A 900 square foot roll does not cover 900 square feet
A 9 by 100 foot roll is 900 square feet of material. What it covers depends on the lap. With a 6 inch horizontal lap the usable width of each course is 8.5 feet, not 9, so the roll covers 850 square feet, and that is before the vertical joints where one roll ends and the next starts. On a 136 foot perimeter at 9 feet of wall, the wall area is 1,224 square feet but you will buy closer to 1,500 square feet of wrap. Ordering by bare area is the standard way to come up one roll short on the last wall, which is the wall you do at four in the afternoon.
The course arithmetic has a sharp edge in it. A 9 foot wall with 8.5 feet of usable width needs two courses, and the second course is almost entirely wasted. Six more inches of usable width would have done it in one. That is worth noticing before you buy, because a 10 foot roll changes the answer completely on a 9 foot wall, and the calculator flags it when the top course is barely used.
Laps go the way water goes
Every lap in a weather-resistive barrier works by shingle logic: the upper piece laps over the lower one, so water running down the face crosses the joint on the outside and keeps going. Get one course reversed and you have built a funnel that directs water into the wall at that line, along the entire length of the house. The same applies at vertical joints, which lap in the direction that suits the prevailing detail, and at every penetration.
This is the reason the head flashing at a window is not optional and the reason the order of taping an opening matters. Sill first, then jambs over the sill, then head over the jambs, so each layer drains onto the one below and nothing drains behind. The sequence is on the manufacturer instruction sheet for the tape and the wrap, and it is worth reading even if you have done it before, because products differ on whether the wrap is cut and folded into the opening or lapped over a flashed sill.
Tape, fasteners and the things that get forgotten
The tape footage surprises people. Two courses around a 136 foot house is 136 feet of horizontal seam by itself, and then every vertical joint and every corner adds its own height. Openings are separate again, and they use the wider flashing tape rather than seam tape. Buying one roll of tape for a whole house is how a wrap job ends up with untaped seams.
Cap fasteners rather than staples are what most manufacturers call for, because a plain staple tears through under wind and the sheet flogs itself to pieces before the siding goes on. The count here comes from the fastener grid: studs at 16 inches across and fasteners every 16 inches up gives about 0.56 fasteners per square foot, which is around 700 for a typical small house. Spacing requirements tighten in windy regions, and both the manufacturer instructions and the local building department have a say in what applies to your wall. Neither is something a calculator decides.
What the wrap is and is not doing
House wrap is a drainage plane and an air barrier, in that order of reliability. It is not a vapor barrier in the sense people usually mean, it is not waterproofing, and it does not fix a wall that gets soaked from the inside. Its job is to give any water that gets past the siding somewhere to run down and back out, which means it only works if the water can actually get out at the bottom and if the laps and penetrations are detailed to keep it on the face.
The one failure that undoes all of it is leaving the wrap exposed for months. UV degrades it, wind works it loose at every fastener, and by the time the siding goes on the sheet is brittle and torn at the fasteners. Manufacturers print an exposure limit and it is usually measured in months, not seasons. Order the siding before you wrap, not after: the siding calculator will tell you what that order looks like, and the soffit and fascia calculator covers the top of the wall where the wrap terminates. If the reason you are opening a wall is water that got in somewhere, work backwards from the symptoms with the water leak guide first.
Questions people ask
How much house wrap does a 2,000 square foot house need?
Square footage of floor does not tell you, which is why this page asks for perimeter and height instead. A 2,000 square foot two-storey with a compact footprint might have 1,600 square feet of wall; a sprawling single storey with the same floor area can have more. Measure the perimeter, multiply by the wall height, add the gables, then add roughly fifteen to twenty percent for the laps. For most modest houses that lands between two and four rolls of 9 by 100.
Which way does the lap go?
Upper over lower, always, at horizontal joints. Water runs down, so the joint has to be crossed on the outside of the sheet below. At vertical joints the next sheet laps over the last in the direction you are working, and at corners the sheet carries around onto the adjacent wall rather than being cut at the corner, so there is no joint at the most wind-driven point on the building. If you find yourself taping a horizontal joint where the lower sheet is on the outside, stop and redo it. Tape does not make a reversed lap safe.
Do I need to tape the seams at all?
For water management, a correctly lapped seam sheds water without tape. For air sealing, it does not, and air leakage carries far more moisture into a wall assembly than liquid water usually does. Manufacturers also generally require taped seams for the product to perform as an air barrier and sometimes for the warranty. Where the wrap is being installed as part of an air-sealing strategy, the tape is the whole point rather than an accessory.
Can I use staples instead of cap fasteners?
Most manufacturers call for cap nails or cap staples, and the reason is mechanical rather than fussy. A plain staple has almost no bearing area, so wind loads concentrate on the two legs and tear through the fabric. Once the sheet is torn at a hundred fasteners it flaps, and flapping wrap abrades itself and pulls further. The cap spreads the load. If the siding goes on the same day the exposure is short, but the specification does not change because your schedule is tight.
Does this work for a rainscreen or furred-out wall?
The wrap quantity is the same, because the wrap still covers the same wall. What changes is everything after it: furring strips, the gap, the insect screening at top and bottom, and fastener lengths that have to reach framing through the gap. A drained and vented gap behind the cladding is a substantial improvement on wrap alone in wet climates, and it is worth deciding about before you order siding, since it changes fastener lengths, trim depths and the detailing at every opening.