How to Measure a House for Siding
Walls, then gables, then a decision about windows and doors — the published rules disagree — then waste, and you have squares.
To measure a house for siding, take each wall as a rectangle and each gable as a triangle, add them up, decide what to do with the windows and doors, add waste and divide by 100. The tape work is the same for every siding; the one real choice is the openings, and the published guides give 3 different answers (sources below).
1. Walls
Measure each wall's width along the base and its height from the bottom of the siding to the eave, not counting the gable. CertainTeed:
Measure height (excluding gables). Measure width
(CertainTeed). On a plain box house two walls share the length and two the
width, so four numbers become two: a 40 by 30 ft house with 8 ft walls has 1,120.0 sq ft of
wall.
2. Gables
Measure the base of each gable and either its height at the peak or the roof pitch. Area is half the base times the height (LP:
For triangle areas, measure from the base of the triangle to the top and multiply that number by half of the base length.
,
LP). Two 30 ft gables at 6 in 12 add 225.0 sq ft; Gable
siding works the pitch through.
3. Windows and doors
Count the windows and doors and measure one of each size. Then pick a rule; the published ones disagree:
- Deduct them all:
Subtract square footage of openings (windows, doors) to determine wall area for siding.
(Western Red Cedar Lumber Association);Remember to deduct window and door areas from coverage calculations.
(Cedar Shake & Shingle Bureau);Subtract the uncovered areas’ square footage from the total amount you calculated in step four.
(LP). - Deduct nothing:
Windows and doors are not usually deducted. Including them will provide an allowance factor for waste.
(VSI installation manual); CertainTeed measures width(including doors and windows).
(CertainTeed). - Deduct half of the large ones:
Large areas not to be covered:
(garage doors/sliding doors)
x 0.50 =
(VSI estimating worksheet).
One house under all three rules
A two-story house 30 by 24 ft with 18 ft walls, two gables at 4 in 12 and an attached 12 by 9 ft wall: 2,148.0 sq ft of walls and gables. It has 12 windows of 3 × 5 ft, one 3 × 7 ft door and a 16 × 7 ft garage door, 313.0 sq ft of openings in all. With 10 % waste:
| Rule | Published by | Net sq ft | To order | Squares |
|---|---|---|---|---|
| Deduct every window and door | WRCLA, Cedar Shake & Shingle Bureau, LP | 1,835.0 | 2,018.5 | 20.19 |
| Deduct nothing | VSI, CertainTeed | 2,148.0 | 2,362.8 | 23.63 |
| Deduct half of the garage door only | VSI estimating worksheet | 2,092.0 | 2,301.2 | 23.01 |
Between deducting everything and deducting nothing the order differs by 3.44 squares on this house. The calculator deducts by default and has a switch to keep the openings in; the half rule you can apply by entering only half of the garage door's height as an opening. Open this house in the calculator.
4. Waste and squares
Add a waste percentage to the net area, then divide by 100. VSI's worksheet: * Add a factor of 10 percent to all material
estimates to allow for waste.
(VSI worksheet); LP: Add 5-10% extra siding to account for mistakes.
(LP).
The whole measurement, worked
The one-story house from steps 1 and 2, with 8 windows of 3 × 5 ft and 2 doors of 3 × 7 ft deducted:
- Walls: 2 × 40 × 8 + 2 × 30 × 8 = 1,120.0 sq ft
- Gables: 2 × ½ × 30 × 7.5 = 225.0 sq ft
- Openings: 8 × 3 × 5 + 2 × 3 × 7 = 162.0 sq ft
- Net: 1,120.0 + 225.0 − 162.0 = 1,183.0 sq ft
- With 10 % waste: 1,301.3 sq ft = 13.01 squares
The second house comes to 20.19 squares with its openings deducted. Squares are the size of the job; the pieces, cartons and trim come next, and Open in calculator does them from the same numbers.