Gable Area for a 40 Foot Wide House
Gable area
533 sq ft
Two 40 ft gables at 8/12, on a house with a 150 ft perimeter and 8 ft walls.
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How that number is worked out
These are the figures this page was built with and every line the calculator produces from them. Change anything above and all of it moves.
- Wall perimeter150 ft
- Wall height to the eave8 ft
- Gable ends2 each
- Gable width40 ft
- Roof pitch, rise per 12 in8 /12
- Windows11 each
- Doors2 each
- Outside corners4 each
- Waste allowance12 %
Siding needed18.3 squares
| Wall area | 1,200 sq ft |
|---|---|
| Gable area | 533 sq ft2 gables at 8/12 |
| Openings deducted at half | 104 sq ftof 207 sq ft total |
| Net area | 1,630 sq ft |
| With 12% waste | 1,825 sq ft |
| Squares | 18.3 |
| Vinyl cartons at 2 squares | 10 |
| Starter strip | 150 ft |
| J channel | 212 ftaround windows and doors |
The material list this produces
| Siding, cartons at 2 squares | 10 |
|---|---|
| Starter strip, ft | 150 |
| J channel, ft | 212 |
| Outside corner posts, 10 ft | 4 |
| Undersill trim, ft | 150 |
| House wrap, sq ft | 1,734 |
| Siding nails, lb | 5 |
Roof pitch compared
Pitch only moves the gable triangles, but it moves them fast. Going from 4/12 to 12/12 triples the gable area on the same house, which is about three extra squares of siding on a 30 foot wide elevation and closer to six on a 40 foot one.
| Roof pitch | Siding needed | Gable area |
|---|---|---|
| 4/12 | 15.3 squares | 267 sq ft |
| 6/12 | 16.8 squares | 400 sq ft |
| 8/12 | 18.3 squares | 533 sq ft |
| 10/12 | 19.7 squares | 667 sq ft |
| 12/12 | 21.2 squares | 800 sq ft |
What else matters here
Five hundred and thirty three square feet for the pair, which is over five squares of siding sitting in two triangles that a tape measure never reaches.
The arithmetic is width squared times pitch over 48. Forty times forty is 1,600, times 8 is 12,800, divided by 48 is 267 per gable. The 48 is doing two jobs at once: dividing by 2 for the triangle and by 24 to turn the pitch into a height, so you never have to climb up and measure the peak.
Width is the number that dominates, because it is squared. A 40 foot gable is not a third bigger than a 30 foot gable, it is nearly twice as big. That is why gables are the line that blows a siding estimate on a wide house and barely matters on a narrow one.
What this calculator assumes
- A gable is a triangle, so its area is the width squared times the pitch divided by 48. Two gables on a 30 foot wide house at 6/12 add about 225 square feet, which is more than two squares of siding that people routinely forget.
- Openings are deducted at half, not in full. You still cut siding around a window and the offcut is usually scrap, so the trade rule is to take off half the opening and leave anything under 10 square feet in.
- 12 percent waste is normal for vinyl on a straightforward house. Lap siding, a cut up elevation or a staggered shake course runs higher.
- A carton of vinyl siding covers two squares, 200 square feet. Fiber cement and wood lap are sold by the piece, so use the square figure and convert at your supplier's coverage.
Questions people ask
How do you calculate gable area?
Width squared times the pitch divided by 48. A 30 foot wide gable at 6/12 is 30 times 30 times 6 divided by 48, which is 112.5 square feet each, 225 for a pair. That shortcut is the triangle area formula with the rise already substituted out, so you never have to measure the height of the peak.
How tall is the peak of a 40 foot gable at 8/12?
Thirteen feet four inches. Half the width is 20 feet, which is 20 runs of 12 inches, and each of those rises 8 inches, so the rise is 160 inches. You do not need that number for the area, but you do need it to know whether the top course is reachable.
How many squares are in a box of vinyl siding?
Two. A standard carton covers 200 square feet, which is why siding is quoted in squares rather than square feet. Fiber cement and wood lap are sold by the piece, so take the square figure here and convert at whatever coverage your supplier lists.
The siding calculator takes any dimensions you like and hands back the same list.