A Gambrel or "barn style" roof is one that includes a slope change (or multiple slope changes) on each side of the roof. When installing asphalt shingles on this type of roof, you can proceed as you would with a regular roof. Choose a warm summer day to complete this job, as the tarred sides of the asphalt shingles are more flexible and easier to work with when warm. As you lay courses of asphalt shingles up the Gambrel roof and encounter slope changes, just let the shingles sit up like cowlicks. The heat will eventually lay the shingles down, every time. Use three-tab asphalt shingles (the standard, tar-backed shingles covering most residential roofs) for all parts of the roof except for the ridge (the horizontal edge at the top), for which you will use ridge caps (square asphalt shingles.)
Determine the number of squares of asphalt shingles you will need for covering the Gambrel roof. A "square" of shingles is usually three bundles, but it is sometimes four or five bundles if the shingles are manufactured to one of the smaller standard sizes. In any case, a square is enough to cover a 10 ft. by 10 ft. area, or 100 square feet of roof. Measure and add the square footage of every section of the roof and add 5 percent of this total to compensate for waste. Divide the grand total by 100 to determine the number of squares you will need.
Lay the first asphalt shingle at an outer corner of the lowest sloping face of the Gambrel roof. Use a nail gun to secure the shingle to the decking with three nails through the upper, nondecorative portion of the shingle.
Lay the next asphalt shingle next to the first one. Secure it (and all ensuing shingles) with the nail gun as you did in Step 2. Continue laying shingles end to end until you reach the other end of the roof face. Trim the last shingle to fit by scoring the tar side with a razor blade and cleanly snapping the shingle in your hands.
Trim the first asphalt shingle of the second tier in half and lay it so that it overlaps the first tier according to the manufacturer's specifications. (The shingles have little ridges on their undersides that make it easy to determine how much to overlap.) Lay the rest of the shingles of the second tier, trimming the last shingle to fit.
Lay the third course starting with a whole shingle. Continue laying courses of shingles, alternating between starting with a half shingle and a whole one.
When you reach a slope change in the Gambrel roof, just keep going as normal.
When you reach the ridge (horizontal "peak") of the Gambrel roof, you are done with the three-tab asphalt shingles. Cap off the ridge with asphalt ridge caps (square shingles).