Setting out, explained

HAP: the number that keeps your roof planes true

What the height above plate is, where it comes from, and why it is the figure to check before the rest of the roof goes on.

HAP is the height above plate: the vertical distance from the top of the wall plate to the top edge of the rafter, measured at the plate.

On a 47 by 147 rafter at 35 degrees with a 44 notch, it is 126mm.

HAP = (d − notch) ÷ cos θ = (147 − 44) ÷ cos 35° = 126 mm

Why it matters more than the notch depth

The roof plane is defined by the top edges of the rafters, not by their undersides. Two rafters with identical notches but different HAP will not sit in the same plane, and the difference shows up as a hollow or a hump under the battens.

Anything landing on that plane has to agree with it. The hip has to arrive at the same height at the corner. The jacks have to come up to the same line where they meet the hip. If the HAP is out on one member, that member fights the others for the whole length of the roof.

That is why the figure is worth writing on the wall plate and checking with a tape rather than trusting it. Measure from the plate top to the rafter's top edge at the plate face, plumb, on the first rafter down. If it is not the number on the schedule, the notch is wrong and the rest of the set is about to be wrong the same way.

What changes it

Two things move the HAP, and neither of them is obvious from the outside.

Rafter depth. The same roof in 47 by 195 at 35 degrees, on a 150 wall plate, takes a 59 notch and gives a HAP of 167 instead of 126. Deeper rafter, more timber above the notch, higher plane. Mixing sections on one roof means mixing HAPs, which is a good reason not to.

A notch that got capped. When the seat cut will not fit on the plate, the app shortens the seat and reduces the notch to suit. A shallower notch leaves more timber above it, so the HAP goes up rather than down. The schedule flags the cap and prints the HAP it actually used.

Hips and HAP

A hip is a wider member sitting at 45 degrees under two roof planes, so its top arrises want to sit above them. The fix is to drop it or to back it, and both treatments are aimed at the same thing: getting the hip's top edges into the planes the commons have already set.

That decision is convention C5, and it has its own article. Whichever way it is set, the number to check on site is still the height at the plate.

On the schedule

CutRoof prints HAP against the common rafter and again in the roof figures, because it is the one number that ties every member to every other one. It is the first thing to check when a roof looks right on the schedule and wrong on the wall.

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

Try the rafter calculator

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