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Pergola/Trellis Rafter Spacing Calculator

Calculate the number of rafters needed to span a pergola or trellis structure at a given on-center spacing.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax
13.12 ft
Schematic, drawn to the proportions you entered — not to scale on screen.

Rafters needed

8 rafters

High confidence

Maximum rafter spacing depends on the rafter material, size, and any planned cover/planting load — confirm the spacing against the specific rafter species/size's span table or a designer's calculation for your load case.

At the values currently entered, the rafters needed works out to 8. Figures are shown for United States, where IRC 2024 is the governing residential reference; switch the market above if you are building elsewhere.

Estimated cost — your price

This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Rafter count = ceil(Pergola Width ÷ Maximum Rafter Spacing) + 1 — the standard support-count method (identical in principle to this site's joist/stud spacing calculators) applied to pergola or trellis rafter layout.

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Frequently asked questions

Why does the calculator add 1 to the divided width?
A rafter is needed at both ends of the span as well as at every interior spacing interval, so the count is the number of spacing intervals (rounded up) plus one for the starting rafter.
Why round up the number of spacing intervals?
Rounding up (ceiling) ensures the actual on-center spacing never exceeds the maximum allowed — if the width doesn't divide evenly, the extra rafter tightens the spacing slightly rather than leaving a gap that's too wide.
What determines the maximum allowable rafter spacing?
It depends on the rafter material, size, and any planned cover or planting load — always confirm the spacing against the specific rafter species/size's span table or a designer's calculation for your load case rather than assuming a default.