Where this applies
Georgia IRC Appendix U (Disaster Resilient Construction) is optional; it governs only where a local jurisdiction adopts it by ordinance. Section AU408 sets out a two-layer secondary water barrier. The figures below come from the DCA Appendix U text.
Laying the sheets (AU408.1)
- Start at the eave with a 19-inch strip.
- Then lay 36-inch-wide sheets, each overlapping the sheet below by 19 inches.
Do not mix up the widths: 19 in. is the starter strip (and the overlap); 36 in. is the full sheet. The 24-inch figure you may see alongside is the minimum width of a metal open-valley lining under IRC R905.2.8.2, not an Appendix U dimension.
The coverage math
Each successive full sheet adds new roof depth, measured up the slope, of:
new coverage = sheet width − overlap = 36 − 19 = 17 in.
Traps:
- 19 in. is the overlap, not the added coverage.
- 36 in. ignores the overlap entirely.
- 32 in. (36 − 4) subtracts only the 4-inch minimum lap, but AU408.1 calls for a 19-inch overlap between successive sheets.
Physically, overlapping 19 of every 36 inches is more than half the sheet, which is exactly what makes the barrier two layers everywhere.
Fastening
The DCA text calls for fasteners approximately 6 in. on center along the laps and approximately 12 in. on center in the field. The tighter spacing belongs at the laps, where the sheet edges need holding. Reversing it (12 at laps, 6 in the field) is wrong, and so is a uniform 6 in. everywhere, which ignores the 12-inch field spacing. The 4-inch and 19-inch figures are laps, never fastener spacings.
Laps and cap nails
- All laps: minimum 4 inches (Appendix U text).
- Cap nail heads: at least 1 inch (AU408.2.1).
Distractors to reject: a 2-inch lap is the drip edge lap from IRC R905.2.8.5, not the water-barrier lap. A 3/4-inch head is not an Appendix U cap-nail size. A 19-inch lap is the sheet overlap, and 2-inch cap heads are not the stated minimum.