Before installing asphalt shingles on a low-slope roof, check to make sure the slope is 2-in-12 pitch or higher. A lower pitched roof requires a different roofing material. Unless the underlying roof is in good shape, remove the original asphalt shingles before installing new ones.
Start by installing a double layer of asphalt felt paper over the roof sheathing. The first layer of felt paper should run parallel to the eaves, be 19 inches wide and extend approximately 3/4 inch beyond the drip edge. Use only a few nails to secure the felt paper to the sheathing. Cover the piece of felt paper with asphalt lapping cement, and place a full 36-inch-wide sheet of felt paper over the first sheet of felt paper. Nail down the felt paper only along the top edge.
Apply asphalt lapping cement over the top 19 inches of the 36-inch sheet of felt paper. Apply another layer of 36-inch-wide felt paper by covering the cement with felt paper, and let the top of the felt paper extend onto the sheathing where it is nailed down. Snap chalk lines to keep the rows straight and to use as a guide. Repeat this process until all the sheathing is covered with felt paper.
Shingle the roof using standard roofing techniques by laying a row of starter shingles along the eaves using a bead of roof cement, nailing through the shingles with roofing nails. Add the next row of full shingles over the starter shingles, making sure to cover the nail holes on the starter shingles and stagger the vertical notches in each shingle sheet. Secure each sheet of shingles with roofing nails, and continue laying rows until the felt paper is covered completely. Install ridge cap shingles along the roof peak.