Attic Space Calculator
Cross-section, attic volume and standing-height floor width for a gambrel roof — with a live gable comparison at the same span and height, so you can see exactly what the extra pitch break buys you.
Cross-section, gambrel vs. gable
Gambrel (filled) Gable (same W & H)
| At these inputs | Gambrel | Gable (same W & H) |
|---|---|---|
| Roof height | 13.2 ft (13′ 2 5/16″) | |
| Attic cross-section | 203.8 ft² | 158.3 ft² |
| Floor width at threshold | 17.1 ft | 13.1 ft |
| Attic volume (24 ft building) | 4,892 ft³ | 3,799 ft³ |
At 24 ft span and 13.2 ft height, this gambrel gives 45.6 ft² more cross-section and 4.0 ft more standing-height floor width than a gable of the same footprint.
Why floor width — not raw volume — is what matters
Every gambrel attic quote eventually leads with a big volume number, because it is the easiest one to compute and the most flattering to show. It is also the least useful figure if what you actually want is a finished room. A 24 ft span, 12 ft tall gambrel encloses roughly 197 ft² of cross-section and, over a 32 ft building, close to 6,300 ft³ of attic volume — but a large share of that volume sits in the two triangular wedges tight against the eaves, where the ceiling is a foot or two off the floor. You cannot put a bed there, and you cannot stand up in it either.
The number that actually predicts whether a loft conversion works is floor width at a standing-height threshold — how wide a strip of continuous floor has a ceiling at or above the height a person can stand, walk, or place furniture under, usually 6 to 7 ft. The calculator above solves for exactly that: it walks the roofline out from the ridge, finds where it crosses your chosen threshold on whichever rafter the crossing happens to land on (see the formulas below), and doubles the distance to get a single width figure.
That is also why the gable comparison column matters more than it looks like it should. Hold a gable to the exact same span and ridge height as a gambrel — not a shallower, cheaper gable, the same height — and its single unbroken slope pinches the ceiling down far faster than the gambrel's steep lower section does. At 24 ft and 12 ft, that difference is 17.1 ft of usable width against 12.0 ft: 5.1 more feet of floor, for a roof that costs more to frame but not more to heat or roof over in proportion to the gain. If a dormer is on the table as an alternative or an addition, work the dormer calculator next — it estimates the same width-and-rise trade-off for a single dormer cut into either slope.
How the cross-section and floor-width numbers are derived
Both figures fall out of the same half-span geometry the rest of the site uses: x1 and
y1 for the lower rafter's run and rise, x2 and height for the upper
rafter and the total roof height. Cross-section is two trapezoids, doubled for symmetry:
Attic cross-section
crossSection = x1·y1 + x2·(y1 + height)
both terms already account for both sides of the roof — this is the total void above the wall plate line, in ft² (or m²).
Floor width at a standing-height threshold h is a piecewise function of which rafter the
threshold line crosses — the lower, steeper one if h is still below the knuckle height
y1, or the upper, shallower one if it has already cleared the knuckle:
Floor width at height h
if h ≤ y1: half = halfSpan − h / tan(θ)
if h > y1: half = x2 − (h − y1) / tan(φ)
floorWidth = 2 · max(0, half)
θ is the lower pitch, φ the upper pitch — the site's gambrel.ts engine runs this exact piecewise check on every input change.
The gable comparison uses the same two ideas with one triangle instead of two trapezoids. Held to the same
width W and height H, a gable's single pitch angle is fixed by that width and
height — it is not a free input, because a triangle with a given base and apex height has no other degree
of freedom:
Gable, same W and H
angle = atan( H / (W/2) )
crossSection = 0.5 · W · H
floorWidth = 2 · max(0, W/2 − h / tan(angle))
at W = 24 ft, H = 12 ft this angle works out to exactly 45°, which is what makes the 144 ft² and 12.0 ft figures below come out so evenly.
Plug W = 24, H = 12, h = 6 into both sets of formulas and you get 197 ft² / 17.1 ft for the gambrel against 144 ft² / 12.0 ft for the gable — the same figures already published on the homepage's comparison table, and exactly what the calculator above reproduces when you set it to those inputs.
Cross-section and floor width by span
Every row below uses the half-circle method at a 60° lower pitch — the same construction the homepage's own span reference table uses — so the roof height always falls out to exactly half the span, and the gambrel and gable columns stay directly comparable at every size. Full worked spans, framing notes and standard proportion sets live on the dimensions reference; traditional barn-specific spacing and loft layouts are on the barn roof page.
| Span | Height | Gambrel cross-section | Gable cross-section | Gambrel floor width | Gable floor width |
|---|---|---|---|---|---|
| 16 ft | 8.0 ft | 87 ft² | 64 ft² | 9.1 ft | 4.0 ft |
| 20 ft | 10.0 ft | 137 ft² | 100 ft² | 13.1 ft | 8.0 ft |
| 24 ft | 12.0 ft | 197 ft² | 144 ft² | 17.1 ft | 12.0 ft |
| 30 ft | 15.0 ft | 307 ft² | 225 ft² | 23.1 ft | 18.0 ft |
| 40 ft | 20.0 ft | 546 ft² | 400 ft² | 33.1 ft | 28.0 ft |
The gap widens in absolute terms as the span grows, because both the extra cross-section and the extra floor width scale with the size of the roof — but as a percentage against the gable, it stays close to the 24 ft span's 37% cross-section gain and 42% floor-width gain across the whole range shown here. The full cost and complexity trade-off behind that gap is worked through on the gambrel vs gable comparison page.
