Roof Truss Calculator
Plan a trussed roof. Enter building length and truss span (width) in feet, the pitch as rise per 12 in, spacing (16 or 24 in on center), and overhang in feet to get the truss count, peak height, top chord length, and roof area.
Example: with Building length (ft) 36 · Truss span / building width (ft) 24 · Pitch (rise per 12 in) 4 · Spacing (on center) 24 in OC (standard) · Overhang, horizontal (ft) 1 → Trusses needed: 19 trusses at 24 in OC.
- Peak height4.00 ft above the top plate
- Top chord length13.70 ft per side (overhang included)
- Roof area987 sq ft of roof plane
Computed by the calculator below using its default values. Change any input to see your own numbers.
Count = building length ÷ spacing + 1 (a truss closes each end). Peak = half-span × pitch/12; top chord = (half-span + overhang) × √(1 + (pitch/12)²).
Counting trusses like the lumberyard does
Trusses set on a regular on-center spacing along the building's length, with one at each gable end — so the count is length ÷ spacing, rounded up, plus one. A 36 ft building at the standard 24 in OC is 18 spaces and 19 trusses. Girders at hips, gable-end trusses with vertical studs for siding, and any attic-access or mechanical framing are ordered as specials on top of the common count.
Spacing at 24 in OC is the residential default because engineered trusses easily handle it and it saves a third of the units versus 16 in; 16 in shows up under heavy roofing like tile or where the ceiling finish wants tighter support.
The geometry you can check by hand
A common (Fink) truss is two right triangles back to back: half the span is the base, pitch sets the rise. Peak height above the top plate is half-span × pitch/12 — a 24 ft span at 4/12 peaks at 4 ft, which matters when you are checking truck clearance or attic room. The top chord runs the slope: (half-span + overhang) × the slope factor √(1 + (pitch/12)²). Doubling chord length times building length gives the roof plane area your shingle order comes from.
How it’s calculated
Truss count = ceil(building length × 12 ÷ spacing) + 1. Peak height = (span ÷ 2) × (pitch ÷ 12). Slope factor = √(1 + (pitch/12)²); top chord = (span ÷ 2 + overhang) × slope factor. Roof area = 2 × top chord × building length. Overhang is entered as a horizontal projection.
Covers straight gable roofs with common trusses only — hips, valleys, girder locations, and all member sizing are the truss manufacturer's engineering, not a count.
Trusses for a 36 ft long building
| Spacing | Trusses |
|---|---|
| 12 in OC | 37 |
| 16 in OC | 28 |
| 24 in OC (standard) | 19 |
Computed as ceil(length ÷ spacing) + 1; gable-end and girder specials extra.
Common mistakes
- Ordering length ÷ spacing without the +1 — the far gable still needs a truss, and being one short stalls a crane day.
- Confusing span with rafter length: truss span is the full building width, outside of bearing to outside of bearing.
- Entering pitch in degrees; 4/12 is about 18.4°, and typing 18 as the pitch nearly doubles every slope number.
- Measuring overhang along the slope — this tool wants the horizontal projection, which is what truss drawings specify.
Frequently asked questions
How many roof trusses do I need?
Building length in inches ÷ on-center spacing, rounded up, plus one for the closing gable end. A 36 ft building at 24 in OC needs 19 trusses.
What spacing do roof trusses use?
24 in on center is standard for residential engineered trusses; 16 in OC appears under heavy roof loads (tile, slate) or where sheathing and ceiling spans demand it. Wider than 24 in is possible commercially but needs purlins.
How is peak height calculated?
Half the span times pitch/12. A 24 ft span at 4/12: 12 × 4 ÷ 12 = 4 ft from the top plate to the peak, before the ridge and sheathing thickness.
Can I build trusses myself from this?
No — these numbers give counts and overall geometry for planning and pricing. Truss member sizes, plates, and bracing are engineered by the manufacturer for your loads, and most codes require sealed truss drawings.
Trusses or stick framing — which needs more lumber?
Stick framing (individual rafters and ceiling joists) uses bigger sawn members and more labor; trusses use engineered webs of 2x4s and drop in fast. For spans over about 24 ft or any complex load path, trusses usually win on cost.