House wrap & weather barrier — how much you need

By Francesco Zinghinì

What house wrap does

House wrap — a weather-resistive barrier — goes on the sheathing before the siding. Its job is to shed any water that gets past the cladding while letting water vapor escape from inside the wall, so the sheathing and framing stay dry. It is a required step in a proper siding installation, not an upgrade or a line to value-engineer out. Skipping or shortchanging it is how a wall that looks finished quietly fails behind the siding. So the first fact about “how much house wrap” is that the answer is never zero.

The roll-count formula

Sizing the wrap is the familiar coverage ceiling, with one twist — an overlap allowance instead of a waste factor:

rolls = ceil( net area × (1 + overlap%) ÷ roll coverage )

The overlap (~10% as a default) accounts for the horizontal seams, the corners and the returns into openings, where the wrap laps over itself and consumes more than the bare wall area. The roll coverage is the manufacturer’s stated figure — a common roll covers about 1,000 sq ft, but they vary widely, so read the label. The net area is the same wall area you measured for the siding; the wrap covers the field, and openings are typically wrapped and returned rather than cut out cleanly, which is part of why the overlap allowance exists.

Worked example

Reference house, 1,336 sq ft net, 10% overlap, a 1,000 sq ft roll: rolls = ceil(1,336 × 1.10 ÷ 1,000) = ceil(1,469.6 ÷ 1,000) = ceil(1.47) = 2 rolls. The ceiling is doing real work here — 1.47 rolls means you buy 2, and the leftover is your insurance against a torn section or a re-lap. Run your own wall in the house-wrap calculator. Because roll coverage varies so much between products, always plug in the actual coverage from the roll you are buying rather than assuming 1,000 sq ft.

Why the overlap is not the siding waste factor

They look similar — both are a percentage on top of the net area — but they cover different things and should not be conflated. The siding waste factor pays for offcuts from cutting rigid pieces to length. The wrap overlap pays for the material that laps at seams and corners in a continuous sheet. A simple house can have a low siding waste factor but still needs its full wrap overlap, because the seams exist regardless of how few cuts the siding takes. Size the two independently.

Details that are installation, not arithmetic

The quantity is straightforward; the performance depends on installation details this guide does not engineer. The wrap must be lapped in the right direction (upper courses over lower, so water sheds outward), taped or sealed at seams per the manufacturer, and integrated with the flashing at windows, doors and penetrations so water is directed out over the cladding, not behind it. That flashing-and-integration work is a moisture-management matter for the manufacturer’s instructions and a qualified installer — get the roll count here, get the lap-and-flash sequence there. A correctly sized roll installed in the wrong lap order does not protect the wall.

Wrap, felt and integrated options

House wrap is the common weather-resistive barrier, but it is not the only one — building felt (tar paper) and integrated water-resistive-barrier products serve the same function, and some rigid insulation and sheathing systems carry a barrier built in. The roll-count formula applies to whatever sheet-good barrier you use; only the roll coverage changes. What does not change is the requirement for a continuous, correctly lapped barrier behind the cladding. If your wall assembly already includes an integrated barrier, you are sizing less separate wrap, not skipping the function.

Cheap layer, expensive to omit

Put the cost in proportion. On the reference house, two rolls of wrap are a small fraction of a five-figure siding job — and they protect the sheathing and framing that the whole wall depends on. Undersizing the wrap to save a roll, or skipping seam tape and flashing to save a little more, trades a trivial saving against the cost of a wall that fails from behind. Size it honestly, buy the compatible tape and flashing, and treat the barrier as structural insurance rather than a line to trim.

What to confirm before ordering

• Actual roll coverage — the single most important input; it varies far more than the overlap does.
• Overlap allowance — ~10% is a reasonable default; a house with many openings and corners may warrant a touch more.
• Seam tape and flashing — separate items the wrap system needs to actually perform; budget them alongside the rolls.
• Compatibility — the wrap, tape and flashing should be a compatible system per the manufacturer.

The roll count is a planning quantity from your measured area and the roll’s stated coverage; treat it as an estimate, round up, and defer the lap-and-flash installation details to the manufacturer’s instructions and a qualified installer. House wrap is cheap relative to the wall it protects — size it honestly and do not skip it.