Things You'll Need
Tar-based felt paper
Hammer tacker with staples
Galvanized roofing nails
Tin or copper step flashing
Shingling a roof is a process that is similar across all types of roofs and with all types of shingles, which means that as long as you understand the basics, you can install shingles anywhere they are needed. Bay windows are similar to dormers in that they extend outward from the building, but while dormers are usually part of the roof itself, bay windows normally extend outward from the walls. As a result, you are shingling against the siding of the home, which means you need to tie the shingling into the flashing for the home.
Layer the roof of the bay window with the tar-based felt paper. Cut the pieces off the roll with a utility knife. Lay them in place and staple them down with the hammer tacker. Overlap each piece by a few inches, and overlap them from the lowest section up, so that the upper sections always overlap the lower.
Install your drip flashing around the eaves of the bay window roof. Apply silicone caulking to the backsides of the flashing and place it so that the vertical section covers the overhang of the lip of the roof while the horizontal section lays flat on the roof. Nail it in place.
Run your starter row of shingles along the bottom section of the bay window roof. Overhang it by 1/2- to 3/4-inch over the eaves of the roof. Nail it along the markings defined by the manufacturer, which varies depending on the brand of shingles you purchased.
Install your second row on top of the first so that the bottom half of the second row overlaps the top half of the first row that was nailed into the roof. Also stagger the second row so that it doesn't line up vertically with the first. Nail this second row of shingles down. Repeat the layering as you work your way up the roof, alternating the vertical rows so that they repeat every other row.
Flash the edge of the roof where it meets the wall of the house with the step flashing. Start on the lowest section of the roof and work your way up to the peak of the roof, overlapping each lower piece of flashing with the next piece higher so that it has a waterfall effect. Install the pieces so that the horizontal sections slide in between the layers of shingles, but not against the felt paper, and the vertical sections slide up underneath the siding for the home.
Cover any peaks with ridge shingles, which are designed to cap the "peaks" where two rows of shingles working up different planes meet at a cap. Nail them in place just like you did with the other rows of shingles.
You can cut any shingles down to size with the utility knife.