On recovers we start by removing the old ridge cap, plumbing pipe boots, cutting back the old edges, then we install the new shingles, cap, boots, and remove all the trash. On the average, house labor & materials start at $110.00 per sq for 3-tab shingles and $125.00 per sq for dimensional shingles.
Although it is better to remove your old roof if you only have one layer it is more cost efficient and acceptable to install a second layer over the old shingle as long as you do it right. The old shingles must be lying flat and you must have good ventilation to your attic. By placing a second layer of shingles on your house you can expect to knock 5 years of life off the new shingles