IRC R703.2 water-resistive barrier, with both of its lap dimensions
Lap siding covers the wall at its exposure and is sold at its face width, so an 8 inch board at a 7 inch exposure means buying 1.14 square feet of board for every square foot of wall. That 14% is not waste and no allowance covers it. Underneath it, R703.2 requires a water-resistive barrier with two different laps, two inches between courses and six at every joint, and almost no siding calculator counts it at all.
Siding, barrier and flashing
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ft
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no.
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no.
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%
Exposure is a manufacturer figure, not a code one. Read the installation instructions for the siding you actually bought.
Specification summary
Generated from on .
Reopen that address to reproduce these figures exactly.
Entered
Result
Courses in section, with the lapped part shaded
Worked example
A 40 by 28 ft two-storey house with 9 ft walls, two 7 ft gables, 8 in board at a 7 in exposure, two doors and fourteen 3 by 4 ft windows, with a 36 in by 144 ft barrier roll and 10% waste.
Wall. 136 ft of perimeter by 18 ft is 2,448 sq ft, plus 196 of gable, less 208 of openings: 2,436 sq ft net.
Courses. 18 ft of wall at a 7 in exposure is 31 courses.
Board per wall. 8 ÷ 7 = 1.14, so 2,436 becomes 2,784 sq ft of board before any waste.
Lapped out of sight.348 sq ft, permanently behind the course above.
To buy. 2,784 plus 10% = 3,063 sq ft.
Barrier. A 36 in roll gives 34 in per course after the 2 in lap, so 7 rows, plus 6 in at every roll joint.
The 348 square feet lapped out of sight is the number worth sitting with. It is 14% of the wall, it is not waste, and no allowance you add covers it, because it is doing the job the siding exists for. A calculator that takes your wall area and adds 10% for waste has under-ordered by roughly the amount of the lap, which is how a siding job runs out one wall short.
The formula
Two ratios, and both of them are laps:
board = A × (face / exposure) barrier rows = ⌈h / (w − 2)⌉
A
net wall area, gables in and openings out, sq ft
face / exposure
board bought per wall covered. An 8 in board at 7 in is 1.14
h
wall height in inches
w
barrier roll width, in. Less the 2 in course lap from R703.2
6 in
the other lap, taken at every vertical joint between rolls
Both terms are the same idea: material that overlaps material is bought twice and counted once. The siding lap is 12 to 20% depending on the product, and the barrier lap is a few per cent on a plain wall and more on a long one with many roll joints.
Two materials, and the second one gets forgotten
A siding order has two parts and almost every calculator gives you one of them.
The siding itself is the visible part, and the arithmetic that matters is exposure rather than area. Lap siding covers less wall than its width because the next course laps over it, so the ratio of board bought to wall covered is the face width over the exposure. On common products that is 1.12 to 1.20, and it is not a waste allowance: it is the lap, permanently hidden, doing the thing the product exists to do.
The second part is the water-resistive barrier, which IRC R703.2 requires over studs or sheathing on all exterior walls before any siding goes on. It has its own laps, and it has two of them: two inches between courses and six inches at joints. A roll therefore covers less wall than its dimensions suggest, in both directions.
Neither of these is subtle and neither is optional. What they have in common is that they are invisible once the job is finished, which is exactly the kind of quantity that gets left off an order.
Face width against exposure, and what the lap costs
Board face width
Typical exposure
Covered by the lap
Courses per ft of wall
Sq ft of board per sq ft of wall
6 in
5 in
17%
2.40
1.20
8 in
7 in
12%
1.71
1.14
8 in
6.75 in
16%
1.78
1.19
10 in
8.75 in
12%
1.37
1.14
12 in
10.75 in
10%
1.12
1.12
Read 2026-09-08. Exposures are typical manufacturer figures, not code values. The last two columns are derived from the first two.
Lap siding is bought by its face width and installed at an exposure, the amount left showing once the next course laps over it. The exposure is always smaller, and the wall is covered at the exposure while you pay at the width.
