You may be able to lay new shingles over existing shingles and avoid the cost of tear-off.
If conditions are right, new shingles laid over old shingles can be just as attractive and durable as shingles laid on bare sheathing.
Installation is easier than for a tear-off job because you can use the existing shingles as guides for laying the new ones. But you must take care to install the shingles correctly so they lie flat. And a good job involves installing new flashings rather than relying on the existing ones.
Reroof jobs are sometimes done without replacing flashings. However installing new flashings -- including special drip-edge flashings made for reroofing -- will ensure a tight seal and a long life. Some roofers install a layer of waterproof shingle underlayment (WSU) over the existing shingles at the eave end, as would be done for a roof laid on bare sheathing. This provides added protection against ice dams.
Asphalt shingles can be laid over cedar shakes or shingles, a job best left to pros. Beveled wood pieces called "horsefeathers" must be laid along the thick edges of the shakes to make a fairly flat surface.
With a helper, two days to flash and roof 700 square feet
Power nailer or roofing hatchet, tape measure, roofing shovel or flat pry bar, hammer, drill, carpenter's square, tin snips, utility knife, chalkline, broom
Marking, measuring, cutting, fastening
Gather the materials and set up ladders or scaffolding.
Roof cement, flashing, nails for flashing, perhaps a WSU sheet, shingles, nails long enough to penetrate sheathing
Remove air vents and pipe flashings by prying out or unscrewing the fasteners holding the fixture. If you damage shingles while doing this, repair the shingles (see next step). Reuse a vent or flashing only if it is like new; otherwise replace it with a new one that will fit the hole or pipe.
Repair broken shingles. If a shingle is torn or cracked, glue the broken piece back in place using roofing cement. If the broken piece is lost, cut a piece to fit. The goal is to provide a reasonably flat surface for the new shingles to lie over, with no gaps greater than 1/2 inch.
Rip-cut the shingles for the first course so they butt up against the third course of existing shingles and are even with the front edge of the starter course. Their tab slots should not line up with the tab slots of the starter course, if there are any. Nail the shingles just above the tab slots.
Once you have installed shingles just past a plumbing vent, install the flashing piece so it will lie on top of roofing at its bottom but be covered with roofing at its top. Depending on the width of the pipe, you may need to tear away a segment or two of the rubber boot. Apply roofing cement, slide the flashing over the pipe, and push so it lies flat on the roof.
Where you meet a chimney or side wall, install step flashing. Apply a shingle, then a piece of flashing, then a shingle, and so on, so that each piece of flashing rests on top of the lower course and is covered by the upper course. You'll need to pry the siding outward to slip in the flashing; in some cases you'll have to remove the siding.
Protect the step flashing with counterflashing. On a chimney use a grinder to cut an indentation into the mortar. Cut and bend the counterflashing to fit snugly into the cut mortar and to cover at least 3 inches of the step flashing. Apply mortar with a caulking gun (inset) and set the flashing into the mortar.