30 Foot Gambrel Roof Span
Thirty feet is large-garage-with-a-second-floor territory, or a small livestock barn built for stalls below and hay above. It is also where this site's framing logic changes register: the lower rafter is 15.00 ft and the upper is 7.76 ft — real leverage at the knuckle — roughly the span where a basic plywood gusset stops being an automatic choice.
The 30 ft span, solved
Pre-filled at a 30 ft span, half-circle method, 60° lower pitch. Try dropping the lower pitch toward 50° and watch both rafters shorten as the roof flattens, at the cost of headroom.
Cross-section
Lower rafter Upper rafter
- Lower rafter R₁
- 15.00 ft 15′ 0″ · 60.0° · 20.8:12
- Upper rafter R₂
- 7.76 ft 7′ 9 3/16″ · 15.0° · 3.2:12
- Total height H
- 15.00 ft 15′ 0″ to the ridge
- Roof area (both slopes)
- 990.6 ft² incl. 1.00 ft overhang
- Attic / loft volume
- 6,147 ft³ 307.4 ft² cross-section
- Clear floor width
- 23.07 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 30 ft span, 60° lower pitch and 15° upper pitch, a 20 ft building length and a 1 ft overhang:
- W
- 30 ft span, wall plate to wall plate.
- R₁
- 15.00 ft lower rafter, 60.0° (20.8:12).
- R₂
- 7.76 ft upper rafter, 15.0° (3.2:12).
- H
- 15.00 ft total height, half the span.
- Area
- 990.6 ft² of roofing, both slopes, 20 ft long with a 1 ft overhang.
- Vol.
- 6,147 ft³ of enclosed loft space.
Clear floor width at 6 ft of standing height: 23.07 ft, and 307.4 ft² of cross-section per foot of building length — this is genuinely second-storey volume, not attic space.
This span's real question
Site-built gusset or engineered connector?
Both options exist at 30 ft; the honest answer depends on load and jurisdiction, not span alone. What changes here is that the question is worth asking in the first place:
| Site-built plywood gusset | Engineered connector | |
|---|---|---|
| Typical comfort range | Up to roughly 24–26 ft span | No practical upper limit |
| Design basis | Rule-of-thumb plate size and nailing schedule | Engineered to actual load case |
| Materials | Exterior plywood, construction adhesive, nails | Galvanized truss plates or bolted steel straps |
| At 30 ft, our take | Workable with a documented, load-checked design | Worth pricing, often the safer default |
Neither column replaces an engineer's sign-off where code requires one — this table is a starting point for that conversation. The truss design tool works out the joint's actual geometry either way.
Worked example: framing this 30 ft truss
Halve the span
W = 30 ft, so the half-span is 15 ft.
Fix the lower rafter
At 60°, R₁ = 15.00 ft — just over a 16 ft board.
Fix the upper rafter
The half-circle rule forces 15°, giving R₂ = 7.76 ft up to the ridge.
Check the height
y₁ + y₂ = 12.99 + 2.01 = 15.00 ft — exactly W ÷ 2.
Compare to one long rafter
A single-slope gable rafter to the same ridge height would run 21.21 ft — the two-member gambrel breaks that into two boards a crew can lift.
Practical guidance for a 30 ft garage or barn gambrel
Genuine second-storey space
With 23.07 ft of clear width at 6 ft headroom and 6,147 ft³ of volume, plan real floor loading — joists for live load, not just rafters for roof load.
Get the knuckle load-checked
"We've always built gussets this way" starts to need a load calculation behind it, especially under snow or hay storage above the loft floor.
Order to the next stock length up
15.00 ft rounds up to a 16 ft board with 6% waste; the upper rafter fits a 8 ft board with room to spare.
Tighten spacing before adding lumber size
16 in on-centre is a common step here before jumping to larger lumber — check both against your actual snow load case.
30 ft next to its neighbours
| Span | R₁ | R₂ | Height | Volume | Headroom width |
|---|---|---|---|---|---|
| 24 ft span | 12.00 ft | 6.21 ft | 12.00 ft | 3,934 ft³ | 17.07 ft |
| 30 ft span (this page) | 15.00 ft | 7.76 ft | 15.00 ft | 6,147 ft³ | 23.07 ft |
| 40 ft span | 20.00 ft | 10.35 ft | 20.00 ft | 10,928 ft³ | 33.07 ft |
30 ft span FAQ
Do I need engineered connectors at a 30 ft gambrel span?
Worth pricing, and worth having the truss design reviewed, even if the final choice is a heavy site-built gusset. At 15.00 ft and 7.76 ft, both rafters carry real leverage at the knuckle — roughly where builders shift from plywood on both faces to galvanized truss plates or bolted steel straps.
What lumber size do I need for a 30 ft gambrel truss?
2×8 or 2×10 stock is typical, sized against actual snow and wind load. The lower rafter needs 15.00 ft, just over a 16 ft board; the upper rafter needs 7.76 ft, comfortably inside an 8 ft board.
How does a 30 ft gambrel compare to one long rafter?
Two members of 15.00 ft and 7.76 ft, against one 21.21 ft rafter for an equivalent-height gable — the traditional barn-framing argument for the shape: two shorter boards are far easier to lift and handle by hand.
How much roofing material does a 30 ft barn need?
990.6 ft² across both slopes for a 30 ft wide, 20 ft long building with a 1 ft overhang. Run that through the metal panel calculator or the build cost breakdown.