20 Foot Gambrel Roof Span
Twenty feet is two-car-garage width, or a small barn built for equipment rather than just hay. It is the first span here where the loft stops being an afterthought — 13.07 ft of clear width at 6 ft of headroom suits a workbench or storage-and-workspace combination, not just shelving on a ladder. At this size, how many trusses the building needs starts to matter as much as the rafter lengths do.
The 20 ft span, solved
Pre-filled at a 20 ft span, half-circle method, 60° lower pitch. Everything below reads off these same numbers — change the width or pitch and the worked example updates with it.
Cross-section
Lower rafter Upper rafter
- Lower rafter R₁
- 10.00 ft 10′ 0″ · 60.0° · 20.8:12
- Upper rafter R₂
- 5.18 ft 5′ 2 1/8″ · 15.0° · 3.2:12
- Total height H
- 10.00 ft 10′ 0″ to the ridge
- Roof area (both slopes)
- 687.1 ft² incl. 1.00 ft overhang
- Attic / loft volume
- 2,732 ft³ 136.6 ft² cross-section
- Clear floor width
- 13.07 ft at 6 ft standing height
Solved with the half-circle (barn) method: the knuckle and ridge sit on a semicircle of radius W ÷ 2 struck from the centre of the wall-plate line, which forces θ − φ = 45° and H = W ÷ 2 for every span.
Numbers at a glance
At a 20 ft span, 60° lower pitch and 15° upper pitch, a 20 ft building length and a 1 ft overhang:
- W
- 20 ft span, wall plate to wall plate.
- R₁
- 10.00 ft lower rafter, 60.0° (20.8:12).
- R₂
- 5.18 ft upper rafter, 15.0° (3.2:12).
- H
- 10.00 ft total height, half the span.
- Area
- 687.1 ft² of roofing, both slopes, 20 ft long with a 1 ft overhang.
- Vol.
- 2,732 ft³ of enclosed loft space.
Clear floor width at 6 ft of standing height: 13.07 ft, or about 65% of the full 20 ft width — the loft floor plan is starting to look like a room, not an attic.
This span's real question
Truss layout for a 20 ft mid-size building
Truss count depends on building length and spacing, not the 20 ft width itself — but this is the first span where getting it right is worth working through before ordering lumber:
| Building length | 24 in o.c. | 16 in o.c. |
|---|---|---|
| 20 ft long | 11 trusses | 16 trusses |
| 24 ft long | 13 trusses | 19 trusses |
24 in centres with 2×6 rafters is the usual rule of thumb up to roughly 24 ft of span, so this page sits comfortably inside it. Drop to 16 in for heavy snow load or a loft floor carrying storage racking; the framing calculator runs this for any building length.
Worked example: framing this 20 ft truss
Halve the span
W = 20 ft, so the half-span is 10 ft — also a standard stock board length.
Fix the lower rafter
At 60°, R₁ = 10.00 ft — a full 10 ft board, 0% waste.
Fix the upper rafter
The half-circle rule forces 15°, giving R₂ = 5.18 ft up to the ridge.
Check the height
y₁ + y₂ = 8.66 + 1.34 = 10.00 ft — exactly W ÷ 2.
Plan the run
For a 20 ft long building at 24 in centres, that is 11 trusses across 687.1 ft² of roofing.
Practical guidance for a 20 ft garage or small barn gambrel
Plan it as a room, not an attic
With 13.07 ft of clear floor at 6 ft headroom, decide the loft's job before framing — fixed stair vs. ladder, flooring vs. exposed joists.
Confirm spacing before the lumber order
Truss count swings by 30–50% between 24 in and 16 in centres — settle spacing before pricing lumber.
Lower rafters land on 10 ft stock
Order 10 ft boards for the lower rafters; the 5.18 ft upper rafters need a 8 ft board each, with material left for gusset blanks.
Still within site-built range
Plywood gussets at the knuckle remain standard at 20 ft; engineered connectors become worth pricing at the 30 ft page.
20 ft next to its neighbours
| Span | R₁ | R₂ | Height | Volume | Headroom width |
|---|---|---|---|---|---|
| 16 ft span | 8.00 ft | 4.14 ft | 8.00 ft | 1,749 ft³ | 9.07 ft |
| 20 ft span (this page) | 10.00 ft | 5.18 ft | 10.00 ft | 2,732 ft³ | 13.07 ft |
| 24 ft span | 12.00 ft | 6.21 ft | 12.00 ft | 3,934 ft³ | 17.07 ft |
20 ft span FAQ
Is a 20 ft gambrel loft usable as a real workspace?
Yes — roughly this is the width where that becomes true. 13.07 ft of clear floor at 6 ft headroom and 2,732 ft³ of volume is enough for a bench or small work area, not just shelving.
How many trusses does a 20 ft long, 20 ft wide gambrel building need?
At 24 in on-centre, 11 trusses. Tighten to 16 in on-centre — common under heavier snow load — and that rises to 16. Use the framing calculator for your actual building length.
Should I use 16 in or 24 in truss spacing at this span?
24 in on-centre with 2×6 members is standard at 20 ft in most snow zones. Move to 16 in if ground snow load is high, the loft floor carries live load, or a snow load check comes back tight.
How much roofing material does a 20 ft span two-car garage need?
687.1 ft² across both slopes for a 20 ft wide, 20 ft long building with a 1 ft overhang. Feed that into the shingle or metal panel calculator.