Attic Space Calculator

Roof & Building

Live Cross-Section

Details

How this works

The tool models your roof profile above the attic floor: ceiling height at any point equals your floor offset plus the rise of the roof from the wall inward. The "usable room" is the strip of floor where the ceiling clears your chosen minimum height. For attic trusses, the room is additionally capped at about half the span (typical engineered room width) — the webs own the rest. The gambrel model uses the classic symmetric profile: lower and upper slopes each cover a quarter of the span per side.

What drives the usable space

Pitch does almost all the work. Usable attic room grows far faster than the pitch number suggests, because the ceiling has to clear your minimum height across a meaningful width of floor before any of it counts. Below roughly a 7/12, a conventional gable produces a triangle with almost no standing room no matter how wide the building is. Push into 10/12 or 12/12 and the same footprint suddenly yields a genuine room. This is why steep roofs persist in snow country for reasons beyond shedding snow — they're the cheapest square footage in the building.

Span sets the ceiling on what pitch can deliver. A steep roof over a narrow building still gives you a narrow room. Widen the span and the peak rises, which is what turns headroom into floor area. The interaction between the two is exactly what the cross-section above is for — it's much easier to see than to calculate.

Truss type is the decision that actually costs money. Standard trusses are the cheapest way to roof a building and they fill the attic with webs, leaving you storage at best. Attic trusses are engineered with a clear room inside, typically about half the span wide, and cost substantially more per truss. Stick framing gives full freedom but takes far longer on site. The important part: this is decided when you order the roof, not later. Converting a standard-truss attic into a room afterward means replacing the roof structure, which almost never makes economic sense.

The floor offset is the most overlooked lever. Where the attic floor sits relative to the top plate shifts every ceiling height in the model. Designs that drop the attic floor below the top plate — or raise the walls a few courses — buy headroom far more cheaply than increasing the pitch, because they don't change the roof geometry or the truss price at all.

Roof style changes the shape of what you get. A gambrel is the classic answer to this problem: by breaking the slope into a steep lower section and a shallow upper one, it produces near-vertical walls and a much larger usable rectangle than a gable of the same height. A hip roof does the opposite — slopes on all four sides cut into the space from the ends as well as the sides, which is why hips are rare where attic space is the goal.

What this tool does not tell you

This is a geometry model. It answers "what shape is the space?" and nothing else. Everything below decides whether that space can legally or practically be a room:

• Whether the floor can carry it. Storage loads and habitable-room loads are different design cases. An attic framed for light storage is not automatically safe to occupy, and the difference is a structural question for your truss engineer.
• Code minimum ceiling heights. The minimum you enter here is your choice — your provincial building code sets the real one for habitable space, along with rules about how much of the floor area has to meet it. Sloped-ceiling rooms have specific provisions.
• Stairs. A compliant staircase needs run, headroom, and landing space, and it consumes floor area on both levels. This is the constraint that most often defeats an otherwise workable attic room, and it's invisible in a cross-section.
• Egress and windows. Bedrooms generally require an egress opening. Dormers or gable-end windows add cost and change the roof geometry this tool models.
• Insulation and ventilation. Insulating at the roofline instead of the ceiling changes the assembly entirely, and the insulation depth eats into the clear dimensions shown above.
• Heating, wiring, and finishing costs, none of which are in scope here.
• Whether it counts as floor area. Municipal zoning may include habitable attic space in gross floor area or lot coverage calculations, which can affect what else you're allowed to build.

Use this to decide what pitch and truss type to ask for, then confirm the room with your truss supplier and building department. The roof framing guide covers the trade-offs in full, and the Roof Cost Estimator prices what the pitch decision does to your roof budget.