How to count a roof skydome: kerb, waterproofing and fixings

Updated October 11, 2026

A skydome is a dome that sits on a concrete kerb (costière) cast around an opening in the roof slab. You order the dome from its overall size, you cast the kerb, and the waterproofing is turned up the kerb and over its top. This guide counts quantities; it does not size the kerb or choose the dome.

1. Opening, kerb and overall size

The clear opening is measured inside the kerb. The kerb outside is the opening plus the kerb thickness on each side. The overall (hors tout) size of the dome adds the overhang of the dome frame beyond the kerb on each side; it is the size you give the maker.

2. Concrete and formwork of the kerb

The plan area of the kerb is its outside area minus the opening. Multiply it by the kerb height for the concrete, and add the waste. The formwork covers both faces: the outside plus the inside perimeter, times the height.

Worked example: 1.00 × 1.00 m opening

Kerb 10 cm thick and 30 cm high, dome overhang 2 cm, 10 % waste, 4 fixings per metre:

Kerb outside 1.20 × 1.20 m; dome overall 124 × 124 cm.
Kerb plan area 1.44 − 1.00 = 0.44 m²; concrete 0.44 × 0.30 × 1.10 = 0.145 m³.
Formwork (4.80 + 4.00) × 0.30 = 2.64 m².
Waterproofing (4.80 × 0.30 + 0.44) × 1.10 = 2.07 m²; sealant bead 4.80 m; fixings 4.80 × 4 = 19.2, so 20.

What this does not cover

The count does not design the kerb or its reinforcement, check the wind uplift, or choose the dome (skins, glazing, thermal and fire performance). Those come from the dome maker, the waterproofing system and the engineer.

General information, checked at the date shown. It does not replace advice from a professional for your situation.

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Tools for this guide

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