Design factors
What actually changes your usable floor width
Four inputs move the floor-width number, and they do not all move it the same way.
Steeper walls out further
A steeper lower rafter keeps the wall closer to vertical for longer, which is most of why a gambrel beats a gable in the first place.
60° vs 45° adds real feetDiminishing returns past ~60%
Moving the knuckle out lengthens the steep run, but push it too far and the upper section shortens until the roof behaves like a gable again.
sweet spot ~45-60%6 ft vs 7 ft is not a rounding error
Raising the threshold from 6 to 7 ft moves the crossing point further up the roofline and can shave a foot or more off the usable width on each side.
check both, code and comfortScales the whole roof
Both cross-section and floor width grow with span, but cross-section grows faster (it is roughly proportional to W²) than floor width (roughly proportional to W).
bigger spans favour volumeThe knuckle and pitch interactions are exactly why this is a calculator and not a lookup table: two roofs with the same span and the same total height can have meaningfully different floor width depending on where the knuckle sits.
Worked example: a 24 ft, 32 ft-long loft conversion
Here is the full path from inputs to a usable-floor-width answer, using the same 24 ft span shown on the homepage's own comparison table.
-
Set the roof
24 ft span, 60° lower pitch, 25° upper pitch, knuckle at 50%, 32 ft building length. Height falls out to 13.2 ft.
-
Read the cross-section
Cross-section comes out to 203.8 ft², for a total attic volume of 6,523 ft³ over the 32 ft length.
-
Apply the 6 ft threshold
At 6 ft, the roofline crosses the threshold on the lower rafter (well below the 10.4 ft knuckle height), giving 17.1 ft of continuous floor width.
-
Compare to a gable
A gable at the same 24 ft span and 13.2 ft height manages only 158.3 ft² of cross-section and 13.1 ft of floor width at the same threshold — 4.0 ft less than the gambrel.
-
Decide if a dormer is worth adding
17.1 ft of floor width down the centre of a 32 ft-long attic is a workable single room. If the eaves-side triangles are still bothering you, the dormer calculator estimates exactly how much more a shed dormer would recover on each slope before you commit to framing one.
Related tools
Each of these takes one piece of this calculation further.
Attic Space Questions
Is attic cross-section the same thing as usable floor space?
No — cross-section counts every square foot above the wall plate, including the wedge-shaped space too low to stand in. A 24 ft span, 12 ft tall gambrel encloses 197 ft² of cross-section, but only the part at or above your chosen standing-height threshold (17.1 ft of floor width at 6 ft, in that example) is space you can actually furnish and walk through. Cross-section and volume are still useful for insulation and HVAC sizing; floor width is what tells you whether a loft conversion works.
How much more usable width does a gambrel give over a gable?
At a matched span and ridge height, a gambrel typically gives 35-45% more standing-height floor width than a gable. At 24 ft span and 12 ft height it is 17.1 ft against 12.0 ft — a 42% gain — because the steep lower slope keeps the walls close to vertical instead of pinching the ceiling down immediately, the way a gable's single slope does. See the full breakdown across five spans on the gambrel vs gable comparison.
Does raising the knuckle position increase usable floor width?
Usually yes, up to a point. Moving the knuckle further out (a higher knuckle percentage) lengthens the steep lower rafter run, which pushes the point where the roofline crosses your standing-height threshold further toward the eave. Past roughly 60-65%, the upper rafter run gets short enough that the roof starts looking gable-like again and the gain flattens out. The calculator above updates live as you drag the knuckle slider, so it is faster to try it than to estimate it.
What standing-height threshold should I use for a code-compliant loft?
Most U.S. residential codes require at least 7 ft of ceiling height over at least 50% of a habitable room's floor area, measured from finished floor to finished ceiling, with a 5 ft minimum wherever a sloped ceiling is counted at all. The 6 ft default here is a conservative planning threshold, not a code citation — raise the threshold input to 7 ft to see how much floor width survives at the standard most building departments actually check, and confirm the exact figure with your local code before framing.
Does a dormer add to this cross-section number?
Yes, and it is usually the cheapest way to add standing-height floor width without changing the main roof's span or pitch. A dormer boxes out a section of the sloped ceiling with a flat or near-flat one, which is a different calculation than the whole-roof cross-section above. Work through the rise and width trade-offs on the dormer calculator.