Attic Truss Calculator
Reviewed by the My Roof Pitch editorial team · Updated · Free, no sign-up, works offline after first load
An attic truss room is as wide as the space where the roof is higher than your required headroom. Room width = span − 2 × (headroom − knee wall) ÷ (pitch ÷ 12), within the walls. A 28 foot span at 12:12 with 7.5 ft headroom and 4 ft knee walls gives a 21 foot room.
Key takeaways
- Usable width depends on pitch far more than on span.
- Most attic truss rooms need 10:12 to 12:12 to be worthwhile.
- IRC R305 requires 7 ft ceiling height over at least 50 percent of a habitable room.
- Attic trusses carry a floor load, so they must be engineered for it.
Your measurements
Enter the values you know — results update live.
Outside wall to outside wall.
Rise per 12 in of run. Attic rooms usually need 10:12 or steeper.
IRC habitable minimum is 7 ft over at least half the room.
Height of the short side walls of the room.
Analysis
Live results from your measurements.
Pitch and span give a usable attic room.
Show the working
- Peak height:
14 × 12/12= 14 ft - Knee wall offset:
4 ÷ 1= 4 ft - Floor width:
28 − 2 × 4= 20 ft - Full-headroom width:
28 − 2 × 7.5= 13 ft
What this calculator does
An attic truss, also called a room-in-attic or bonus room truss, is an engineered truss with an open rectangle in its centre that forms a living space. Its usable size is set by geometry: the room floor runs between the knee walls, and full standing headroom only exists where the rafters are higher than the target ceiling. This calculator turns span, pitch, knee wall height and target headroom into the floor width, full-headroom width, peak height and floor area per foot of building length, so you can see whether a bonus room is realistic before ordering trusses.
How to use it
- Enter the truss span from outside wall to outside wall.
- Enter the roof pitch — try 10:12, 12:12 and 14:12 to compare.
- Set the headroom you want (7 to 8 ft) and the knee wall height (often 4 to 5 ft).
- Read the floor width and full-headroom width.
- Multiply the area per foot by the room length for the floor area.
The formula
Peak height
peak = (span ÷ 2) × pitch ÷ 12
Rise at the centre, ignoring truss depth.
Width at a height h
width = span − 2 × h ÷ (pitch ÷ 12)
The rafter line climbs pitch/12 feet per foot of run from each wall.
Worked example
A 28 ft garage with a 12:12 roof, 4 ft knee walls and 7.5 ft target headroom.
- Peak = 14 × 1 = 14 ft
- Floor width = 28 − 2 × 4 = 20 ft
- Full headroom width = 28 − 2 × 7.5 = 13 ft
A 20 ft wide floor between knee walls with 13 ft of full standing height in the middle.
Floor width between 4 ft knee walls by pitch (28 ft span)
| Pitch | Peak | Floor width | 7.5 ft headroom width |
|---|---|---|---|
| 8:12 | 9.33 ft | 16.0 ft | 5.5 ft |
| 10:12 | 11.67 ft | 18.4 ft | 10.0 ft |
| 12:12 | 14.0 ft | 20.0 ft | 13.0 ft |
| 14:12 | 16.33 ft | 21.1 ft | 15.1 ft |
Common mistakes
- Ignoring the depth of the truss chords and insulation, which lowers the finished ceiling 10–14 in.
- Forgetting stairs — a stairwell eats roughly 3 × 10 ft of the room.
- Ordering standard trusses and trying to cut webs later.
- Assuming a 6:12 roof will give a room; it usually will not.
Assumptions and limits
- Symmetrical gable with equal pitch both sides.
- Heights measured from the top of the attic floor to the underside of the top chord.
- Geometry only; truss design and floor loads must be engineered.
Frequently asked questions
What pitch do I need for an attic truss?
10:12 is the practical minimum for most spans; 12:12 gives a comfortable room on 24–32 ft spans.
How wide is the room in a 24 ft attic truss?
At 12:12 with 4 ft knee walls, 24 − 8 = 16 ft of floor, with 9 ft of full 7.5 ft headroom in the centre.
What is the minimum ceiling height for an attic room?
IRC R305.1 requires 7 ft over at least 50 percent of the floor area; areas under 5 ft do not count toward the room area.
Do attic trusses cost more?
Yes, typically 2 to 3 times a common truss, because the bottom chord carries a floor load and the members are larger.
Can attic trusses be 24 inches apart?
Often yes, but floor stiffness may push the design to 16 in or require thicker subfloor.
How do I add a dormer to an attic truss room?
Dormers need headers and doubled trusses designed by the truss engineer; plan them before ordering.
Is knee wall height included in headroom?
Knee walls are the short side walls; headroom is measured in the middle. Both determine usable area.
What span can an attic truss do?
Common room-in-attic trusses run about 20 to 40 ft; longer spans are possible with engineering.
Last updated 2026-10-08. Results are estimates for planning and are not a substitute for a licensed engineer or local code review.