Attic Truss Calculator
Calculate the usable floor width inside an attic truss from its height, roof pitch, and your minimum headroom requirement.
How this calculator works
An attic truss’s peak gives you height at the center of the room, but the roof slopes down on both sides. This calculator finds how far out from center, on each side, the sloped roof still clears the minimum headroom you’re willing to accept — using your truss height, your headroom target, and the roof pitch to work out how quickly that slope eats into the space.
Formula: Usable width = 2 × (truss height − minimum headroom) ÷ (pitch ÷ 12).
Worked example
An 8 ft truss height at the center, 6-in-12 roof pitch, wanting 6.5 ft of headroom:
- Height above headroom: 8 − 6.5 = 1.5 ft
- Pitch as a ratio: 6 ÷ 12 = 0.5
- Distance to each side: 1.5 ÷ 0.5 = 3 ft
- Total usable width: 6.0 ft
Notes
The formula handles the trade-off between pitch and usable width for you — just enter your truss height, your pitch, and your headroom target, and it returns the total width.
Attic trusses (also called room-in-attic trusses) trade some roof framing simplicity for genuinely usable floor space, unlike common trusses which leave the attic as storage-only crawl space behind internal webbing.
How to use
Enter your truss height at the peak, your roof pitch (rise per 12 in of run), and the minimum headroom you want in the finished room. The calculator returns the total usable floor width across the room, split evenly to each side of center.
A planning estimate for usable floor width. Attic truss engineering (structural capacity, ceiling height at the ridge) should be confirmed with your truss manufacturer.
Frequently asked questions
What counts as 'usable' floor width in an attic truss?
It's the horizontal distance, measured out from the room's centerline on each side, where the sloped roof above still clears your chosen minimum headroom. Beyond that point the ceiling drops below your headroom target, so that floor area is typically left for storage rather than living space.
Why does the calculator ask for a minimum headroom, not just truss height?
Truss height alone tells you the tallest point of usable space, at the center of the room. Minimum headroom is the shortest ceiling height you're still willing to stand or walk under, and it's the number that actually limits how far the usable floor extends before the slope cuts it off.
How is an attic truss different from a common roof truss?
A common truss is webbed with internal framing members that fill the middle of the triangle, leaving the attic as storage-only crawl space. An attic truss (also called a room-in-attic truss) is designed with an open center, trading some framing simplicity for genuinely usable floor space upstairs.
Should I trust this number for a finished room's building permit?
Treat it as a planning estimate only. Local building codes set their own minimum ceiling height rules for habitable rooms, and the truss manufacturer needs to confirm both the structural capacity and the actual ridge height before this becomes a permit-ready number.
Estimates only. Verify quantities with your supplier before purchasing.