Why Important
Cupolas are a feature of the Georgian period and Edwardian period and are a very elegant way of letting light down through the centre of the building, usually to a stairwell. They are often elliptical or round in plan and constructed of a timber kerb with thin timber astragals bridging from the kerb to a central high point.
What to look for
Cupolas always sit in a flat area of roofing finished in lead sheets.
The roof is an exposed position for this complex joinery and decay can go unnoticed from inside, so don’t assume that because it looks sound from underneath it doesn’t require external inspection.
Cupolas are particularly susceptible at the base of the astragals which have exposed timber end grain.
Cupolas are single-glazed so some condensation on the underside of the glass is inevitable. If the glazing putty cracks, water can sit in the crack and rot the timber hidden from view underneath. Glass can crack due to impact or movement (for example due to timber rot) or if a piece of glass slips.
Traditionally, it is held in place at the base of the pane by lead straps, fixed to the kerb and turned up over the glass. However, due to thermal movement, the lead can slump and the glass creeps.
Cupolas often go unpainted because they are so awkward and dangerous to get to.
When to look
How to fix
A solution to this problem is to flash the cupola by dressing thin lead sheet over the astragals. Although initially expensive, this will be relatively maintenance-free and save paint work and timber repair costs in the longer term.
Lead sheet dressed up over timber ring and secured with copper clips formed by the thickness of the lead strips at the base of the pane is enough to allow air movement across the underside of the glass and form a weep-hole for water to get away.
Modern safety glass should always be used when replacing a pane or reinstating a cupola.
Avoiding creating problems
Do not block this gap or condensation will cause corrosion of the lead (seen as a white powder) at the foot of the kerb or rot the timber kerb or astragals.
Cupolas and lanterns generally have a vent at the head which must also remain open.