Lap Siding Coverage by Exposure and Length
How many lap boards cover 100 sq ft at each exposure, in 12 and 16 ft lengths, and why two published charts give different counts for the same wall.
Lap siding coverage charts list boards per area for each exposure, the part of the board left showing. Two widely used charts disagree on how to round. LP's SmartSide lap chart rounds every count up: all 90 of its 90 lap cells equal the area divided by one board's coverage, rounded up (LP chart). The 2016 fiber-cement lap chart in James Hardie's installation instructions rounds to the nearest plank: 160 of its 180 cells equal the nearest whole plank, only 126 the round-up (fiber-cement chart).
The two charts, cell by cell
On the fiber-cement chart, rounding to the nearest plank gives a lower count wherever the fraction is under one half. At a 7 in exposure the chart and the round-up agree in 11 of 20 rows. The column for the 9-1/2 in board at an 8-1/4 in exposure prints the same numbers as the 9-1/4 in board at 8 in, in all 20 of its 20 rows, although a wider exposure needs fewer planks (fiber-cement chart).
| Squares | 8 in: printed | 8 in: round up | 8-1/4 in: printed | 8-1/4 in: round up | 8-1/4 in: nearest |
|---|---|---|---|---|---|
| 1 | 13 | 13 | 13 | 13 | 12 |
| 2 | 25 | 25 | 25 | 25 | 24 |
| 3 | 38 | 38 | 38 | 37 | 36 |
| 4 | 50 | 50 | 50 | 49 | 48 |
| 5 | 63 | 63 | 63 | 61 | 61 |
| 6 | 75 | 75 | 75 | 73 | 73 |
| 7 | 88 | 88 | 88 | 85 | 85 |
| 8 | 100 | 100 | 100 | 97 | 97 |
| 9 | 113 | 113 | 113 | 110 | 109 |
| 10 | 125 | 125 | 125 | 122 | 121 |
| 11 | 138 | 138 | 138 | 134 | 133 |
| 12 | 150 | 150 | 150 | 146 | 145 |
| 13 | 163 | 163 | 163 | 158 | 158 |
| 14 | 175 | 175 | 175 | 170 | 170 |
| 15 | 188 | 188 | 188 | 182 | 182 |
| 16 | 200 | 200 | 200 | 194 | 194 |
| 17 | 213 | 213 | 213 | 207 | 206 |
| 18 | 225 | 225 | 225 | 219 | 218 |
| 19 | 238 | 238 | 238 | 231 | 230 |
| 20 | 250 | 250 | 250 | 243 | 242 |
The printed columns are quoted from the chart (fiber-cement chart). The chart itself warns This coverage chart is meant as a guide.
Actual usage is subject to variables such as building design.
The calculator rounds up, as LP does; How
we calculate explains why.
Boards per square, one table
Each chart covers part of the range: LP one 12 ft reveal and four 16 ft reveals, the fiber-cement chart nine exposures in 12 ft planks only (LP chart, fiber-cement chart). The table puts them in one grid, computed the same way for every cell, rounded up, with no waste. A mark shows which chart has a cell there; where the fiber-cement chart rounds to the nearest plank, its printed number can be one lower than the cell here.
| Exposure | One 12 ft board covers | 12 ft boards | 16 ft boards |
|---|---|---|---|
| 4 in | 4 sq ft | 25 F | 19 |
| 4 7/8 in | 4.875 sq ft | 21 | 16 L |
| 5 in | 5 sq ft | 20 F | 15 |
| 5 7/8 in | 5.875 sq ft | 18 | 13 L |
| 6 in | 6 sq ft | 17 F | 13 |
| 6 1/4 in | 6.25 sq ft | 16 F | 12 |
| 6 3/4 in | 6.75 sq ft | 15 F | 12 |
| 6 7/8 in | 6.875 sq ft | 15 L | 11 L |
| 7 in | 7 sq ft | 15 F | 11 |
| 8 in | 8 sq ft | 13 F | 10 |
| 8 1/4 in | 8.25 sq ft | 13 F | 10 |
| 10 3/4 in | 10.75 sq ft | 10 F | 7 |
| 10 7/8 in | 10.875 sq ft | 10 | 7 L |
The formula, worked
Boards = area ÷ (exposure ÷ 12 × board length), rounded up. For 1,000 sq ft of a 7 in exposure in 12 ft boards:
- One board covers 7 ÷ 12 × 12 = 7 sq ft
- 1,000 ÷ 7 = 142.86 → 143 boards
- Linear feet: 1,000 × 12 ÷ 7 = 1,714.3 ft
Add the waste to the area before dividing: LP's chart says Disclaimer: The above Tables do not account for waste factors. Remember to
include your expected jobsite waste.
(LP chart). Multiplying squares by a per-100 count from a table rounds every square up,
so it can come out up to almost one board per square higher: 10 squares × 15 =
150 boards, against 143 from the single division. The calculator divides once. Open in calculator with
your own exposure and board length.