House wrap is the weather-resistive barrier that goes over the wall sheathing and under the siding. It sheds any water that gets behind the cladding while letting water vapor escape from the wall. Ordering the right number of rolls means accounting for the walls, the gables, the openings and the laps between courses.
The calculation
- Wall area = perimeter × wall height + gable area − openings
- Material = wall area × roll width ÷ (roll width − lap), plus waste
- Rolls = material ÷ (roll width × roll length), rounded up
- Seam tape = (number of courses − 1) × perimeter, plus waste
Many installers wrap straight over window and door openings and cut them out afterwards, then fold the flaps into the rough opening. Deducting the openings is still reasonable for ordering, since the cut-outs are small compared with the waste allowance.
Example
A house with 160 ft of walls 18 ft high, 100 sq ft of gables and 150 sq ft of openings has 2,830 sq ft to cover. With 9 ft wide rolls, 6 in laps and 10% waste, that is about 3,300 sq ft of material: 4 rolls of 9 × 100 ft, laid in 3 courses, with about 350 ft of seam tape.
Roll sizes
| Roll | Coverage |
|---|---|
| 3 × 100 ft | 300 sq ft |
| 9 × 100 ft | 900 sq ft |
| 9 × 150 ft | 1,350 sq ft |
| 10 × 100 ft | 1,000 sq ft |
Installation basics
- Start at the bottom corner and work up, so each course overlaps the one below by at least 6 in, like shingles.
- Overlap vertical seams by 6 to 12 in.
- Fasten with cap nails or cap staples, typically every 12 to 18 in on the studs.
- Tape horizontal and vertical seams with the maker’s tape.
- Flash windows and doors in the right order: sill flashing first, then jambs, then the head, tucked under the course above.
House wrap or felt?
Asphalt felt and building paper are older alternatives; synthetic house wraps are lighter, tear less and come in larger rolls. Both are allowed by building codes as weather-resistive barriers when installed per the instructions. In cold climates, the wrap must be vapor-permeable so the wall can dry outward.
Limits
Corners, bays, dormers and two-story gables add waste. Check the wrap maker’s instructions and local code, which can require a specific barrier behind stucco or stone veneer.