Attic Truss Calculator
An attic truss (also called an attic storage truss or a room-in-attic truss) is engineered to create a usable room inside the roof structure, not just dead storage above a flat ceiling. This attic truss calculator takes your building span, roof pitch, and knee wall height and returns the clear room width, peak headroom, rafter length, and total lumber needed. Use it to sanity check a design before ordering from a truss plant.
How an Attic Truss Is Different
A conventional roof truss has a triangular interior filled with webs and is not walkable. An attic truss replaces that web pattern with a rectangular opening: vertical knee walls on each side, a flat ceiling across the top, and sloped cathedral sections above the knee walls. The result is a room inside the roof.
Three inputs drive the geometry:
- Span (building width): sets the maximum possible room width
- Roof pitch (rise:run): steeper pitch = more headroom and narrower room
- Knee wall height (interior vertical wall): sets how tall the side walls are inside the attic
Attic Room Width Formula
The usable attic floor width between the knee walls is:
Attic Room Width = Span - 2 x (Knee Wall Height / Pitch Ratio)
Where Pitch Ratio = pitch / 12 (so an 8:12 pitch is 0.667, a 12:12 pitch is 1.0).
Example: a 28 ft span with 10:12 pitch and 5 ft knee walls:
- Inset from each exterior wall = 5 / (10/12) = 6 ft
- Attic room width = 28 - 2(6) = 16 ft
Headroom at the Peak
Peak Height = (Span / 2) x (Pitch / 12) Headroom = Peak Height - Knee Wall Height
For the same 28 ft span, 10:12 pitch, 5 ft knee wall:
- Peak height = 14 x (10/12) = 11.67 ft
- Headroom = 11.67 - 5 = 6.67 ft at the peak
US building codes (IRC) require at least 7 ft ceiling height over at least 50% of the usable floor area for a habitable room. Conditioned space also requires at least 7 ft clear at 5 ft off the floor line, which means the knee wall + pitch combination must give you that clearance partway in from each knee wall, not just at the peak.
Common Design Rules of Thumb
- 24 ft span, 8:12 pitch, 4 ft knee walls: marginal, tight headroom, storage only
- 28 ft span, 10:12 pitch, 5 ft knee walls: typical for a second-story living space
- 32 ft span, 12:12 pitch, 5 ft knee walls: spacious, comfortable, higher material cost
- Below 8:12 pitch: not enough headroom unless span is very wide
Lumber and Truss Count
For a 40 ft long building at 24 in on-center spacing, you need 21 trusses (40 / 2 + 1). Each attic truss contains about 85-120 linear feet of lumber depending on span and pitch. Factory-built attic trusses are far stronger and cheaper than site-built framing for this geometry; never frame a usable attic with stick rafters without an engineer's design.
When to Use This Attic Truss Calculator
- Planning a 1.5-story or "Cape Cod" style roof
- Adding conditioned living space inside a garage roof
- Estimating lumber before getting quotes from a truss plant
- Deciding whether your current span and pitch support a room-in-attic design
Always confirm final dimensions with a licensed structural engineer. Attic trusses carry floor loads in addition to roof loads and require specific lumber grades and steel connector plates.