A polycarbonate rooflight replacement usually starts with a phone call about a cracked dome, and the useful question is not the dome size but the condition of the kerb underneath it.
A school in Coventry had six domes replaced after storm damage. Four were finished in a day; two needed the upstand rebuilt because the original membrane had been dressed over wet timber and the deck had rotted under the kerb.
Polycarbonate rooflight panel, kerb and upstand
- Panel: 10 mm solid or 16 mm multiwall
- Kerb: continuous, minimum 150 mm above roof deck
- Gasket: EPDM compression, not sealant only
- Expansion: 3 mm per metre of length
The kerb is the weathertightness. A rooflight panel can be perfectly made and still leak because the membrane stops at the kerb face instead of dressing over it, and rainwater tracks back under the flashing.
Glare, condensation and walk-on loading
Glare is what occupants complain about, not light level. A clear 10 mm panel over a classroom gives a hard bright patch; an opal diffuser or a prismatic multiwall scatters it and the room reads as evenly lit.
Condensation on a single skin panel is unavoidable in a heated building, which is why multiwall or a twin skin with a sealed cavity is the right answer over occupied space. The cavity also adds the insulation value that a single sheet cannot.
For UV and weathering performance in strong sun, UV resistant polycarbonate sheet explains the co-extruded cap layer that keeps a panel from yellowing.
Cleaning a rooflight from above is best done dry, and the method sits in polycarbonate care.
Send your upstand dimensions, roof pitch and required U-value to oversales@rpplexiglass.com for a rooflight quote.
Frequently Asked Questions
What thickness polycarbonate rooflight panel?
10 mm for a walk-on-safe access roof, 16 mm where the panel spans over 1.5 m between kerbs. Span and snow load set the gauge; visual size does not.
Why do rooflights leak after replacement?
The new panel is fixed to an old upstand without a new membrane detail. A rooflight seal needs a continuous kerb, a compression gasket and a dressed membrane, not sealant alone.
How much expansion should be allowed?
About 3 mm per metre of panel length for a 40 C temperature swing. A panel fixed tight to a kerb on all four sides bows or cracks at the fixing line in the first hot spell.