Roof Truss Calculator
Enter the span, pitch and overhang for a common, mono or scissor truss. You get the top and bottom chord lengths, the height at the peak, the roof angle, how many trusses your building needs and a material cost range. Every formula is shown. It gives geometry, not engineering: the truss maker designs the webs, lumber and connections.
Your truss
Fink, Howe and king post are all common.
Outside of wall to outside of wall.
Top chord rise per 12 in of run.
Scissor only: bottom chord slope.
Level, out from the wall.
Optional. Truss depth at the wall.
From your plans or truss maker.
Along the ridge. For the count.
(14 ft + 12 in) × 1.1180
the full span
14 ft × 6 ÷ 12, plus your heel height
atan(6 ÷ 12)
40 ft × 12 ÷ 24 in, rounded up, + 1
21 trusses × $60 to $700
Chords per truss: 61.5 ft. All 21 trusses: 1,292 ft of chord lumber. Webs are not included; their layout is set by the truss designer.
From the frame to the roof on top
The same span and pitch give you the roof area to cover and what that roof will cost.
How the roof truss calculator works
A roof truss is a triangle of top chords, a bottom chord and the webs between them. Its outside shape is fixed by three numbers: the span, the pitch and the overhang. The calculator works out that shape with the same arithmetic as a rafter length calculator:
Take the starting values: a 28 foot common truss at 6/12 with a 12 inch overhang. The run is 14 feet, the 6/12 multiplier is 1.1180, and each top chord is (168 + 12) × 1.1180 = 201.25 inches, or 16′ 9-1/4″. The truss rises 7′ 0″ at the peak, and the bottom chord spans the full 28 feet. The roof pitch chart lists the multiplier and angle for every pitch.
Lengths are line lengths measured from the outside of the wall, to the nearest 1/8 inch. They tell you the size of truss to ask for and how much room it takes up; the truss maker’s drawing gives the cut lengths, which depend on the lumber depth and how the chords meet.
Common, mono and scissor trusses
- Common truss. Two top chords meeting at a ridge over a flat bottom chord, the shape of a gable roof. Fink (or W), Howe and king post are names for the pattern of webs inside it. They change how the truss carries load, not its outside dimensions, so the calculator treats them as one shape.
- Mono truss. A single slope, high at one wall and low at the other, used for sheds, porches and additions against a house. The run is the whole span, so a 12 foot mono truss at 3/12 rises 3′ 0″, and the tall end is that rise plus the heel.
- Scissor truss. The bottom chord slopes up toward the middle to make a vaulted ceiling. Enter the ceiling pitch, which must be lower than the roof pitch. On a 28 foot span at 6/12 with a 3/12 ceiling, the ceiling rises 3′ 6″ at the center and each bottom chord is 14′ 5-1/8″.
Attic, gambrel and hip trusses have more complicated outlines than one pitch and one span can describe, so this calculator does not draw them.
How many roof trusses you need
Divide the building length by the spacing, round up and add one for the last truss:
A 40 foot building at 24 inches on center has 20 spaces and needs 21 trusses. The calculator also adds up the chords: 61.5 feet per truss in the example, 1,292 feet for the whole roof, which is a quick check on the size of the delivery. Spacing is part of the truss design, so take it from your plans or the truss maker rather than choosing it to save trusses.
What roof trusses cost
HomeGuide puts roof trusses at $60 to $700 each for materials, and replacing trusses at $5 to $14 per square foot installed. The calculator multiplies your truss count by the per-truss range, so the example’s 21 trusses come to $1,300 to $14,700. That range is wide because it covers every size and type in one figure; the source does not publish prices by span or by truss type, so the calculator does not pretend to narrow it. A truss maker quotes on the design drawing, and delivery, a crane and the framing crew to set the trusses are usually separate lines.
Spans and engineering
This calculator does not tell you whether a truss can carry your roof. The 2024 International Residential Code, Section R802.10, sets the rules:
- Wood trusses must be designed in accordance with accepted engineering practice, and metal-plate-connected trusses must be designed and made to ANSI/TPI 1.
