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Construction & renovation

Rafter calculator

Calculate rafter length, count, and total timber for a simple gable roof. Calculations stay local in the browser and the assumptions are adjustable.

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Project settings

Calculate rafter length and estimate pairs and total timber for a simple gable roof.

Half span
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Roof rise
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Run with overhang
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Rafter length
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Rafter pairs
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Total rafters
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Length with waste
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Quantity estimate only: adapt the values to the product, the site, and the applicable technical rules.

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How does this tool work?

Calculate rafter length and estimate pairs and total timber for a simple gable roof. Adjust the assumptions to compare several scenarios before preparing a material order.

The roof rafters calculator runs entirely in the browser from the dimensions and parameters you enter. The engine uses half span and roof pitch to calculate rise and sloped length, includes the horizontal overhang, and derives the number of rafter pairs from spacing along the building. Every result is recalculated when an input changes, so different assumptions can be compared before ordering. The calculation remains a quantity estimate: it does not verify building-code compliance, structural design, or product-specific installation requirements.

Geometric rafter length comes from half span, pitch and overhang and can be checked with trigonometry or Pythagoras. Timber section, allowable span, loads and connections remain a separate structural-design task.

How to use this tool

  1. Describe the project

    Enter the dimensions and parameters that match the real site.

  2. Adjust assumptions

    Match spacing, densities, stock sizes, and waste to the products you plan to use.

  3. Check the quantity

    Use the result to prepare an order, then verify it against the relevant technical requirements.

Use cases

Prepare an order

Turn project dimensions into a first roof rafters quantity to compare with supplier pack sizes and stock lengths.

Compare assumptions

Change dimensions, spacing, density, or waste and immediately see how the required quantity changes.

Check a quotation

Use the intermediate results to verify the order of magnitude of a material quantity in a quote.

Tips and best practices

  • Measure the actual site or finished support instead of relying only on drawings.
  • Replace the suggested values with the manufacturer or supplier specifications.
  • Use a waste allowance that reflects cuts, offcuts, overlaps, and site constraints.

Frequently asked questions

Does this replace structural or technical design?

No. The calculator estimates quantities from geometric inputs. Strength, sections, reinforcement design, spans, and code-required thicknesses must be checked separately.

Why include a waste allowance?

Waste covers some cuts, breakage, offcuts, overlaps, and measurement variation. The appropriate percentage depends on the material, layout, and site.

Can I order from the default values?

The defaults are examples only. Before ordering, replace them with the real dimensions and the technical specifications supplied for the product you will use.

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