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Wood Siding Calculator

R703.2 barrier laps and the R703.3.2 wind limits

Wood siding is bought by the linear foot and sold by the board, and the two are not the same number. The lap is 15% of every plank and it is the reason the material never matches the wall area.

Wood siding takeoff

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Board face and exposure are the manufacturer's. The barrier laps come from R703.2 and the wind limits from Table R703.3.2, both quoted from the 2018 Seattle Residential Code.

Specification summary

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Table R703.3.2, and where it stops answering

Prices for exterior wall finishes

The published figures for exterior wall finishes are on the siding calculator, with the byline, the publication date and the caveats that go with them. They are not repeated here, because the same table on two pages is worth less than one table you can find.

This site publishes no price of its own. Why, and what that means for these figures.

Worked example

A 40 ft by 28 ft two-storey house in nominal 1x8 bevel siding, actual face 7 1/2 in, at 6 in exposure.

  1. The board is not eight inches. A nominal 1x8 is 3/4 by 7 1/4 dressed, and a bevel board is resawn from it, so the face you are laying is around 7 1/2 at most and often less. Every figure below starts from the board you measured rather than the one on the invoice.
  2. At 6 in exposure the lap is 1 1/2 in, which is 20% of the board. That compounds to 25% more board than wall: 3,068 square feet bought to cover 2,455.
  3. 4,909 linear feet, or 5,401 with waste, which is 451 twelve-foot planks.
  4. Thirty-six courses up an 18 ft wall. Wood is the cladding where that number gets adjusted on site, because the courses have to land sensibly at window heads and sills, and shaving a quarter inch off the exposure across a wall is how that is done.
  5. And the wind gate is the same as every other cladding. At 115 mph in Exposure B the fastening is a lookup; at 140 in Exposure C there is no prescriptive answer at all.

Wood is the one cladding here that moves after it is installed, so the exposure is a decision rather than a specification. Everything on this page follows from whatever you settle on.

The formula

The same two divisions as any lap siding, on a board whose real size is not its name:

linear feet = area × 12 / exposure    board bought = area × face / exposure
face
the actual dressed face, measured. A nominal 1x8 is 7 1/4 dressed before it is bevelled
exposure
what you choose to show, which on wood is genuinely a choice
face / exposure
the multiplier on the material, always above one and often 1.25

Wood differs from the other claddings here in that the exposure is not handed to you. Within the manufacturer's or the trade's minimum lap you can set it, and people do, to make courses land at window heads. A quarter inch off the exposure across a whole wall is several extra courses and several percent more board.

Nominal, dressed and what you actually lay

A nominal 1x8 is 3/4 by 7 1/4 inches dressed, which is the same nominal-to-actual gap that runs through every framing page on this site. Bevel siding is then resawn from that, so the board you lay is thinner at one edge and its face is whatever the mill left.

Which is why this page asks you to measure. Publishing a face width for 1x8 bevel would be publishing a figure that varies by mill, and the whole order scales directly off it.

The code has very little to say about wood siding specifically. R703.2 requires the water-resistive barrier and its laps, Table R703.3(1) gives minimum nominal thicknesses and the fastening schedule, and Table R703.3.2 says when that schedule may be used. The exposure, the lap and the finish are trade practice and manufacturer guidance.

And the thing that matters most is not in the code at all: it moves. Wood takes up and gives off moisture with the weather, across the grain, forever. The lap exists partly to shed water and partly to let each board move without opening a gap to the outside.

Back-priming, which is not optional and is not in the code

The most consequential thing you can do to wood siding happens before it goes on the wall, and no inspector will ask about it.

Wood takes up moisture through whichever face is unsealed. A board primed and painted on the front and bare on the back does not stop absorbing water; it absorbs it from one side only, which is exactly the condition that makes it cup.

Cupping pulls the board away from the wall at its edges, which opens the lap, which lets more water in behind, which accelerates the problem. It is the commonest way a wood wall fails that has nothing to do with the paint failing.

The fix is to prime all six surfaces before hanging, including the cut ends, and to re-prime every cut made on site. Cut ends are end grain and end grain drinks far faster than a face does.

None of that is in IRC Chapter 7, which is why it needs saying here. The code is about water getting past the cladding; this is about the cladding surviving the water it is supposed to shed.

Setting the exposure so the courses land

Wood is the one cladding on this site where the exposure is genuinely yours to set, within the minimum lap, and experienced installers use that.

The goal is to make a course line land at the head and the sill of the windows, so the siding meets the trim in a full board rather than a thin rip. On a wall with windows at consistent heights that is achievable; on one with mixed heights it is a compromise.

The method is to divide the wall height by a trial exposure, round to whole courses, and divide back. A 18 ft wall at 6 in exposure is 36 courses exactly, which is lucky. At 6 1/4 it is 34.6, so 35 courses at 6.17 in, and that quarter-inch adjustment is invisible on the wall and lands the line where you want it.

What it does to the order is real though. Reducing the exposure increases the board bought in direct proportion, so going from 6 in to 5 3/4 adds about 4% to the material on the whole house.

Which is the argument for settling the exposure before ordering rather than after, and for ordering the waste allowance on the exposure you have actually decided on.

Frequently asked questions

How much wood siding do I need?
Area divided by the exposure gives linear feet: a 2,455 sq ft wall at 6 in exposure needs 4,909 linear feet, or 5,401 with 10% waste, which is 451 twelve-foot planks. You will buy 3,068 sq ft of board to cover 2,455 sq ft of wall.
Why do I buy more board than I have wall?
Because the lap is bought and hidden. A 7 1/2 in board at 6 in exposure laps 1 1/2 in, which is 20% of the board and compounds to 25% more board than wall. It is not waste and no amount of careful cutting reduces it.
Is a 1x8 board eight inches wide?
No. A nominal 1x8 is 3/4 by 7 1/4 inches dressed, and bevel siding is resawn from that, so the face you lay is narrower again. This page asks you to measure because the figure varies by mill and the whole order scales off it.
Can I choose the exposure on wood siding?
Within the minimum lap, yes, and installers do. Adjusting it slightly makes course lines land at window heads and sills so the siding meets the trim in a full board rather than a thin rip. Reducing the exposure raises the material in direct proportion, so settle it before ordering.
Should wood siding be primed on the back?
Yes, on all six surfaces, and re-primed at every cut. Wood absorbs moisture through whichever face is unsealed, so a board finished on the front and bare on the back absorbs from one side and cups. Cupping opens the lap, which lets more water behind. None of this is in the code.
What goes behind wood siding?
A water-resistive barrier per R703.2, lapped not less than 2 in horizontally and 6 in at vertical joints. Allow roughly 15% over the wall area once those laps are counted.
When does wood siding need engineered fastening?
Same gate as every other cladding: whenever the design wind pressure exceeds 30 psf or Table R703.3.2's height limits are exceeded. Four of that table's twelve cells are Design Required with no prescriptive answer at all.

Check these numbers yourself

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 2026-09-09. Codes are amended locally; confirm against the edition your jurisdiction enforces.

Related calculators

Exposure and board dimensions are the manufacturer's. The IRC does not set them and they vary between products of the same nominal width. Where Table R703.3.2 gives Design Required, or the design wind pressure exceeds 30 psf, no calculator can give you a fastening schedule. Cutting fiber cement produces respirable silica, which is an OSHA matter and outside both this page and IRC Chapter 7.