What is a rhombic roof plane?
A rhombic roof plane is a roof surface shaped like a rhombus: a quadrilateral with four equal sides and corners that are not 90 degrees. In practice the plane often takes the form of an inverted kite, with the long point running up to the apex. The word comes from the Greek rhombos, diamond.
Where it occurs
Rhombic roof planes belong to towers and spires, not to ordinary houses. The shape appears by itself when a roof plane narrows towards the top and then closes in a point again. Two situations:
- The broach or constricted spire. The four planes taper in from the tower edge, kink inwards halfway up and end in a single point. Each plane is then an elongated diamond or kite.
- Compound roofs and tower terminations where two ridges or hips meet in such a way that the plane between them becomes diamond-shaped.
Why the shape is awkward to build
A rectangular roof plane is battened with straight battens of equal length. On a rhombic plane the width changes row by row, and that has practical consequences:
- Every course gets shorter. Battens do not run the full width; almost every one is cut to size.
- Large tiles do not fit. A roof tile is a fixed module and cannot be narrowed. Rhombic planes are therefore nearly always covered in slate, wooden shingles, zinc, copper or lead — materials you can trim piece by piece.
- The junctions are hips, not ordinary ridges. They take lead flashings, folded zinc hip pieces or doubled slate courses; this is the most vulnerable point of the spire.
- Pitch. Spires are very steep. Working on a rhombic roof plane means working from a scaffold or suspended platform with full fall protection — not from a ladder.
Application
For custodians of churches, mills and listed buildings this is chiefly a maintenance issue. A spire with rhombic planes needs periodic inspection of the slating and of the lead or zinc in the hips; because of the pitch, a leak on a spire quickly rots the carpentry without anything showing inside. Have the existing covering surveyed before replacement: the slate size and course spacing determine the silhouette, and on a listed building that silhouette is protected.
Related terms
- Roof plane
- Constricted spire
- Hip rafter
- Slate roof
- Roof slate
Want to know more about building terms? Visit our knowledge base at fredsdiyplans.com.