40 Foot Gambrel Roof Span
Forty feet is a real working farm barn — hay storage filling the loft over an equipment bay wide enough for a tractor below. It is also the span where these numbers stop being about pitch angles and start being about what you can actually buy: the lower rafter works out to exactly 20.00 ft, the longest length most lumberyards carry off the shelf without a special order.
The 40 ft span, solved
Pre-filled at a 40 ft span, half-circle method, 60° lower pitch. Push the width past 40 and watch the lower rafter cross past the point a single stock board can cover.
Cross-section
Lower rafter Upper rafter
- Lower rafter R₁
- 20.00 ft 20′ 0″ · 60.0° · 20.8:12
- Upper rafter R₂
- 10.35 ft 10′ 4 1/4″ · 15.0° · 3.2:12
- Total height H
- 20.00 ft 20′ 0″ to the ridge
- Roof area (both slopes)
- 1,294.1 ft² incl. 1.00 ft overhang
- Attic / loft volume
- 10,928 ft³ 546.4 ft² cross-section
- Clear floor width
- 33.07 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 40 ft span, 60° lower pitch and 15° upper pitch, a 20 ft building length and a 1 ft overhang:
- W
- 40 ft span, wall plate to wall plate.
- R₁
- 20.00 ft lower rafter, 60.0° (20.8:12).
- R₂
- 10.35 ft upper rafter, 15.0° (3.2:12).
- H
- 20.00 ft total height, half the span — a genuine two-storey rise.
- Area
- 1,294.1 ft² of roofing, both slopes, 20 ft long with a 1 ft overhang.
- Vol.
- 10,928 ft³ of enclosed loft space.
Clear floor width at 6 ft of standing height: 33.07 ft — more than 83% of the full width is walkable, over a loft big enough to genuinely store a season's worth of hay.
This span's real question
Lumber sourcing at 40 ft
At 20.00 ft, the lower rafter sits exactly at the edge of a single piece of off-the-shelf lumber. Three ways builders get that length on site:
| Full-length 2× stock | Engineered (LVL / glulam) | Site-scarfed splice | |
|---|---|---|---|
| Availability | At the outer limit of common stock | Special order, most yards | Any shorter stock on hand |
| Straightness / consistency | Variable | Very consistent | Depends on joint quality |
| Best suited to | A build lucky enough to source it locally | A load-checked, code-reviewed truss | A crew experienced with structural splices |
None of these is a purely carpentry decision at this span — the connection detail has to carry real load, which is why a structural engineer's review belongs in the plan before lumber gets ordered.
Worked example: framing this 40 ft truss
Halve the span
W = 40 ft, so the half-span is 20 ft.
Fix the lower rafter
At 60°, R₁ = 20.00 ft — exactly the 20 ft longest common stock length.
Fix the upper rafter
The half-circle rule forces 15°, giving R₂ = 10.35 ft up to the ridge.
Check the height
y₁ + y₂ = 17.32 + 2.68 = 20.00 ft — a full 20 ft rise from wall plate to ridge.
Flag it for review
Confirm rafter stock at 20.00 ft is actually available locally, and get the knuckle and spacing checked against real site loads.
Practical guidance for a 40 ft farm barn gambrel
Plan it like a second building
10,928 ft³ over just a 20 ft section is genuine storage capacity — plan hay-loading access as part of the framing, not an afterthought.
Confirm stock length before design freeze
Call the yard before finalizing the design — a 20.00 ft board is at the edge of what is commonly stocked, and availability varies by region.
Not a DIY spec at this scale
A stamped truss design is standard practice — and usually required by code — for a barn this size, with hay load and a tall, wind-exposed profile.
40 ft next to the size below it
40 ft is the largest span on this site's reference list; the step down is 30 ft, where rafters still come off a single stock board.
| Span | R₁ | R₂ | Height | Volume | Headroom width |
|---|---|---|---|---|---|
| 30 ft span | 15.00 ft | 7.76 ft | 15.00 ft | 6,147 ft³ | 23.07 ft |
| 40 ft span (this page) | 20.00 ft | 10.35 ft | 20.00 ft | 10,928 ft³ | 33.07 ft |
40 ft span FAQ
Can I still use standard dimensional lumber at a 40 ft span?
For the lower rafter, just barely — R₁ comes out to exactly 20.00 ft, the longest length most yards stock off the shelf. Any wider and that rafter needs LVL, glulam, or a scarfed splice instead of a single board. The 10.35 ft upper rafter has plenty of room on a 12 ft board.
Do I need an engineer for a 40 ft gambrel barn?
Strongly recommended, and required by code in most jurisdictions at this scale. Hay storage loading, wind uplift on a tall profile, and snow drift at the knuckle all deserve a real engineer's numbers, not a rule of thumb.
How big is the loft at a 40 ft span, really?
10,928 ft³ over a 20 ft building length, with 33.07 ft of clear floor at 6 ft headroom. Scale that to a typical 40–60 ft barn and volume runs into the tens of thousands of cubic feet.
How much roofing material does a 40 ft barn need?
1,294.1 ft² across both slopes for a 40 ft wide, 20 ft long section with a 1 ft overhang. See the metal panel calculator and the build cost breakdown.