The precise geometry required to waterproof a low-slope roof
In short: Roofing is often dismissed as brute labor, but successful installation relies on meticulous measurements. From calculating the perfect overhang to snapping precise chalk lines, the difference between a dry attic and a leak lies in a few fractions of an inch and a disciplined sequence of steps.
When tackling small-scale roofing projects, such as those for sheds, porches, or doghouses, the process begins with establishing a straight, level foundation. General contractor Tom Silva emphasizes that these guidelines are specifically for low roofs where easy access is possible. For larger, more complex structures, the risks of working at height necessitate a professional crew equipped with proper fall protection.
The first critical layer is the aluminum drip edge. To install it, one must align the edge flush with the roof's perimeter, marking the top ends of the house and using a chalk line to ensure a straight path. A vital nuance here is creating a deliberate gap: the drip edge should be pulled down approximately half an inch from the chalk line to maintain separation from the fascia board. This component is secured using 1 1/4-inch galvanized roofing nails.
Once the drip edge is set, 30-lb. asphalt roofing felt paper is laid and nailed down with the same 1 1/4-inch galvanized nails. The layout of the shingles themselves requires even more precision. Starting at the bottom, a tape measure is used to set a 1/2-inch margin from the roof edge, ensuring the shingles overhang the drip edge correctly.
To maintain a consistent 'reveal,' the installer marks a line 12 inches up the roof for the first course, then marks every 5 inches moving upward. Vertical chalk lines are snapped every 6 inches to guide placement. The process even involves modifying the shingles themselves; a starter course is created by cutting away the three top tabs of a shingle, leaving only the top tar section to initiate the seal.
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Sources
- Summary by Intellect · source: How to Install Roof Shingles | Ask This Old House
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