Once you accept that it's going to be hard labor, re-shingling a composition roof isn't difficult. It's not, as a woodworking friend likes to say, brain surgery--though it's not for the brain-dead either, as you need to keep your wits about you while working on a slanted surface so far above the ground.
Measure the roof to find out how many shingles to order (three bundles per hundred square feet), and remember to order an extra 10% to 20% for waste and miscalculation. If you can get the bundles easily, go for the lower figure; if the shingles have to be delivered from some distant warehouse, add the higher percentage to avoid frustration. Check whether you can return unopened bundles.
Check also whether the supplier has a conveyor-belt delivery system; a truck- or trailer-mounted conveyor can deliver the shingles right to the roof. Having the shingles delivered to the roof is worth paying a few extra dollars. The alternative is carrying dozens of bundles of shingles, each weighing 80 to 100 pounds, to the roof on your shoulder, one bundle at a time.
Keep shingles dry by covering them with a tarp or heavy plastic. If the paper wrapping gets soggy, the bundles could fall apart.
Roofing requires a hammer, a utility knife with lots of extra blades, a tape measure, a carpenter's square and roofing nails. To install new shingles over old, use roofing nails long enough to go through all layers and into the underlying surface at least three-quarters of an inch. Order plenty of nails--each shingle takes four nails, and there are 27 shingles in a bundle.
You also need roofing cement and a caulk gun with silicone caulk. Tin snips, for cutting flashing and trimming shingles, are also useful.
Before you start on the shingles, go up on the roof and check out the flashing, the metal strips that line valleys, and go around anything that protrudes through the roof, such as chimney and pipes. Watch for flashing on the edge of the roof. If it's in good shape, all you have to do is nail down any loose spots. If it's damaged or worn, however, you'll have to replace it. (If the roof is simple, a two-sided peak, for instance, with one chimney protruding, you can probably replace the flashing yourself. If the roof has a lot of peaks and valleys, however, you may want to get professional help with the flashing.)
Once the prep work is done, you're ready to start nailing. Here are the steps.
* First, familiarize yourself with the parts of a shingle. We're assuming you're using a standard three-tab asphalt or fiberglass shingle, 12 inches high by 36 inches wide. The tabs, the bottom part of the shingle, are the part that shows, the part that has the color you have chosen. The top part is usually black with a row of self-sealing adhesive where the nails will be driven in. Read the wrapper carefully; it should tell you how to cut them and suggest a nailing pattern.
* Roofing goes on in rows, called courses, starting along the bottom edge and ending at the ridge. The first strip of shingling, the starter strip, has no tabs. It's a piece trimmed from the black part of the shingle that butts against the bottom of the original second course, providing a flat surface for the new first course. Trim several inches off one end of the first strip so old seams will be covered. Nail every two or three inches.
* Once the entire row of starter strips is in place, start on the first course. This course usually starts with a full shingle. It should butt against the bottom of the old third course, with the ends of the tabs even with the bottom of the starter strips. You may have to trim it an inch or so to make it fit--trim the black part, not the tabs. Put nails one inch from each side in the adhesive, and above each tab cutout (the slot that forms the tab). It's important to stick to this nailing scheme so nails will always be covered.
* The first shingle is always trimmed, to make the cutouts stagger. It should say on the wrapper how much to cut off. Always cut from the same side and always start each course at the same edge of the roof. The cut edge of a shingle should always be on the outside edge of the roof.
* Continue to place each course, fitting it against the bottom edge of the next course of existing roofing, and following the pattern specified on the wrapper. When you come to an obstruction, a pipe or chimney, cut the shingle to fit around it. Nail it down and lap the next course over the nails. The trick is to nail only in the black parts of the shingles, and not in the tabs, where the nails would be exposed. If there's a little piece of shingle you can't nail, because the nail would show, glue it down with roofing cement. Then use silicone caulk around the trimmed-out edges.
* A traditional ridge cap is made by cutting each tab apart, through the black part, then folding the resulting piece sideways across the peak. (You can trim the black part slightly, so there won't be any black showing when the pieces are overlapped; trimmed ridge-cap pieces might look a bit like home plate, with the black being the narrower part.) Make sure the black parts of the last regular course of shingles won't show under the ridge cap. It may mean adding an extra course on one side of the roof. Nail each cap piece on each side, again nailing through the adhesive in the black part.
The ridge cap is the only place on the roof where any nails will show. The last piece you fold and place over the edge will have the black part showing. Cut the black part off of one of the cap pieces and fold it over the black part of the last cap piece. It goes in like the other pieces, with one nail on each side. Start the nail, put a dab of silicone caulk on the spot, then drive the nail through the caulk and smear it around to cover the nail head.
Newer roofs may have a ridge vent. It will have to be removed to get the shingles far enough under it to protect the house. Then it can be replaced after the shingles are nailed down. The original vent may not survive removal; new ones aren't expensive and come with directions on how they should be installed.
Finally, climb down carefully, clean up any mess and take yourself out to dinner. You deserve it.