Gambrel Roof Calculator

Framing Guide & Calculator

Truss count, spacing and the order everything goes up in — from the first end truss to the last sheet of sheathing.

ft
ft
in o.c.
Regular mode: locked at 24" on-center
in
in

Truss layout

Truss count
13
12 bays
Actual spacing
24.00 in
2.00 ft on-center
Seat notch depth
4.33 in
53% of depth remains
Heel height
9.84 in
plumb cut at the rafter tail
Gusseted joints
26
two per truss, left and right of ridge
Purlin runs
2
full building length, each side

Layout aid for planning material counts — confirm spacing and connection details against your local building code before ordering lumber.

Regular layout, and when to go custom

Most of this site's calculators, including the main gambrel tool, default to the same layout: a 60° lower pitch and 15° upper pitch (the half-circle method's classic barn proportion), trusses every 24 inches on-center, and purlins doing the thrust-resisting work at each knuckle. "Regular" mode here just locks that layout so you can plan a straightforward building without re-deciding five things at once.

"Custom" mode exists for the buildings that do not fit that mold: a shallower or steeper pitch, tighter 16" spacing for heavy snow load or a longer unsupported span, or a raised tie instead of a purlin at the knuckle. The truss-count math does not change between the two modes — only which numbers are locked.

Purlin and raised tie are not interchangeable — they resist different forces, and mixing up which one you are relying on is a common framing mistake. A purlin runs the full length of the building, seated in the crook of every knuckle joint, and its main job is bracing the roof PLANE against racking along that length — without it, nothing stops one truss from leaning out of line with its neighbors. A raised tie runs the other direction, rafter to rafter across a single truss below the knuckle, and its job is resisting the two rafters spreading apart under snow and wind load — the same spreading force a ceiling joist resists on a simple gable roof. A gambrel roof with a purlin and no tie has solved racking but left the spreading force unresolved; a roof with a tie and no purlin has done the opposite. Most standing gambrel barns run both, and this site's Regular layout assumes a purlin plus the collar ties called out in the raising sequence below.

At the calculator's Regular defaults — a 24 ft span, 24 ft building length, 24" spacing — the layout works out to 13 trusses in 12 bays, landing exactly on the 24" target with nothing left over. That clean division does not happen at every building length, which is exactly why the calculator reports the actual achieved spacing next to the nominal one you typed.

Switch to Custom with a 40 ft building and 16" spacing instead — the spacing a heavy-snow region or a long unsupported roof deck often calls for — and the same building needs 31 trusses across 30 bays, at an actual spacing of 16.00" on-center. More trusses, more gusseted joints, more lumber — the tradeoff for the tighter deck support.

How truss count and heel height are computed

Truss count from building length and spacing

bays = ceil(buildingLength × 12 / spacing)

trussCount = bays + 1

actualSpacing = (buildingLength × 12) / bays

Rounding bays UP (never down) means the actual spacing is always at or under the spacing you asked for — the frame never ends up looser than requested.

Seat depth and heel height, at the wall plate

notchDepth = seatLength × tan(theta)

heelHeight = (rafterDepth − notchDepth) / cos(theta)

Same seat-cut model as the rafter length page and the birdsmouth page. Heel height is the vertical extent of the plumb cut through whatever rafter depth remains once the notch is removed.

At the Regular defaults — 60° pitch, a 2.5" seat cut, 9.25" (2x10) rafter stock — the notch removes 4.33" measured plumb, leaving 53% of the rafter's depth, and the resulting heel height at the tail is 9.84".

These are the same two numbers every rafter in the run needs, because the seat cut, the pitch and the stock depth do not change from truss to truss on a single roof — only the truss count and the spacing do. Get the seat and heel numbers right once, mark a single pattern rafter, and every other rafter on that span and pitch copies it exactly; there is no reason to recompute a birdsmouth notch truss by truss.

Truss count by building length, 16" vs 24" spacing

Same 24 ft span and 60° pitch throughout — only the building length and the spacing change.

