Speed square rafter layout
Updated: 22 October 2026
You have a UK drawing that says 35°, a common rafter to mark, and a video that says to set 6 and leave degrees alone. Speed square rafter layout is the plumb line, the separate row for a hip or a valley, and the stop before the square designs the roof. A pitch mark chooses an angle. Span, load, and bracing sit outside the tool. Prices stay on the [speed square comparison](/hub/measuring-and-marking-tools/speed-square). Peter F. wrote this on 21 October 2026 from those listings and the public sources named below. No laboratory test. A qualifying purchase can earn us a commission at no extra cost to you.
You have a UK drawing that says 35°, a common rafter to mark, and a video that says to set 6 and leave degrees alone.
You have a UK drawing that says 35°, a common rafter to mark, and a video that says to set 6 and leave degrees alone. Speed square rafter layout is the plumb line, the separate row for a hip or a valley, and the stop before the square designs the roof. A pitch mark chooses an angle. Span, load, and bracing sit outside the tool. Prices stay on the speed square comparison. Peter F. wrote this on 21 October 2026 from those listings and the public sources named below. No laboratory test. A qualifying purchase can earn us a commission at no extra cost to you.
Our top picks
| Badge | Product | Scales | Size | Marking | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  OX OX speed square set, 180 mm and 300 mm | Metric | 180 and 300 mm | Laser-etched | 7.8 | about £29.79 | View offer |
| Alternative |  DeWalt DeWalt speed square, 18 cm | Metric | 180 mm | Stamped | 7.2 | about £16.25 | View offer |
| Alternative |  Swanson Swanson speed square kit, 7 inch and 12 inch | Imperial | 7 and 12 inch | n/a | 8.0 | about £21.84 | View offer |
On this page
A UK pitch is a degree, not a rise per 12 inches
On a UK drawing the pitch is usually a degree. For a speed square roof pitch in degrees, pivot on the degree scale that matches the paper. When the drawing says 35°, set 35 on the degree row and mark the plumb line along the body, with the pivot tight to the top edge of the rafter. That is a mark you can make while the job is still in degrees. DeWalt’s UK page describes top cuts for common, jack, hip, and valley on its metric square. OX’s UK page describes metric marks and conversion tables. Those tools list degree and metric rafter scales. They are a different system from a rise-per-12 square.
A US framing video often says the opposite: stay on the rise number and skip degrees. That advice fits an imperial common scale, where the number is inches of rise per 12 inches of run. It fights a drawing that already says 35°. If the only figure you have is that degree, the degree row is the one that matches the paper. The unit fork is on metric vs imperial speed square. A square with neither a degree row nor a common scale is not ready for this layout. Holes from 3 mm to 15 mm on a Preciva triangle are a drill gauge. They do not carry a pitch.
Plumb cut on the common scale
Swanson’s common-rafter page is the imperial one-number method, for when you have chosen that language on purpose. Pivot on the top edge, set the rise number on the COMMON scale, and mark the plumb line along the body. In that US method a common rafter runs square to the ridge in plan, and the scale assumes 12 inches of run. The number you set is inches of rise for that run. It is imperial teaching, not a UK span in millimetres. No cited table here gives a worked rafter length in millimetres, so none is printed.
Swanson’s pages disagree, and both figures stay as printed. The common-rafter page calls a 5 inch rise 22.5°. The pitch-reference page states 5/12 as 22.62° and 6/12 as 26.57°. That gap is Swanson’s own rounding. Leave it un-averaged. From a UK drawing, mark the degree on the drawing. On the inch-rise scale, set the rise number that scale asks for. The plumb line gives the angle at the ridge. It does not give a millimetre length, and it does not choose C16 or C24. Hip and valley use the other row.
Hip and valley use the other row
A hip or a valley runs diagonally, so its run is longer than the common’s for the same rise. Swanson’s hip-and-valley page says to set the same rise number on the HIP-VAL scale. Their teaching figure for that longer run is 16.97 inches, rounded to 17, against 12 inches for the common. At a 5 inch rise their example is a common length of 13.00 inches per 12 inches of run, and a hip or valley length of 17.69 inches per that 17 inch run. Those figures are imperial teaching numbers from Swanson. They are not a UK span table.
