Setting out, explained

Backed or dropped? The hip decision explained

Two ways to get a hip's top edges into the roof planes, what each costs in work, and which one CutRoof uses by default.

A hip is a rectangular section sitting at 45 degrees under two roof planes that slope away from it in opposite directions. Left alone, its two top arrises stand proud of both planes and its centre line sits below them. Battens would rock on the arrises and the felt would bridge the hollow.

There are two ways to deal with it, and CutRoof asks you which one you want because they produce different members from the same roof.

Backing

Backing means bevelling the top edge of the hip along its whole length, both sides, so the top of the hip becomes a shallow roof shape of its own that matches the two planes meeting over it.

The angle is the backing bevel. On a 35 degree roof it is 23.9 degrees each side. The hip then presents two narrow flat faces in the planes, and the battens land on solid timber the whole way up.

It is the accurate answer and it is real work. That is a bevel run down four hips, on a member deeper than everything else on the roof, usually with a circular saw and a straight edge, and the arris is easy to damage in handling once it is cut.

Dropping

Dropping means leaving the top edge square and cutting the birdsmouth deeper instead, so the whole hip sits lower by the amount its arrises were standing proud.

drop = (bₕ ÷ 2) × tan γ = (47 ÷ 2) × tan 23.9° = 10 mm

Ten millimetres deeper at the birdsmouth and the arrises drop into the planes. The centre of the hip's top face now sits slightly below the plane, which is why battens bear on the arrises rather than across the full width, and on most cut roofs that is accepted.

What actually changes on the schedule

The lengths do not move. On the worked example the hip is 3750 to the long point either way, its plumb cut is 63.7 degrees either way, and the double cheek at the ridge is the same cut.

What changes is one line of work at each end of the decision. Choose backed and the schedule gives you a backing bevel of 23.9 degrees to run down both top edges. Choose dropped and it gives you a 10mm drop to take out of the birdsmouth, and no bevel.

Which to pick

CutRoof ships dropped as the UK default, because it is a deeper birdsmouth on a member you are notching anyway rather than a bevel down four long sticks.

Back the hips when the roof is shallow enough that a dropped hip leaves a visible hollow under the battens, when you are working to a specification that calls for it, or when the hip is exposed and the top face will be seen.

Whichever you choose, the app prints it in words on the schedule, so the person cutting and the person checking are working from the same decision.

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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