Is your roof in disrepair? You might 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 "horse feathers" must be laid along the thick edges of the shakes to make a fairly flat surface.