Common lap siding widths and exposures. Exposure is set by the manufacturer, not by the code
Board face width
Typical exposure
Covered by the lap
Courses per ft of wall
Sq ft of board per sq ft of wall
6 in
5 in
17%
2.40
1.20
8 in
7 in
12%
1.71
1.14
8 in
6.75 in
16%
1.78
1.19
10 in
8.75 in
12%
1.37
1.14
12 in
10.75 in
10%
1.12
1.12
Read the last column. An 8 in board at a 7 in exposure means you buy 1.14 sq ft of board for every square foot of wall, before any waste allowance at all. That 14% is not waste and no allowance covers it: it is the lap, it is doing its job, and it is permanently behind the course above.
Exposure is a manufacturer figure rather than a code one, and it varies with the product. The calculator asks for it rather than assuming, because getting it wrong changes both the course count and the order.
The barrier underneath, and its two different laps
Before any siding goes on, R703.2 requires a water-resistive barrier, and the section is worth reading closely because it contains two lap dimensions and they are not the same:
One layer of No. 15 asphalt felt, free from holes and breaks, complying with ASTM D226 for Type 1 felt or other approved water-resistive barrier shall be applied over studs or sheathing of all exterior walls. No. 15 asphalt felt shall be applied horizontally, with the upper layer lapped over the lower layer not less than 2 inches. Where joints occur, felt shall be lapped not less than 6 inches.
2 inches where an upper course laps the one below it. 6 inches where a vertical joint occurs, which is every time one roll ends and the next starts.
So a roll never covers its own area. A 36 in wide roll laid horizontally delivers 34 in of wall per course, which is 5.6% lost before you reach the end of it, and then another 6 in comes off the length at every vertical joint.
On a plain wall that is a few per cent. On a long low wall with many roll joints it is considerably more, and the calculator counts both.
Note what the section actually requires. One layer, free from holes and breaks, over studs or sheathing, on all exterior walls. Not over the sheathing where convenient. All of them.
Ask the manufacturer for the exposure, not the internet
Exposure is set by the product, not the code. The same nominal 8 in board can be specified at 7 in on one product and 6 3/4 on another, and the difference moves the order by 4%.
It also moves the course count, which matters where courses have to land: at a window head, at a soffit, or where two elevations meet at a corner and the courses need to line up. Adjusting the exposure slightly within the manufacturer's permitted range is the normal way to make courses land, and it is a decision to make before ordering rather than halfway up a wall.
The maximum exposure is a weather-tightness limit, not a preference. Stretching past it to save a course leaves too little lap for water to shed properly.
Gables, and the courses that stop mid-wall
A gable is half its base times its rise, so the area is easy. The material is not, because every course in a gable is cut at an angle on both ends and the offcuts are triangles that fit nowhere.
The waste allowance on this page assumes plain rectangular walls. A house with steep gables genuinely runs higher, and the offcut from one end of a gable course only helps if the pitch happens to mirror on the other side of the ridge, which it does on a symmetrical gable and does not on a hip or a saltbox.
If the elevation is mostly gable, add to the allowance rather than trusting the flat figure.
What the barrier actually has to be
R703.2 names one layer of No. 15 asphalt felt complying with ASTM D226 for Type 1 felt, or other approved water-resistive barrier. The housewraps in common use are that other approved barrier, and they are installed to their own manufacturer instructions rather than to the felt laps.
Which means the 2 in and 6 in figures are the code minimum for felt, and your wrap may specify differently. The calculator uses what you type, and the roll width and length are on the packaging.
The requirement that does not vary is the phrase free from holes and breaks, on all exterior walls, as one continuous layer. A barrier with the staples torn through it or a section missing behind a porch roof is not a lesser barrier, it is a hole in the drainage plane at the point where water is heading.
And the part no quantity page can help with
Approved corrosion-resistant flashing shall be applied shingle-fashion in a manner to prevent entry of water into the wall cavity or penetration of water to the building structural framing components. The flashing shall extend to the surface of the exterior wall finish.
