Attic Truss Calculator
An attic truss calculator finds usable room width as 2 × (center height − headroom) ÷ (pitch ÷ 12). An 8 ft peak at 6/12 pitch gives 6 ft at 6.5 ft headroom.
How wide is the room inside an attic truss?
An attic truss calculator answers one question: how much of the attic floor has enough headroom to stand in. It uses usable width = 2 × (truss height at center − minimum headroom) ÷ (pitch ÷ 12). With an 8 ft center height, a 6-in-12 roof, and 6.5 ft of headroom, the result is 6 ft of usable floor, 3 ft each side of the peak.
The rest of the floor under the slope still exists, but the ceiling there is too low to walk under. It is usually closed off with short knee walls and used for storage.
The formula in plain words
Picture standing at the center of the attic, under the peak. Walk toward the eaves and the ceiling drops by the pitch for every foot you move. A 6/12 roof drops 6 in per foot of travel, or 0.5 ft per ft.
- Spare height = center height − the headroom you want. This is how much the ceiling is allowed to drop.
- Distance to one side = spare height ÷ (pitch ÷ 12). This is how far you can walk before the ceiling hits your headroom limit.
- Usable width = 2 × that distance, because the roof slopes down on both sides.
If the center height is lower than the headroom, the result is zero.
Worked examples
- Calculator defaults: 8 ft center height, 6/12 pitch, 6.5 ft headroom
- Spare height: 8 − 6.5 = 1.5 ft
- Pitch as a slope: 6 ÷ 12 = 0.5
- Distance to each side: 1.5 ÷ 0.5 = 3 ft
- Usable width: 2 × 3 = 6.00 ft
- Bonus room over a garage: 10 ft center height, 10/12 pitch, 7 ft headroom
- Spare height: 10 − 7 = 3 ft
- Pitch as a slope: 10 ÷ 12 = 0.833
- Distance to each side: 3 ÷ 0.833 = 3.6 ft
- Usable width: 7.20 ft
Drop the headroom target to 6.5 ft in the second example and the width grows to 8.40 ft. Half a foot of headroom buys 1.2 ft of floor.
Usable width by center height and pitch (7 ft headroom)
| Center height | 6/12 | 8/12 | 10/12 | 12/12 |
|---|---|---|---|---|
| 8 ft | 4.0 ft | 3.0 ft | 2.4 ft | 2.0 ft |
| 9 ft | 8.0 ft | 6.0 ft | 4.8 ft | 4.0 ft |
| 10 ft | 12.0 ft | 9.0 ft | 7.2 ft | 6.0 ft |
| 11 ft | 16.0 ft | 12.0 ft | 9.6 ft | 8.0 ft |
| 12 ft | 20.0 ft | 15.0 ft | 12.0 ft | 10.0 ft |
Each row holds the center height fixed, so the shallow 6/12 roof wins because its ceiling falls slowly. On a real house the span is fixed instead, and a steeper roof raises the center height, which moves you down the table. Many US codes based on the IRC use 7 ft as the minimum ceiling height for habitable rooms, with rules for sloped ceilings, so check local code for your target.
How to measure for an attic truss
- Center height: measure from the top of the attic floor (the truss bottom chord plus subfloor) to the underside of the finished ceiling at the peak, not to the roof sheathing. Insulation and drywall take several inches.
- Pitch: read it off the plans, or measure rise over 12 in of run with a level. The roof pitch calculator converts angle or rise and run to pitch.
- Headroom: pick the lowest ceiling you would walk under every day. Many people choose 6.5 to 7 ft.
Common mistakes
- Measuring to the roof deck. Insulation, a vent channel, and drywall can take 10 in or more off the height you measured to the sheathing.
- Assuming steeper is always roomier. Steeper only helps when it raises the peak. Compare designs by real center height.
- Forgetting stairs. A stair opening needs full headroom over its whole run, and it often lands where the usable width is narrowest.
- Treating a common truss as convertible. Cutting webs out of a standard truss to make room is unsafe. The room has to be designed into the truss.
Next steps
Count trusses and spacing with the roof truss calculator, and budget them with the truss cost calculator. For stick-framed roofs, the rafter calculator sizes rafter length from pitch and run. Attic trusses carry floor loads and must be engineered by the truss manufacturer; have the final layout reviewed by a licensed structural engineer or your building department.
Frequently asked questions
What counts as 'usable' floor width in an attic truss?
It is the width of floor, centered under the peak, where the sloped ceiling is at least as high as your chosen headroom. With an 8 ft center height, a 6/12 pitch, and 6.5 ft headroom, the ceiling stays above 6.5 ft for 3 ft each side of center, so the usable width is 6 ft.
Why does the calculator ask for a minimum headroom, not just truss height?
Center height only tells you the tallest point. The headroom you pick decides where the slope cuts the room off. On a 10 ft center height at 12/12 pitch, asking for 7 ft of headroom gives 6 ft of usable width, while accepting 6.5 ft gives 7 ft.
Does a steeper roof pitch give me more attic room?
Only if it also raises the center height. At a fixed 10 ft center height and 7 ft headroom, a 6/12 pitch gives 12 ft of usable width but a 12/12 pitch gives 6 ft, because the steeper ceiling falls faster. On a real building, a steeper pitch over the same span raises the peak, which is why bonus rooms usually sit under steep roofs.
How is an attic truss different from a common roof truss?
A common truss fills the triangle with diagonal webs, so the attic is only crawl space. An attic truss (room-in-attic truss) leaves an open rectangle in the middle, framed by a floor chord, knee walls, and a ceiling collar, which needs heavier members designed by the truss maker.
Should I trust this number for a finished room's building permit?
No. It is a planning figure. Codes set minimum ceiling heights and how much of a sloped room counts as floor area, and the truss manufacturer must design the floor chord for living-space loads. Check local code and have the truss design and any structural changes reviewed by a licensed professional.
Estimates only. Verify quantities with your supplier before purchasing.