Attic Truss Calculator
An attic truss (also called a room-in-attic truss) is engineered with a raised bottom chord and open web space so the area inside the roof can be finished as a usable room instead of a crawl space. Enter your span, pitch, and heel height below to get rafter length, roof height, and — unique to this calculator — your attic room's usable height and width.
Size Your Attic Truss
Enter your building specifications below. Results and the roof diagram update live as you type.
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Estimate Your Project Budget
Automatically calculated from your inputs above in the calculator.
Estimates only. Actual costs vary by region, supplier, and site conditions.
How an Attic (Room-in-Roof) Truss Works
Unlike a standard truss, which fills its interior with diagonal web members, an attic truss leaves a rectangular open space in the middle — bounded by a raised bottom chord (the future room's floor) and vertical webs (the future room's walls) — so a finished bedroom, office, or storage room can be built inside the roof cavity.
Attic truss capacity depends heavily on pitch: a steeper roof pitch (commonly 8/12 and up) gives more headroom and a wider usable floor for the same span, since the raised bottom chord sits higher and the roof slope clears it sooner. Low-pitch roofs may not leave enough clear height for a code-compliant room.
Heel height — the vertical riser at the wall before the top chord begins its slope — raises the entire truss and directly increases attic room height without changing pitch. The calculator above uses your heel height and a minimum usable ceiling height (a common reference is around 5 ft, though this varies by local code) to estimate the room's peak height and usable floor width, updating live as you change span, pitch, or heel height.
Because the open center removes the web bracing that a normal truss relies on, attic trusses need larger, more heavily engineered top and bottom chords and are usually spaced and detailed individually by a truss engineer rather than approximated from a generic span table.
Frequently Asked Questions
What is an attic truss?
An attic truss (room-in-attic truss) is a roof truss engineered with an open rectangular space and a raised bottom chord, allowing a finished room to be built inside the roof cavity instead of leaving unusable crawl-space attic.
How much floor space does an attic truss give you?
Usable floor width depends on span, pitch, and heel height — steeper pitches, wider spans, and taller heel heights all increase the usable width, since the sloped top chord clears a given minimum ceiling height sooner. Enter your numbers above for an exact estimate.
What is heel height on an attic truss?
Heel height is the vertical riser at the wall plate before the top chord begins sloping upward. Increasing heel height raises the whole truss and adds usable headroom in the attic room without changing the roof pitch.
Are attic trusses more expensive than regular trusses?
Yes — attic trusses typically cost significantly more than standard trusses because of the larger engineered members needed to span the open center without interior web bracing.