Setting a hip on the common scale changes the plumb angle, because a 12 inch run has been applied to a member whose run, in that method, is about 17 inches. Use HIP-VAL for a hip or a valley, and COMMON for the common. In this set, DeWalt’s 18 cm square, the OX metric pair, and the Tracer 110 mm square are the listings that name hip and valley or rafter scales. They list the scales. The 17 inch run is Swanson’s imperial method, not a description of those metric rows. Swanson SW1201K is the rise-per-12 pair. A correct line still does not pick a 47 mm section in C16 or C24.
The mark is not the timber size
Eden District Council publishes pitched-roof timber sizes in bands of 15–22.5°, 22.5–30°, and 30–45°, for C16 and C24, at centres of 400 mm, 450 mm, or 600 mm, for tiles or slates on laths over sarking felt. Heavier coverings and unusual loads go to Approved Document A, and the note says to ask building control. A pitch mark does not choose the timber, and this page does not reprint Eden’s span table. When the span, the covering, or the centres sit outside that guide, the square has finished its job.
Approved Document A, the England 2013 edition, notes that a traditional cut roof usually has some in-built resistance, and that diagonal bracing should be considered for single-hipped and non-hipped roofs over 40° on detached houses. That is a structure point, not a marking sequence. A birdsmouth in a video is a seat cut on a rafter somebody has already sized. Stop when the job changes the structure, when the span is outside the published table, when you cannot guard a bench saw, or when a failed flip test would land in a structural cut. The flip test is on how to use a speed square. Call a carpenter or building control when the load sits outside the guide. The square marks a line.
Common questions
How do you mark a plumb cut when the drawing says 35°?
Set 35 on the degree scale and mark the plumb line along the body, with the pivot tight to the top edge of the rafter. A UK drawing usually gives the pitch in degrees, so that row matches the paper. The figure 6/12 is rise per 12 inches of run on an imperial common scale. Use it when the job is written in inch-rise. A metric square that prints degrees can take 35° directly.
What happens if you set a hip on the common scale?
The plumb angle comes out wrong for the hip. In Swanson’s method the common run is 12 inches and the hip or valley run is about 17 inches, from 16.97 inches rounded to 17. The same rise number belongs on HIP-VAL. A hip marked on the common row uses the shorter run. The line can look tidy and still miss the cut.
Why do Swanson’s pages say both 22.5° and 22.62° for a 5 inch rise?
The common-rafter page calls a 5 inch rise 22.5°. The pitch-reference page states 5/12 as 22.62°, and 6/12 as 26.57°. Both figures are Swanson’s, on different pages, and they are left as printed. If the drawing states a degree, mark that degree. On the inch-rise scale, set the rise number the scale asks for rather than averaging the two lines.
Does a speed square tell you the rafter length in millimetres?
The sources here do not give one. Swanson’s example at a 5 inch rise is 13.00 inches of common length per 12 inches of run, and 17.69 inches of hip or valley length per 17 inches of run. Those are imperial teaching numbers. No UK millimetre length is copied here. Eden’s notes size timber by pitch band, and the span table stays on Eden’s page.
When do you stop and ask building control?
Stop when the span, the covering, or the arrangement sits outside the published guide. Eden sends heavier coverings and unusual loads to Approved Document A and to building control. A pitch mark does not size the rafter, and a birdsmouth in a video does not change that. Also stop if the work alters the structure, if you cannot guard a bench saw, or if a failed flip test would be cut into the roof.
Related pages
Sources
Public sources listed below. Prices and star lines are from the twelve amazon.co.uk listings captured on 19 October 2026.
- Swanson, Reading the Speed Square (retrieved 19 October 2026)
- OX Tools UK, 180 mm metric rafter square (retrieved 19 October 2026)
- DeWalt UK, DWHT25227-0 180 mm metric speed square (retrieved 19 October 2026)
- HSE, WIS16 Circular saw benches (retrieved 19 October 2026)
- BSI, BS 3322:1981 Carpenters' squares and bevels (retrieved 19 October 2026)
- BSI, BS 939:2007 Engineers' squares (retrieved 19 October 2026)
- legislation.gov.uk, Consumer Rights Act 2015 (retrieved 19 October 2026)
- Eden District Council, Pitched-roof timber sizes (retrieved 19 October 2026)