Attic Truss Calculator

An attic truss calculator must answer a floor question, not just a roof question: how wide and how tall is the habitable room between the knee points? This page's tool converts span, pitch, and floor-chord height into usable room width, center headroom, and floor area per truss bay. A normal calculator stops at ridge height and rake length — numbers that are identical for an attic truss and a plain truss of the same outline.

Each row is a truss span; each column is a roof pitch; each cell gives usable room width and center headroom at a stated floor-chord height (4 ft on this page). For example, at 32 ft span and 12/12 pitch, the chart reads 24 ft wide × 9.3 ft high. To adapt to your floor height, subtract your floor height above 4 ft (or add the difference below) from the headroom figure, and shrink room width by 2 × that difference × (12 ÷ pitch) — the same relationship the calculator above automates.

Common stock spans run 24, 28, 30, 32, 36 and 40 ft, with chords stepping up from 2x6 on short spans to 2x8–2x10 bottom chords on 30 ft-plus where the floor must carry full live load. Room width is the practical sizing driver: expect roughly half the span back as usable width at 12/12 pitch (32 ft span → 22–24 ft room) and only a third at 8/12 (24 ft span → 12 ft room). Manufacturers also mix spans — an attic bay over the garage with standard fink trusses on the rest of the roof is a normal order.

Because the room lives inside the triangle. Usable width equals span minus twice the knee height divided by the pitch slope, so width falls linearly as pitch falls, while headroom at the center equals ridge height minus floor height and falls with it. A 32 ft truss at 12/12 gives a 24 ft-wide room; drop the same truss to 8/12 and the room narrows to 20 ft while center height collapses from 9.3 to 5.3 ft. The roof-area penalty of the steeper pitch is usually the cheaper side of that trade.

A bonus room truss is an attic truss sized for occasional occupied use over a garage or bonus area — it still carries 30–40 psf of floor live load on the floor chord, but designers often accept a compact 10–12 ft room width and modest headroom since the space supplements rather than defines the floor plan. The load modeling is identical; what changes is how small a room you'll tolerate. Any bonus room truss calculator worth using therefore reports the same two numbers this page's tool does: usable width and center headroom.

Yes — the 'Open the preloaded 32 ft attic model' button on this page carries an encoded link that rebuilds a complete attic truss (9 joints, 15 members, 18 kip of combined roof and floor loading) inside the Structural Analysis tab, including the floor chord, studs, and knee diagonals. You can then edit loads, switch to metric, or add section properties to explore displacements.