What is polygonal?
Polygonal means many-sided: made up of a number of straight sides meeting at angles. In building it describes plans, towers, vaults and masonry bonds built from flat facets rather than a curved surface.
Common variants have their own names: triangular, square, pentagonal, hexagonal, octagonal. “Polygonal” is the umbrella term.
Where you meet it
- The apse of a church. The east end of a chancel is often closed not in a semicircle but with three, five or seven sides of a polygon — the polygonal apse, characteristic of the Gothic.
- Towers and stair turrets. An octagonal tower or an octagonal lantern on a square base is a classic way of making the transition to a spire.
- Windmills. The octagonal smock mill is one of the most recognisable polygonal building forms: a timber octagon on a masonry base.
- Domes and vaults. A polygonal dome consists of flat or slightly curved fields between ribs, rather than one continuous shell.
- Bays and conservatories that project with three or five facets.
Polygonal masonry
The term is also used for a stone bond in which irregular, many-sided blocks interlock like a puzzle, with joints running in every direction. That is the classic “cyclopean” or polygonal walling known from Greek and Italian fortification walls and from retaining walls in hilly country. It demands craftsmanship: every stone is dressed individually to fit.
The practical side
- Angles are not 90 degrees. In a regular octagon the internal angles are 135°, in a hexagon 120°, in a pentagon 108°. The internal angle of a regular polygon is (n − 2) × 180° / n. Work this out before you cut; mitre angles are half the angle between two faces.
- Set out from the centre. The most accurate way to lay out a regular polygon is from a circumscribed circle: the corners lie at equal distance from the centre, at a central angle of 360° / n.
- Finishing at the corners. With a polygonal plan, skirtings, roof edges and floor finishes all run in mitre. Allow for the fact that this work goes slower than a right-angle corner.
- Roof planes. A polygonal roof has as many hips as it has sides, and they all converge on a single point. That point is the most vulnerable detail on the whole roof.
Related terms
- Tracery
- Gothic
- Hip rafter
- Natural stone
- Architectural style
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