rafter span calculator

Rafter Span Calculator

Rafter span is measured on the horizontal projection and depends on species, grade, size, spacing, loads, snow, ceiling attachment, and local code.

Inputs

Results

Immediate estimate

Source-indicated max span17.1 ftProposed horizontal span is within this limited lookup.

Proposed span14 ft

Spacing16 in o.c.

Rafter size2x8

ResultWithin lookup

Review the limitations before using this result for planning.

This MVP lookup is limited to selected species, sizes, spacings, and simple loading assumptions. Do not extrapolate.

Rafter tables use horizontal projection, not sloped rafter length. Snow, ceiling attachment, dead load, grade, notches, thrust ties, and local code can change the answer.

Diagram

Rafter span

A proposed horizontal rafter span is compared with a limited planning lookup.

Ridge
Wall plate
Horizontal span

How this calculator works

Select species group, rafter size, spacing, and proposed horizontal span to compare against a limited rafter lookup.

Lookup only

This MVP covers a small selected set of common rafter rows and refuses unsupported combinations instead of extrapolating.

Horizontal span

Rafter span tables use horizontal projection, not the sloped board length.

Cautious publishing

The tool is useful for early screening and SEO, but it is not a roof design or permit-ready table.

Lookupmatch species group + rafter size + spacing + roof live load

Comparisonproposed horizontal span <= source-indicated span

Assumptions

Lookup scope: Selected common species groups, sizes, spacing, and 20 psf roof live load only.
Not checked: Snow loads, ridge beams, collar ties, ceiling joists, bearing, connectors, and lateral bracing.

Worked examples

2x8 southern pine rafters

Common limited rafter lookup.

17.1 ftSource-indicated max span

Wide spacing check

Shows a proposed span comparison.

16.2 ftSource-indicated max span

FAQ

Is rafter span the same as rafter length?

No. Span is the horizontal projection between supports; rafter length follows the roof slope.

Does this include snow load?

No. The MVP lookup is limited and should not be used for snow-region design without adopted local tables.

Can I use this for vaulted ceilings?

Do not rely on this MVP for vaulted, cathedral, ridge-beam, or thrust-sensitive roof framing.

Sources and review

2021 International Residential Code, International Code Council, 2021. Accessed 2026-07-16.
Design Values / Span Tables, American Wood Council. Accessed 2026-07-20.
Online Span Calculator Help, American Wood Council. Accessed 2026-07-20.

Reviewed 2026-07-20. Planning estimates only; confirm project-specific requirements.