- Truss design drawings, showing the span, slope, spacing, joint locations, bearing widths and design loads, go to the building official for approval before installation and arrive with the trusses.
- Trusses must be braced as the drawings require, or by accepted industry practice such as the SBCA’s BCSI guide.
- Truss members must not be cut, notched, drilled or spliced without the approval of a registered design professional.
So the maximum span for a given lumber size, the web layout and the plate sizes all come from the truss designer, working to your snow, wind and dead loads. Use the numbers here to plan the building and to check the drawing you are given.
Sources
- International Code Council, 2024 International Residential Code, Chapter 8, Section R802.10: truss design, design drawings, bracing and alterations, read on November 2, 2026.
- HomeGuide roofing labor cost guide, dated November 7, 2025: truss material cost per truss and truss replacement cost per square foot installed.
- Chord lengths, heights, angles, counts and the cost arithmetic are calculated with the formulas above. Every source is listed on the methodology page.
Related calculators and guides
Roof truss calculator FAQ
What is a roof truss?
A roof truss is a triangulated frame of top chords, a bottom chord and the webs between them that spans from wall to wall and carries the roof. Under the 2024 International Residential Code, wood trusses joined with metal connector plates must be designed and made to ANSI/TPI 1, and they arrive at the job site with their truss design drawings.
How do you calculate the length of a truss top chord?
Take half the span for a common truss, or the whole span for a mono truss, add the overhang, and multiply by the pitch multiplier, the square root of (pitch squared + 144) divided by 12. A 28 foot span at 6/12 with a 12 inch overhang gives (14 feet + 12 inches) x 1.1180 = 201.25 inches per side, or 16′ 9-1/4″.
How many roof trusses do I need?
Divide the building length in inches by the truss spacing, round up and add one. A 40 foot building with trusses 24 inches on center needs 40 x 12 / 24 = 20 spaces, so 21 trusses. Gable-end trusses and any girder trusses are part of that count only if your truss designer says so.
How tall is a roof truss?
The rise of a common truss is half the span times the pitch divided by 12, so a 28 foot truss at 6/12 rises 7 feet at the peak. Add the heel height, the depth of the truss where it sits on the wall, for the overall height.
How far can a roof truss span without support?
There is no single figure. The span a truss can carry is set by its design, which depends on the lumber, the web layout, the pitch and the loads, including snow. Under the 2024 IRC the truss design drawing must state the span, spacing, joint locations, bearing widths and design loads, and the building official approves it before the trusses go up. Ask the truss maker for a design at your span; this calculator gives the geometry, not the capacity.
How much does a roof truss cost?
HomeGuide puts the material cost of a roof truss at $60 to $700, depending on its size and type, and replacing trusses at $5 to $14 per square foot installed. A truss maker's quote for your span, pitch and loads is the only firm price.
Do roof trusses have to be engineered?
Yes. The 2024 International Residential Code requires wood trusses to be designed in accordance with accepted engineering practice, with metal-plate-connected trusses designed and made to ANSI/TPI 1, and requires truss design drawings to be approved by the building official before installation.
What is a scissor truss?
A truss whose bottom chord slopes up toward the middle instead of running flat, which gives a vaulted ceiling. On a 28 foot span with a 6/12 roof and a 3/12 bottom chord, the ceiling rises 3′ 6″ at the center while the roof rises 7′ 0″.
Can you cut or alter a roof truss?
Not on your own. The IRC says truss members must not be cut, notched, drilled, spliced or otherwise altered in any way without the approval of a registered design professional.
How much weight can you hang from a roof truss?
Only what the truss was designed for. Its design drawing lists the bottom chord loads, and the 2024 IRC does not allow added loads such as HVAC equipment or a water heater beyond the truss design load without verifying that the truss can carry them. For anything heavier, ask the truss maker or a registered design professional.