Here's a tip you can use to shingle your own roof. Have a starter course at the edge of the eave, laying the shingles upside down. Trim one-third off your first shingle, so that your joints will be staggered. Then lay your first course over that with tabs down. Form a pyramid of overlapping shingles, trimming each by six inches so that all rows will have overlapping joints. Then continue to lay rows from bottom to top using whole shingles.