Rafter Size Calculator
Enter the building span, the roof pitch, the overhang and the building length, and choose the rafter spacing. The calculator gives the rafter length, the board length to buy, how many rafters and a guide to their size.
- Length with overhang
- Count by spacing
- Lumber size guide
Span is the outside width of the walls the roof sits on. Overhang is measured level, out from the wall.
Rafters are the sloping boards that carry a pitched roof. Each one runs from the wall plate up to the ridge, plus an overhang. The run is half the span, and the length is the run times the pitch factor. Longer runs need deeper boards.
Rafter length chart
6/12 pitch, 1 ft overhang. Search for a span.
| Span (ft) | Run (ft) | Rafter (ft) | Buy (ft) | Size |
|---|---|---|---|---|
| 12 | 5.94 | 7.76 | 8 | 2×4 |
| 16 | 7.94 | 9.99 | 10 | 2×6 |
| 20 | 9.94 | 12.23 | 14 | 2×6 |
| 24 | 11.94 | 14.47 | 16 | 2×8 |
| 28 | 13.94 | 16.7 | 18 | 2×8 |
| 32 | 15.94 | 18.94 | 20 | 2×10 |
| 40 | 19.94 | 23.41 | 24 | 2×12 |
Sizes are a rough guide at 16 in centres. Check local span tables.
Span and rafter
6/12 pitch. Slide the span.
The coloured board is one rafter, from the eave to the ridge.
How to work out rafter length
Run times factor. Step through it.
Half the span
The run.
Cut the birdsmouth notch where the rafter sits on the wall, no deeper than a third of the board.
Rafters, trusses or I-joists
Roof framing. Compare them.
Cut on site.
Factory made.
Engineered timber.
Trusses are made in a factory and arrive ready, but they leave little loft space.
Before cutting
Framing checks. Tick each item.
Tick each item as you go.
Snow and tile roofs weigh much more, so use local span tables for the load in your area.
What you get
Rafter sizing. Each tile explains one result.
Each rafter.
Board to buy.
Both sides.
Angle.
2×4 to 2×12.
Cut one pattern rafter, test it at both ends of the roof, then use it to mark the rest.
Frequently Asked Questions
No question matches that search.
How do I work out rafter length?
Take half the span, add the overhang and multiply by the pitch factor.
What size rafters do I need?
Roughly 2×6 for runs to about 11 feet and 2×8 to about 15 feet at 16 inch spacing, but check local span tables.
How many rafters do I need?
The building length in inches divided by the spacing, plus one, then doubled for both sides.
What is a pitch factor?
How much longer the sloped rafter is than its level run.
What is the plumb cut angle?
The angle at the top of the rafter where it meets the ridge, equal to the roof's pitch angle.
What is a birdsmouth cut?
A notch where the rafter sits on the wall plate.
Should I use rafters or trusses?
Rafters give open loft space. Trusses are quicker to fit and span further.
Rafters cut right first time
Work out the length from the span, pitch and overhang, round up to a board you can buy and check the size against local tables before you cut.
More size calculators
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