Flashing is where siding jobs fail, and it is not a quantity problem. Shingle-fashion is the whole idea: every piece laps over the piece below it so water running down the wall stays on the outside of the assembly.
Get one lap the wrong way round at a window head and the barrier is a funnel pointing into the wall cavity. That is not a material shortfall and no order quantity prevents it.
The section requires flashing at exterior window and door openings among other locations, and requires it to extend to the surface of the exterior wall finish. The calculator gives you the linear feet around openings so it is at least on the order, and stops there.
Where the code text came from
Read 2026-09-08. 2018 Seattle Residential Code, Chapter 7 Wall Covering, Seattle Department of Construction and Inspections, page 439. Prints IRC Section R703 in full, including R703.2 water-resistive barrier with its two lap dimensions and R703.4 flashing. Used because codes.iccsafe.org returns HTTP 403.
Exposure figures are not from the code. They are manufacturer product data and they vary, which is why the table above is labelled typical and the calculator asks you for the number rather than assuming one. Read the installation instructions for the siding you have actually bought.
Corner posts, trim, starter strip, J-channel and soffit are not quantified here. They are linear items driven by the shape of the building rather than its area.
Frequently asked questions
How much siding do I need for a 2000 square foot wall?
More than 2,000 square feet of board. Lap siding covers the wall at its exposure and is sold at its face width, so an 8 in board at a 7 in exposure needs 8/7 = 1.14 sq ft of board per square foot of wall, which is 2,286 sq ft before any waste allowance. The extra 286 is the lap and it is not waste.
What is siding exposure?
The part of each board left showing once the course above laps over it. An 8 in board is typically installed at a 7 in exposure, so 1 in of every board is permanently hidden. Exposure is a manufacturer figure rather than a code one and varies by product, so read the installation instructions.
Do I need house wrap under siding?
IRC R703.2 requires a water-resistive barrier over studs or sheathing on all exterior walls: one layer of No. 15 asphalt felt to ASTM D226 Type 1, or other approved barrier, free from holes and breaks. Housewrap is that other approved barrier and follows its own manufacturer instructions.
How much do you overlap house wrap?
For No. 15 felt, R703.2 sets two different minimums: not less than 2 inches where an upper course laps the one below, and not less than 6 inches where joints occur. Housewraps specify their own laps, which are often larger, and the manufacturer's instructions govern.
How many courses of siding will my wall need?
Wall height in inches divided by the exposure, rounded up. An 18 ft wall at a 7 in exposure is 216 / 7 = 31 courses. Adjusting the exposure slightly within the manufacturer's range is the normal way to make courses land neatly at window heads and soffits.
Does the gable count in siding area?
Yes, at half its base times its rise, and it uses more material per square foot than a plain wall because every course is cut at an angle at both ends. On a symmetrical gable the offcut from one side can start the other; on a hip or a saltbox it cannot.
Is the siding lap included in a waste allowance?
No, and treating it as waste is how siding orders come up short. The lap is 12 to 20% of the board depending on the product and it is permanently behind the next course. A waste allowance sits on top of it, for cuts and offcuts.
Check these numbers yourself
2018 Seattle Residential Code, Chapter 7 Wall CoveringSeattle Department of Construction and Inspections, page 439. Prints IRC Section R703 in full, including R703.2 water-resistive barrier with its two lap dimensions and R703.4 flashing. Used because codes.iccsafe.org returns HTTP 403.
How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.
Figures on this page last checked against the source documents on 2026-09-08. Codes are amended locally; confirm against the edition your jurisdiction enforces.
Exposure is a manufacturer figure, not a code value. The table on this page is typical, and the calculator uses what you enter. IRC R703 was read from the 2018 Seattle Residential Code on 8 September 2026 because codes.iccsafe.org returns HTTP 403. The 2 in and 6 in laps are the code minimum for No. 15 felt; approved housewraps follow their own instructions. Corner posts, trim, starter strip and J-channel are not counted.