Making repairs to an existing asphalt shingle roof is easy when you follow these step-by-step instructions. Use a small pry bar to pull out the nails holding the damaged shingle. Remove both the nails and the shingle. If you cannot reach the nails, use a sharp linoleum knife to cut away the damaged shingle. Always try to replace the damaged shingle with one similar in weight, size and color. Apply asphalt roofing cement to the back of the replacement shingle before putting it into place. After positioning the new shingle, nail it down with 1-3/4″ roofing nails. Position the roofing nails so they are covered by the shingle above. For extra protection against leaks, apply a small amount of roofing cement to the nail heads.