Setting out, explained

Reading plumb and seat cuts off a square

Why an angle on a roof has two right answers, which reference CutRoof states them from, and how to tell the two apart before you set the saw.

Every angle on a roof can be stated two ways: measured against the edge of the timber, or measured from a line squared across it. They add up to 90, they are rarely the same number, and mixing them up puts a cut on the wrong side of square.

Cuts you mark with the square

The plumb cut and the seat cut come straight off a framing square, using the run and the rise as they are.

On a 4800 span at 35 degrees the run is 2400 and the rise is 1680. Scale those down and you have 240 and 168, which is a pair of figures that fits on a square. Hold the square on the rafter with the run figure on the blade and the rise figure on the tongue, both on the same edge of the timber, and the line along the tongue is the plumb cut. The line along the blade is the seat.

Any pair of run and rise numbers in the same ratio gives the same two cuts, which is why a square works on a roof of any size.

The two references

Lay a sliding bevel's stock along the rafter's top edge and the seat cut reads 35 degrees, the same as the pitch. The plumb cut reads 55. Square a line across the timber instead and the two swap over: plumb 35, seat 55.

Nothing has moved. It is the same cut, measured from two lines 90 degrees apart, so you get an angle and its complement.

The check that fixes the direction in your head is a shallow roof. At 5 degrees the seat is a long thin sliver along the rafter's edge and the plumb cut is nearly square across it. Against the edge that reads seat 5, plumb 85. Goss prints SEAT 5 and RIDGE 85, which tells you which reference the book uses.

Which one CutRoof puts in front of you

CutRoof states every angle against the edge of the timber, because that is what the standard UK reckoner does and a schedule that disagrees with the book in your van is a schedule you have to stop and check.

So on the worked example you get:

Plumb cut (ridge): set bevel to 55.0° against the rafter edge

Seat cut (wall plate): set bevel to 35.0° against the rafter edge

Edge bevels come off the same reference. The jack's cut into the hip is 39.3 against the edge, the hip's double cheek at the ridge 41.9. Both are the figures Goss prints.

Convention C6 switches the schedule to the other reference, where those four cuts read 35, 55, 50.7 and 48.1. Nothing is wrong with those numbers; they are the same cuts stated from the square line. Whichever is in force, the schedule prints it in words on every surface, so nobody has to guess whether a figure on a bit of paper means the edge or the square.

A quick check

An angle and its complement add to 90. That is enough to catch most mistakes before they reach the saw.

Be careful with a figure near 45, where both readings look plausible and the label is the only thing telling them apart. Read it before you set the bevel.

Every figure above is worked out by the same engine that produces CutRoof's cut schedules, and is checked against it on every build.

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