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. This calculator sizes an attic truss from your span and pitch and returns rafter length, roof height, and material cost.
Size Your Attic Truss
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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.
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 and pitch — steeper pitches and wider spans give a wider clear floor area, since the sloped top chord clears the raised bottom chord's edges sooner on a steep roof.
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.