Truss count and actual spacing, 16" vs 24" nominal spacing
Building length Trusses @ 24" Actual spacing Trusses @ 16" Actual spacing
16 ft 9 24.0 in 13 16.0 in
24 ft 13 24.0 in 19 16.0 in
32 ft 17 24.0 in 25 16.0 in
40 ft 21 24.0 in 31 16.0 in
60 ft 31 24.0 in 46 16.0 in
80 ft 41 24.0 in 61 16.0 in

Halving the spacing does not double the truss count outright — it roughly doubles the bay count, and truss count is bays plus one, so the difference narrows a little on shorter buildings and widens on longer ones. Either way, every extra truss also means another pair of gusseted knuckle joints; see the truss design calculator for sizing those.

Spacing is not just a truss-count decision — it sets how far the roof deck has to span unsupported between trusses, and every sheathing panel has a span rating that assumes a maximum on-center distance. Standard 7/16" OSB roof sheathing is commonly rated for 24" on-center support; push the spacing wider than the panel's rating and the deck deflects more than it should under snow load, telegraphing a wavy roofline even when every truss underneath is sound. Tightening to 16" is the usual fix on long spans or heavy-snow sites, not a thicker rafter.

Raising order, end to end

Framing failures on a gambrel usually trace back to skipping or reordering one of these steps, not to a bad calculation.

  1. Stand and brace the end trusses

    Raise the two gable-end trusses first and brace them plumb — every other truss and every purlin line takes its reference from these two.

  2. Set the interior trusses at spacing

    Stand the remaining trusses at the on-center spacing the calculator returns, temporarily bracing each one before you move to the next.

  3. Run the knuckle purlins

    Purlins run the full building length, seated in the crook of each knuckle joint, tying every truss together at the one point where the roof most wants to rack.

  4. Add the collar ties

    Collar ties (or a raised tie, in a custom layout) go in below the purlins, resisting the rafters spreading apart under snow and wind load — this is not optional bracing, it is part of the load path.

  5. Square the whole line before sheathing

    Measure diagonals across the standing frame and adjust bracing until the whole run is square — sheathing locks in whatever shape the frame is in when you nail it, mistakes included.

  6. Sheathe lower slopes, then upper slopes

    Sheathe both lower slopes first, working from the eave toward the knuckle, then both upper slopes toward the ridge — this keeps every sheet's bearing edge on framing that is already fastened down.

Every truss is unstable on its own until the purlins and collar ties tie it to its neighbors — brace each one plumb the moment it is standing, and leave that bracing in place until the purlin run is fully nailed off. A gambrel truss that looks solid immediately after standing can still rack sideways under a gust of wind before the frame is tied together; that is a real, common jobsite injury and roof-collapse cause, not a theoretical one.

Framing FAQ

What truss spacing should I actually use?
24 inches on-center is the standard spacing for typical light-frame gambrel outbuildings and matches this site's defaults everywhere. Tighten that to 16 inches on-center for heavy snow country or for longer spans where the roof deck and purlins need shorter unsupported runs between trusses.
What is the difference between Regular and Custom mode here?
Regular mode locks the layout this site assumes everywhere else: 24" truss spacing, a 60/15-degree half-circle split, purlins doing the thrust-resisting work at the knuckle. Custom mode lets you set the pitch, the spacing (12" to 48") and swap the knuckle joint to a raised tie instead of a purlin.
Why does the actual spacing sometimes differ from what I typed?
Truss count has to be a whole number, so the calculator rounds UP to a whole number of bays and then spreads the building length evenly across them. That actual on-center spacing is always equal to or tighter than what you asked for — you never end up further apart than your input, only closer.
Purlin or raised tie — which one at the knuckle?
A purlin seats in the crook of the knuckle joint and mainly braces the roof plane against racking along the building's length. A raised tie sits below the knuckle, spanning rafter to rafter, and mainly resists the two rafters spreading apart. Many gambrel roofs use both; a purlin alone without any tie leaves the spreading force unresolved.
How does the seat cut and heel height here relate to the birdsmouth page?
Identical formulas, different focus. This page reports seat depth and heel height as part of planning truss count and stock sizing; the birdsmouth calculator is where you check that same notch against the IRC 1/3-depth structural limit in detail.