16 Foot Gambrel Roof Span
Sixteen feet is single-car-garage width, or a large storage shed built with real workspace in mind, not just a place to park a mower. By coincidence of the geometry rather than by design, it is also one of the tidiest spans on this site to build: the lower rafter comes out to exactly 8.00 ft, precisely the shortest length dimensional lumber is stocked in.
The 16 ft span, solved
Pre-filled at a 16 ft span, half-circle method, 60° lower pitch. Move the width slider to 12 or 20 ft and watch the lower rafter reading drift off a clean number — 16 ft is the sweet spot between them.
Cross-section
Lower rafter Upper rafter
- Lower rafter R₁
- 8.00 ft 8′ 0″ · 60.0° · 20.8:12
- Upper rafter R₂
- 4.14 ft 4′ 1 11/16″ · 15.0° · 3.2:12
- Total height H
- 8.00 ft 8′ 0″ to the ridge
- Roof area (both slopes)
- 565.6 ft² incl. 1.00 ft overhang
- Attic / loft volume
- 1,749 ft³ 87.4 ft² cross-section
- Clear floor width
- 9.07 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 16 ft span, 60° lower pitch and 15° upper pitch, a 20 ft building length and a 1 ft overhang:
- W
- 16 ft span, wall plate to wall plate.
- R₁
- 8.00 ft lower rafter, 60.0° (20.8:12).
- R₂
- 4.14 ft upper rafter, 15.0° (3.2:12).
- H
- 8.00 ft total height, half the span.
- Area
- 565.6 ft² of roofing, both slopes, 20 ft long with a 1 ft overhang.
- Vol.
- 1,749 ft³ of enclosed loft space.
Clear floor width at 6 ft of standing height: 9.07 ft — more than half the building's width is walkable at head height, the point where a loft stops being a curiosity and starts being storage you actually use.
This span's real advantage
Lumber efficiency at 16 ft
R₁ always equals W ÷ 2, so whether a span wastes lumber depends on whether half its width happens to match a stock board length. 16 ft is one of the ones that works out clean:
| Span | R₁ | Stock board | Waste |
|---|---|---|---|
| 12 ft span | 6.00 ft | 8 ft | 25% |
| 16 ft span (this page) | 8.00 ft | 8 ft | 0% |
| 20 ft span | 10.00 ft | 10 ft | 0% |
The upper rafter does not get the same clean fit — at 4.14 ft it still needs an 8 ft board, wasting 48% — but two upper rafters nest onto a single 8 ft board, the practical fix carpenters use at every span where the upper rafter is this short.
Worked example: framing this 16 ft truss
Halve the span
W = 16 ft, so the half-span is 8 ft — also a standard stock board length.
Fix the lower rafter
At 60°, R₁ = 8.00 ft — one 8 ft board, no offcut.
Fix the upper rafter
The half-circle rule forces 15°, giving R₂ = 4.14 ft up to the ridge.
Check the height
y₁ + y₂ = 6.93 + 1.07 = 8.00 ft — exactly W ÷ 2.
Take off materials
One 8 ft board per lower rafter, one shared board per pair of upper rafters, 565.6 ft² of sheathing.
Practical guidance for a 16 ft garage or shed gambrel
Real storage, real access
9.07 ft of clear width at 6 ft headroom suits a centre walking aisle with shelving both sides — plan a fixed stair or pull-down ladder rather than a hatch.
Buy lower rafters to length
Order 8 ft stock for the lower rafters — nothing is wasted by ordering exact-length boards.
Standard spacing applies cleanly
24 in on-centre with 2×6 members is the normal call here in most snow zones.
Site-built gussets, no special hardware
Plywood gusset plates at the knuckle are standard here; nothing about 16 ft pushes you toward engineered plates.
16 ft next to its neighbours
| Span | R₁ | R₂ | Height | Volume | Headroom width |
|---|---|---|---|---|---|
| 12 ft span | 6.00 ft | 3.11 ft | 6.00 ft | 984 ft³ | 0.00 ft |
| 16 ft span (this page) | 8.00 ft | 4.14 ft | 8.00 ft | 1,749 ft³ | 9.07 ft |
| 20 ft span | 10.00 ft | 5.18 ft | 10.00 ft | 2,732 ft³ | 13.07 ft |
16 ft span FAQ
Why does a 16 ft gambrel waste no lumber on the lower rafter?
Because R₁ always equals W ÷ 2 under the half-circle method, and 16 ÷ 2 = 8 — the shortest length dimensional lumber is stocked in. Cut 8.00 ft off an 8 ft board and nothing is left over, unlike a 12 ft roof, which wastes 25% of the same board.
Is the loft usable at a 16 ft span?
Yes. 9.07 ft of clear floor at 6 ft headroom, with 1,749 ft³ of volume over a 20 ft building — enough for shelving both sides with a walkable centre aisle, real storage rather than crawl space.
What truss spacing works for a 16 ft garage or shed?
24 in on-centre with 2×6 rafters is standard at this span in most snow zones — see the framing guide, or drop to 16 in centres under heavy snow load.
How much roofing material do I need for a 16 ft span, 20 ft long?
565.6 ft² across both slopes, including a 1 ft overhang. Run that through the shingle or metal panel calculator.