A dome rooflight is a curved, translucent plastic cover set on a flat or shallow-pitched roof to bring daylight into the space below. It almost always sits on an upstand — the kerb — which lifts the dome above the water level of the roof plane.
The curved form has two practical reasons: it is stiffer than a flat sheet of the same material, and rain and dirt run off it instead of standing on it.
Build-up
- The kerb (upstand). In plastic, timber or insulated board. The roof covering is dressed up the kerb and terminated there. The kerb must be high enough to hold back ponding water and snow; roofing practice generally works to at least about 15 cm above the roof surface.
- The dome itself. In acrylic (PMMA) or polycarbonate (PC). Acrylic is clearer and more scratch resistant, polycarbonate considerably more impact resistant.
- The number of skins. Single, double, or triple to quadruple. Each extra skin improves insulation but costs light transmission.
Types and sizes
Dome rooflights come clear (maximum light transmission, but with a view in and hard shadows) and opal (diffuse light, no view in, slightly less transmission). Variants also exist with:
| Acrylic (PMMA) | Clearer and more scratch resistant |
|---|---|
| Polycarbonate (PC) | Considerably more impact resistant |
| Clear | Maximum light transmission, but with a view in and hard shadows |
| Opal | Diffuse light, no view in, slightly less transmission |
- an opening section, hand operated with a winding pole, electrically or by spindle — particularly useful for shedding summer heat;
- a smoke and heat exhaust function (SHEV), which opens automatically in a fire;
- ventilation grilles in the kerb for permanent background ventilation.
Common clear sizes (the internal dimension of the roof opening) run from around 60 × 60 cm up to elongated units of about 100 × 250 cm. Always order to the clear opening size, not the external size of the dome.
Insulation and condensation
A single-skin dome insulates poorly; a multi-skin unit is clearly better but still behind good insulating glass. More important in practice is the kerb: an uninsulated kerb is a thermal bridge on which condensation forms in winter. That drips inside and is then blamed on a leak while the roof covering is entirely sound. Choose an insulated kerb and make sure the internal finish is airtight, so that warm indoor air cannot get into the construction.
Safety
A dome rooflight is not walkable. Falling through a daylight opening is a well-known cause of serious accidents in roof work. Where people go onto the roof for maintenance, fall protection belongs there — a grille below or within the kerb, or a physical barrier around the dome.
Maintenance and replacement
- Clean the dome annually with water and a soft cloth; abrasives and aggressive cleaners scratch and attack plastic.
- Check the junction between roof covering and kerb at every roof inspection; that is where leaks begin.
- Plastic ages. Yellowing, dulling and crazing are normal signs of age. A dome is a wearing part with a considerably shorter life than the roof covering beneath it.
- Replacement is usually simple. With a sound kerb, the dome alone can be swapped — a limited job, provided you take the size correctly and can work safely on the roof.
Related terms
- Rooflight kerb
- Continuous rooflight
- Light hood
- Flat roof
- Light transmission
- Roof window
- EPDM
Want to know more about building terms? Take a look at our knowledge base on fredsdiyplans.com.