. The flashing is a piece of metal bent at 90 degrees. One side of its L-shape fits flat against the roof while the other side of it tucks up under the lip on the skylight base. The pieces are short and overlap each other when installed, with the upper pieces overlaying the bottom for water flow.
Measure the width and length of the skylight and add 36 inches to each measurement. The extra 36 inches allows the membrane to extend beyond the dimensions of the skylight by 18 inches on each side. Roll out the membrane and cut four pieces with a utility knife, one for each side.
Peel the back off the bottom piece of membrane first. Place it centered at the bottom of the skylight, allowing it to bend and extend up the side of the skylight base to the top -- about 2 inches. Make a slit at each corner to allow the membrane to lie flat where it bends up on both sides.
Place the membrane on the sides of the skylight next employing the same procedure used for the bottom piece. Install the top piece last in the same manner. Carefully stamp the membrane into place with your feet to bond the membrane to the roof.
Place new flashing around the perimeter of the skylight over the membrane. Start on the bottom, overlapping the short pieces by 3/4-inch to flush on both sides. Tuck the top edge up under the lip of the skylight, with the 90-degree corner pressing against the membrane sides. Repeat on both sides, finishing with the flashing along the top.
Replace the shingles, nailing them in place in the order you took them off. Dab a bit of asphalt or roof caulk on the nail head to seal around it.