Calculate roof trusses
Work out the number of roof trusses before you order timber or request a quote. Enter the roof length and centre spacing (c/c) and you get the count straight away – a quick material estimate, free and no account.
Roof trusses – number from c/c
Enter the roof length and the spacing (c/c) and we work out the number of roof trusses.
An estimate (count = length ÷ c/c + 1). A common c/c is 1200 mm, but follow the engineer’s dimensioning for your truss and snow load.
This tool gives an estimate and is an aid – not a finished calculation. Always check the result against drawings, applicable regulations, supplier data and your professional experience before giving a binding price, ordering material or using the file. ByggExp is not responsible for decisions made solely on the basis of the tool.
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How to calculate the number of trusses
The truss count uses a simple formula: count = roof length / c/c + 1. The extra truss is so that both gables also get a truss.
- Measure the roof length in metres.
- Enter the spacing (c/c) in mm.
- See the number of trusses instantly.
Example truss calculation
A roof 10 metres long at c/c 1200 mm: 10,000 / 1200 = 8.33, rounded up to 9, plus 1 = 10 trusses. At a closer c/c of 600 mm it becomes about 18 trusses for the same roof – so the spacing drives both the count and the material.
Which c/c spacing should the trusses have?
The spacing (c/c) depends on the roof covering, sub-roof and snow load. A common spacing is 1200 mm for prefabricated truss rafters, but closer c/c (e.g. 600–900 mm) occurs with heavy covering or high snow load. Always use the c/c specified by the engineer for your roof.
Count vs. dimensioning of trusses
This calculator does a truss count – not a structural calculation. Dimensioning of trusses (choice of timber sizes, span and fixings from snow and wind load) must be done by an engineer per Eurocode. Use the count here for material and quotes, and always follow the engineer’s drawing for the execution.
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Frequently asked questions
- How do you calculate the number of trusses?
- Take the roof length divided by the spacing (c/c) and add one truss. A 10 m roof at c/c 1200 mm gives 10 trusses. Enter your measurements and the calculator does it for you.
- How many trusses do I need?
- It depends on the roof length and the c/c you choose. Always round up and add one truss for the gable – that is why the formula ends in +1. The calculator handles the rounding.
- What c/c spacing do trusses have?
- Common is 1200 mm, but it depends on the covering, snow load and dimensioning. Follow the engineer’s specification.
- What is the difference between counting and dimensioning trusses?
- This calculator counts trusses from c/c. Dimensioning – determining timber sizes from span and snow load – is done by an engineer per the applicable load.
- Are gable ends and noggins included?
- No, the calculator counts the ordinary trusses. Gable construction and trimming come in addition.
- Does it cost anything?
- No, the roof-truss calculator is free and requires no account.