Roof Windows? They are all the Same, Right?
The quick answer is that they aren’t. What you might consider a roof window someone else may regard as it to be something totally different. You might want to consider the terms roof light and skylight too.
Firstly, are you looking for a roof window for a traditional flat roof or a pitched roof?
This can be confusing because flat roofs aren’t flat (they should have a fall) and also pitched roofs could be covered with flat roof materials. Add to this that some pitched roofs are too shallow for a traditional pitched roof window to be used.
Roof Windows for Pitched Roofs
So what roof covering do you have? Profiled tiles, plain tiles, slates, stone, lead, zinc, copper, etc. Then these would be the material for a traditional pitched roof in the UK.
Roof windows for these types of construction are normally flat glass and would almost certainly be fitted in-plane (in-line) with the roof materials. However low roof pitches may require roof windows to be out-of-plane (out of the line of the roof materials) so water cannot pond or enter at the head or the sides of the roof window.
There are also occasions when larger bespoke glass or polycarbonate roof windows are used along the ridge line of the pitched roof. These would be out-of-plane by design too.
Would we class a tubular skylight as a roof window? Well yes, because the result is the same, it lets light into the property for both pitched and flat roofs.
Ventilation can be included within the roof window but these tend to be hinged in some way using a manual or electrical type of operations.
Roof Windows for Flat Roofs
So what roof covering do you have? Mastic asphalt, bituminous membranes (roll and pour, torch-on), single-ply rubber/EPDMs, GRP, liquid solutions etc. These would be the material for a traditional flat roof in the UK.
Roof windows for these types of construction are normally glass or polycarbonate and would certainly be fitted out-of-plane. They may also be profiled such as a dome, barrel vault, pyramid, ridge light or just flat. The roof window would also require a manufacturer’s upstand or builder’s kerb, which has to be waterproofed using the materials above.
Ventilation can be included within the roof window. This could be a hinged option using a manual or electrical type of opener or it could also be permanent or controllable ventilation within the upstand (hit and miss or trickle vents). There are sliding opening options available as well as access roof windows and automatic smoke ventilators too.
If your roof will be a warm roof i.e. the insulation is above the roof deck then the manufacturer’s upstand or builder’s kerb must still protrude 150mm from the finished waterproofing level to the designed termination point. This is normally just under the point where the glazing starts but can also be the underside of any vent section. If you have a cold roof construction then it still must project 150mm from the finished waterproofing level to the intended termination point.
If your roof is a green or upside-down roof i.e. the insulation, flags, stones, sedum, filtration, pebbles etc. is on top of the roof deck then the manufacturer’s upstand or builder’s kerb must still protrude 150mm from the horizontal finished top surface to the designed termination point. Again, this could be where the glazing starts but can also be the underside of any vent section.
The 150mm is a good code of practice requirement to allow for snow build up and potentially ponding around the roof window.
Walk-on roof windows have become popular but these still require the 150mm rule. You need to create a trough around the roof window to allow fast water run-off etc.
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