How to Calculate Roof Truss Size

What Determines Truss Size?

Truss size isn't a single number — it's a combination of member length (how long the top and bottom chords are), member depth (how tall the lumber is, like a 2x4 versus a 2x6), and the web configuration bracing between them. All three are driven by the same four inputs: how far the truss has to span, how steep the roof pitch is, how much load it has to carry, and how closely the trusses are spaced. Change any one of those inputs and the others move with it, which is why truss design is always a calculation, not a guess.

Step 1 — Measure Your Span

Span is the horizontal distance between the outside edges of the load-bearing walls — not the roofline, and not the building footprint including overhangs. Measure wall-to-wall at the top plate. This single number sets the ceiling for everything downstream: a 24 ft span needs a smaller truss than a 40 ft span, holding pitch and load constant.

Run, which shows up in the rafter length formula a few steps from now, is simply half the span for a standard gable or hip roof — the horizontal distance from the outside wall to the center ridge. The roof truss span calculator works out run and rafter length automatically once you enter your own span.

Step 2 — Choose Your Roof Pitch

Roof pitch is expressed as a ratio: inches of rise for every 12 inches of horizontal run. A 6/12 pitch climbs 6 inches for every foot it runs, which works out to a 26.57° roof angle. Steeper pitches — 8/12, 9/12, 12/12 — shed snow and water faster and buy more usable attic headroom, but they also lengthen the rafter and increase the total roof area you're covering, and paying to frame.

Pick your pitch based on climate, architectural style, and local code minimums before calculating anything else, since every number that follows — rise, rafter length, roof area — depends on it.

Step 3 — Calculate Rise, Rafter Length, and Roof Height

With span and pitch fixed, rise is just run multiplied by the pitch ratio: a 30 ft span (15 ft run) at a 6/12 pitch rises 7.5 ft from the wall plate to the ridge. Rafter length — the actual sloped board length you'll cut — comes from the Pythagorean theorem: the square root of run squared plus rise squared. Add your eave overhang to the run before squaring, so the tail of the rafter is accounted for.

• Run = Span ÷ 2
• Rise = Run × (Pitch ÷ 12)
• Rafter Length = √(Run² + Rise²), adjusted for overhang

Step 4 — Factor In Your Loads

A truss sized only for its own weight will fail the first time it snows. Total design load stacks four separate load types: dead load (the permanent weight of roofing material, decking, and the truss members themselves — typically 15 to 17 psf depending on material), live load (temporary weight from maintenance workers and equipment, usually a 20 psf code minimum), snow load (climate-dependent, pulled from your local building department's ground snow load data), and wind load (uplift pressure, also code- and region-specific). These four combine into a single total design load, expressed in pounds per square foot, that determines chord depth and web spacing.

Heavier loads call for deeper chords, tighter web bracing, or closer truss spacing — often all three at once. See the roof truss load calculation guide for a full breakdown of how these four load types are worked out and combined.

Step 5 — Set Truss Spacing and Count

On-center spacing — measured center-to-center between adjacent trusses — is usually 16, 19.2, or 24 inches. Wider spacing means fewer, larger trusses; tighter spacing means more, lighter ones. Truss count follows a simple formula: divide your building length in inches by the spacing, then add one more truss to close out the run. A 40 ft long building at 24-inch spacing needs 21 trusses; the same building at 16-inch spacing needs 31. The roof truss quantity calculator runs this formula for you and also estimates total board footage.

How Truss Type Changes the Calculation

The steps above assume a standard gable truss, but the shape you're building shifts the math. A hip roof needs jack and hip-girder trusses in addition to the common trusses, adding pieces for the same footprint. A gambrel truss breaks the slope partway up, effectively running two pitch calculations per side instead of one. A scissor truss slopes the bottom chord as well as the top, which changes the interior ceiling height formula without changing the exterior span calculation. Whatever the shape, span, pitch, and load still drive the underlying numbers — the type just changes how many distinct truss profiles a full roof needs.

How Overhang Changes Rafter Length

Overhang is easy to forget and it quietly throws off a rafter length calculation more than most other inputs. The eave overhang extends the rafter tail beyond the wall line at the same slope as the rest of the roof, so it has to be added to the run before the Pythagorean calculation, not tacked onto the final rafter length afterward. A 12-inch overhang on a 6/12 pitch adds roughly 13.4 inches of actual rafter material once the slope is accounted for — a small difference per rafter that adds up across an entire roof's board footage.

Wider overhangs, common in hot climates for shading or in regions with heavy rain for wall protection, have a proportionally larger effect on steep pitches than shallow ones, since the slope multiplier grows with the roof angle.

Working in Metric Units

The same five steps apply whether you're working in feet and inches or meters and centimeters — only the unit conversion changes. Span and length are typically entered in meters, pitch can be expressed either as the same rise-over-run ratio or converted directly to a roof angle in degrees, and load figures switch from pounds per square foot to kilograms per square meter. A calculator that accepts both systems side by side, and lets you switch a single field's unit without retyping the whole form, saves the most time when a project mixes imperial framing lumber with metric site drawings, which happens more often than most builders expect on international or architect-specified jobs.

Worked Example

Take a 30 ft span, 40 ft long building, 6/12 pitch, 12-inch overhang, 24-inch spacing, and a 20 psf snow load. Run is 15 ft, rise is 7.5 ft, and rafter length — including the overhang — works out to roughly 17.9 ft. Roof area comes to about 1,340 sq ft. At 24-inch spacing, the building needs 21 trusses. Total design load (roughly 15 psf dead plus 20 psf live plus 20 psf snow, for about 55 psf) applied across that roof area gives a rough total roof weight for planning — the figure a supplier uses to confirm chord depth and check the design against code.

Once you know your truss size and quantity, the next natural question is what it will cost to build — see how to estimate the cost of roof trusses using these same span, spacing, and material numbers.

Common Mistakes to Avoid

• Measuring span along the roofline instead of wall-to-wall at the top plate
• Guessing snow or wind load instead of pulling the real figure from local code maps
• Forgetting to add the overhang before calculating rafter length
• Assuming 24-inch spacing works for every load condition without checking the total design load first