What is a dihedral angle in roofing?
A dihedral angle is the angle between two planes that meet along a common line. In Dutch building terms it is called a standhoek. It is measured at right angles to the line where the planes meet, like the angle you see when you look down on the spine of an open book. In construction it usually refers to the angle between two roof slopes, for example at the hip rafter of a hipped roof or at a valley rafter where two roofs meet in an internal corner.
The term is also used for the position of that intersection line relative to a third plane, such as the horizontal. With a hip rafter you are dealing with three planes: two roof slopes and the horizontal plane on which the roof sits.
Why the dihedral angle matters
- Backing the hip or valley rafter. The top edges are bevelled so that both roof slopes sit flat on it. Carpenters call this the backing bevel.
- Fitting roof boarding and battens against the hip or valley.
- Bending lead or zinc, such as a valley gutter, to the correct angle.
- Cutting mitres for fascias, soffits and trim.
Application
- Draw it full size. In roof setting-out, the angle is found by drawing the slopes true to size in an auxiliary view, so you can measure it directly.
- Equal pitches give a symmetrical angle. If both slopes have the same roof pitch, the backing is the same on both sides; with unequal pitches it differs.
- Measure with a sliding bevel and transfer the angle to the workpiece.
- Make a test piece from offcuts before bevelling the expensive rafter.
Common mistake: using the roof pitch itself as the backing angle. The dihedral angle at a hip is a different angle; using the pitch leaves gaps or a hump in the roof surface.
Related terms
- Hip rafter
- Valley rafter
- Hip roof
- Roof pitch
- Valley gutter
Want to know more about building terms? Visit our knowledge base at fredsdiyplans.com.