If the old shingles aren’t badly curled, you can nail new shingles directly over them. Use 3-foot-long shingle strips the same width as the old shingles; secure them with galvanized roofing nails long enough to penetrate 3/4 inch into the roof deck. If your roof already has two layers of shingles, check with a building engineer to be sure that it can carry more weight.
Use a utility knife to trim the old overhanging shingles flush with the edge of the roof. It is best to cut shingles from the back, or smooth, side. If the roof lacks aluminum drip edges, install them over the trimmed shingles: use aluminum nails and dab the heads with roofing cement.
For the starter row, trim off the notched edges of some new shingles to a width of about 5 inches. This unnotched starter row should overhang the drip edge by about 1/4 inch when it is butted against the second row of old shingles. Position the nails 12 inches apart, 3 inches from the shingles’ upper edge.
Starting at the left side of the roof, lay the new shingles one row at a time, notched edges downslope, so that each row butts against the lower edge of the old shingles above. The notches should alternate from row to row. Fasten each strip with four nails; one 1 inch above each notch, and one 1 inch from each side.
Before you reach the peak of the roof, carefully remove the row of hip shingles or the metal ridgecap from the peak. Nail a final row of new shingles on each side so that their upper edges meet at the peak. Then reinstall the ridgecap or lay new hip shingles, using the old ones as patterns. Let the sun warm the shingles before you fold them over the peak.