Truss Size Calculator
Enter the span, the roof pitch, the building length, the truss spacing and the overhang. The calculator gives the number of trusses, the height at the peak, the top chord length, the roof angle and a suitable truss type.
- Number of trusses
- Truss height
- Truss type
Spacing is usually 24 in (600 mm) centres, or 16 in for heavy roofs and snow.
Trusses are spaced along the building at regular centres. At 24 in centres, a 40 ft long building needs 21 trusses. A 30 ft span at a 6 in 12 pitch rises 7.5 ft at the peak, and a Fink or W truss is the usual choice for that span.
Truss type chart
Typical choices by span. Search for a span.
| Span (ft) | Span (m) | Height at 6/12 (ft) | Type |
|---|---|---|---|
| 20 | 6.1 | 5 | Fink (W) |
| 24 | 7.3 | 6 | Fink (W) |
| 28 | 8.5 | 7 | Fink (W) |
| 32 | 9.8 | 8 | Fink (W) |
| 36 | 11 | 9 | Fink (W) |
| 40 | 12.2 | 10 | Fan |
| 48 | 14.6 | 12 | Fan |
Final design by the truss maker.
Span and truss
6 in 12 pitch. Slide the span.
The truss is drawn with the web pattern for that span.
How to plan roof trusses
Span to count. Step through it.
Span
Wall to wall.
Gable end trusses sit flush with the end walls, and count in the total.
King post, Fink or attic
Truss types. Compare them.
Short spans.
Most homes.
Room inside.
Attic trusses leave a room in the roof, but they need deeper members and cost more.
Before ordering
Truss checks. Tick each item.
Tick each item as you go.
Brace trusses as you set them. Unbraced trusses can topple like dominoes.
What you get
Truss planning. Each tile explains one result.
Trusses.
At the peak.
Length.
Roof.
For the span.
Pitch in 12 is the rise for every 12 of run, so 6 in 12 is about 26.6°.
Frequently Asked Questions
No question matches that search.
How many trusses do I need?
Divide the building length by the spacing and add one.
How tall is a roof truss?
Half the span times the pitch divided by twelve. A thirty foot span at six in twelve is seven and a half feet tall.
What spacing do roof trusses use?
Usually twenty-four inches or six hundred millimetres apart.
What truss type suits a thirty foot span?
A Fink or W truss is the usual choice.
What does six in twelve pitch mean?
The roof rises six inches for every twelve inches across, about twenty-seven degrees.
Can I design my own trusses?
Trusses should be engineered by the maker for your loads. Use this to plan and price.
How long is the top chord?
Work out the slope from the half span plus overhang and the rise, using the right-angle rule.
Plan the roof frame
Count your trusses, check the height and choose the type, then let the truss maker engineer them for your loads.
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