Gambrel Roof Dimensions Reference
Fourteen spans, 10 ft to 60 ft, each solved at the traditional 60° lower / 15° upper pitch pairing — rafter lengths, ridge height, roof area, attic volume and 6 ft headroom width, precomputed so you can read off a close match without running the calculator yourself.
Dimensions by Building Width, 10–60 ft
Solved with the half-circle method at a 60° lower pitch, which fixes the upper pitch at 15° and the total height at exactly half the span. Roof area and attic volume assume a 20 ft building length with no overhang, counting both slopes. Widths with their own dedicated page link through to the full truss layout and cutting list for that span.
| Width | Lower rafter R1 | Upper rafter R2 | Total height H | Roof area, 20 ft length | Attic volume | Headroom at 6 ft |
|---|---|---|---|---|---|---|
| 10 ft | 5.00 ft | 2.59 ft | 5.00 ft | 303.5 ft² | 683 ft³ | 0.0 ft |
| 12 ft | 6.00 ft | 3.11 ft | 6.00 ft | 364.2 ft² | 984 ft³ | 0.0 ft |
| 15 ft | 7.50 ft | 3.88 ft | 7.50 ft | 455.3 ft² | 1,537 ft³ | 8.1 ft |
| 16 ft | 8.00 ft | 4.14 ft | 8.00 ft | 485.6 ft² | 1,749 ft³ | 9.1 ft |
| 20 ft | 10.00 ft | 5.18 ft | 10.00 ft | 607.1 ft² | 2,732 ft³ | 13.1 ft |
| 24 ft | 12.00 ft | 6.21 ft | 12.00 ft | 728.5 ft² | 3,934 ft³ | 17.1 ft |
| 25 ft | 12.50 ft | 6.47 ft | 12.50 ft | 758.8 ft² | 4,269 ft³ | 18.1 ft |
| 30 ft | 15.00 ft | 7.76 ft | 15.00 ft | 910.6 ft² | 6,147 ft³ | 23.1 ft |
| 35 ft | 17.50 ft | 9.06 ft | 17.50 ft | 1,062.3 ft² | 8,367 ft³ | 28.1 ft |
| 40 ft | 20.00 ft | 10.35 ft | 20.00 ft | 1,214.1 ft² | 10,928 ft³ | 33.1 ft |
| 45 ft | 22.50 ft | 11.65 ft | 22.50 ft | 1,365.9 ft² | 13,831 ft³ | 38.1 ft |
| 50 ft | 25.00 ft | 12.94 ft | 25.00 ft | 1,517.6 ft² | 17,075 ft³ | 43.1 ft |
| 55 ft | 27.50 ft | 14.24 ft | 27.50 ft | 1,669.4 ft² | 20,661 ft³ | 48.1 ft |
| 60 ft | 30.00 ft | 15.53 ft | 30.00 ft | 1,821.2 ft² | 24,588 ft³ | 53.1 ft |
How to Read This Table
Each row is one building width, solved independently — nothing carries over from the row above it. R1 is the lower, steeper rafter measured from the wall plate to the knuckle; R2 is the upper, shallower rafter from the knuckle to the ridge. H is the total height from the wall plate to the ridge, which under this method always works out to exactly half the span: a 12.00 ft height on a 24 ft span, a 30.00 ft height on a 60 ft span.
Roof area and attic volume both assume a 20 ft building length with no overhang, so they scale with width but not with length — double the building length and both figures double with it. Headroom width is the floor width available at 6 ft of standing height, and it is genuinely zero for the two narrowest rows: a 10 ft span only reaches 5.00 ft to the ridge, and a 12 ft span reaches 6.00 ft — both at or under 6 ft, so there is no point in the attic where a 6 ft-tall person can stand upright. Headroom only becomes usable from around 15 ft of span upward at this pitch.
Why 60°/15° Is the Reference Pairing
60° and 15° are not arbitrary — they are the half-circle method's closest common pair to the traditional 67.5°/22.5° barn split, and they are the pairing this entire site uses for its worked examples, so every page's numbers agree with each other.
- θ
- Lower pitch, fixed at 60° for this table.
- φ
- Upper pitch, always θ − 45° under the half-circle method — 15° here.
- H
- Total height, always W ÷ 2 under this method, regardless of θ.
φ = θ − 45° = 60° − 45° = 15°
H = W ÷ 2
Both hold for any knuckle position on the semicircle — the relationship comes from the construction, not from a chosen knuckle point.
Change the lower pitch and every figure in this table changes with it — a steeper lower pitch trades attic width for height, without changing the span. The angle calculator lets you try any lower pitch from 46° to 84°.
When Your Actual Dimensions Will Differ
This table gives one specific design — 60°/15°, knuckle position irrelevant because the half-circle method fixes it geometrically, 20 ft building length, no overhang. Real buildings deviate from it in a few predictable ways.
Pitch choice. A steeper lower pitch pushes more of the height into the lower section and shrinks the upper section; a shallower lower pitch does the opposite. Every dimension in this table — rafter lengths, height, area, volume, headroom — shifts with it. Use the angle calculator for other half-circle pitch pairs, or the home page calculator for the two-pitch method, where the upper pitch and knuckle position are chosen independently rather than derived.
Knuckle position. Under the half-circle method used here, the knuckle sits wherever the pitch puts it on the semicircle — there is no separate choice to make. Under the two-pitch method, the knuckle can be moved along the span independently of either pitch, which changes how much of the total height comes from the lower section versus the upper section, without changing the overall height formula used here.
Overhang. This table uses zero overhang. Every foot of horizontal overhang adds a sloped length of overhang ÷ cos(θ) to the roof area calculation on both sides of the building — small on its own, but not zero once shingles or panels are being ordered. The rafter length calculator and roof area calculator both take overhang as a direct input.
Gambrel Roof Dimensions Questions
What size rafters does a gambrel roof need for a given span?
At the traditional 60°/15° reference pitch, a 24 ft span needs a 12.00 ft lower rafter and a 6.21 ft upper rafter, and the table above lists the same figures for every width from 10 ft to 60 ft. These are rafter lengths along the top edge, before any birdsmouth or ridge cut is subtracted — the rafter length calculator applies those cuts for a specific lumber size.
Why does this table use 60° and 15° instead of my own pitch?
60°/15° is a reference pairing, not a requirement. It is the closest common rise-in-12 pair to the traditional 67.5°/22.5° barn split, easy to lay out, and it is the same pairing the home page calculator uses for its own worked examples — so the numbers here match everything else on this site. If your design uses a different lower pitch, or the two-pitch method instead of the half-circle method, use the home page calculator to resolve your own dimensions; the shape of the table is identical, only the numbers change.
How much headroom does a gambrel attic have at a given span?
Headroom at 6 ft standing height only exists once the ridge clears 6 ft. A 10 ft span tops out at 5.00 ft to the ridge under this reference pairing — below 6 ft entirely, so the headroom column reads 0 ft. A 24 ft span, by contrast, gives 17.1 ft of clear floor width at 6 ft standing height. The table above lists this figure for every width, and the attic space calculator works it out for any pitch and any standing height, not just 6 ft.
Does a bigger gambrel span need a steeper pitch?
No — pitch and span are independent choices. The table above holds pitch fixed at 60°/15° and only changes the span, which is why rafter lengths, height, area and volume all scale up smoothly with width rather than jumping around. A wider building can use the same 60°/15° pitch pair as a narrow one; it simply needs longer rafters and encloses more attic volume for the same standing headroom. Pitch is instead chosen for appearance, snow performance or headroom preference — covered on the home page and the angle calculator.