Free Tool
Roof Rafter Span Calculator
Look up the maximum horizontal span of a wood roof rafter from the Ontario Building Code span table. Choose the species, grade, size, spacing and roof snow load.
Roof Rafter Span Calculator
Result
All sizes with these settings
Preliminary planning only. This tool does not replace the drawings. The designer or engineer of record and the building permit set govern every size, span and detail.
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Change the numbers in the link to match your job. The full list of tools is at /api/tools/.
Where the numbers come from
- Source. Table 9.23.4.2.-F, Maximum Spans for Roof Rafters, Specified Roof Snow Loads 1.0 to 2.0 kPa, in Division B Part 9 of the 2024 Ontario Building Code. The values are read directly from the table.
- Horizontal span. Appendix A note A-9.23.4.2. explains that for sloping members the span is the horizontal distance between supports, measured from the inside face of one support to the inside face of the other, and that snow loads are on the horizontal projection of the roof.
- Snow load. Pick the column at or above your specified roof snow load. A load between two columns takes the higher column, which is the safe side.
- Not covered. Snow loads above 2.0 kPa (Table 9.23.4.2.-G), roof joists that carry a ceiling, heavy roofing under Article 9.23.4.5., ridge beams, trusses and engineered rafters.
- Conversion. Feet and inches are converted at 25.4 mm per inch. The maximum span is shown rounded down to the whole inch.
This tool is for preliminary planning. The designer or engineer of record and the building permit set govern.
Frequently asked questions
Is the rafter span measured along the slope?
No. In the Part 9 span tables the span of a sloping rafter is the horizontal distance between supports, and the snow load is on the horizontal projection of the roof. Measure the run in plan, not the length of the rafter.
Which snow load should I choose?
Use the specified roof snow load from the drawings or the climatic data in Supplementary Standard SB-1 for your municipality. This tool covers the 1.0, 1.5 and 2.0 kPa columns of Table 9.23.4.2.-F. If your load falls between two columns, use the higher one.
What is the difference between a rafter and a roof joist?
In the span tables a rafter is a sloping member that supports the roof sheathing and encloses an attic space without supporting a ceiling. A roof joist supports the sheathing and a ceiling finish and does not enclose an attic space. This tool covers rafters.
Does a collar tie or ridge beam change the span?
The table gives the maximum span of the rafter between supports. A ridge beam, a purlin or a wall below creates a support point. Ridge beams are sized from Table 9.23.4.2.-L, which this tool does not cover.
Can I use this for trusses or engineered rafters?
No. Trusses and engineered lumber come with their own design and shop drawings. The table is for dimension lumber.
Is this a quote?
No. It is a planning tool. Contact us with your plans and we will arrange a free site visit.
For the trade-offs between rafters and trusses, read Stick-Built vs Truss Roofs.
Related: Roof and truss framing, Roof pitch and rafter length calculator, Floor joist span calculator.
More free tools are on the framing tools page.
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