A flat doghouse roof that is not shingled properly will leak, allowing water to settle in the doghouse. Most doghouses have blankets in them or a doggy bed. Wet bedding eventually moulds, making the doghouse unusable for the dog. The mould could cause respiratory and skin problems for the dog if it spends a lot of time in the doghouse.
Fit the drip edge along all edges of the flat doghouse roof by placing the widest surface on the top of the roof and allowing the curved overhang to hang over the edge of the roof. Cut the drip edge so that it has a 1-inch overhang on each end of the roof. Cut a horizontal slit in the drip edge, so that the overhang can be bent around the corner of the roof.
Nail the drip edge with roofing nails to the roof, placing a nail every 12 to 18 inches.
Unroll enough tar paper to cover the entire roof. If the doghouse is large, unroll enough for the first row, at the bottom or back of the roof. Nail the tar paper to the roof, using the hammer and roofing nails, placing nails 6 to 8 inches apart.
Unroll enough tar paper for the second row, if a second row is needed. Overlap the first row with the second row by at least 3 inches. Nail the second row of tar paper down.
Cut the tabs plus 2 inches off the first row of shingles. Working from left to right, lay the shingles across the roof, so that the edge lines up with the bottom edge of the drip edge. Use three nails per shingle: one in the middle and one on each side, about 4 inches from the middle nail.
Start the second shingle row, working left to right, by placing the tabs over the top of the first row. Nail the second row of shingles just above the cutouts. Use four nails, evenly spaced, for each shingle. Repeat this step for each of the rows, overlapping only the tabs over the previous row, until you reach the far edge of the roof. If there is too much shingle on the last row, cut the top of the shingle, so that it fits (do not cut the overlapping tabs).
Use the utility knife and straight edge to create a straight cut on the shingles.