What is the minimum roof pitch for a skylight?

Roof pitch affects drainage, determines how the units should be installed and even what type of products you can use. Below, we explain the different requirements for skylights and roof windows; this should help you to decide which product is most appropriate for your project.

When installing a skylight on a flat roof make sure the product is not fitted completely flat itself. Glass inherently has a degree of flex when installed flat (as opposed to vertically as you would find in traditional windows and doors).

This is known as ‘deflection’ and means that without pitching the skylight up slightly at one end, rainwater will accumulate and begin to ‘pool’ on the glass. When this evaporates it can leave unsightly marks and stains behind.

Manufacturers therefore recommend a ‘minimum pitch’ to install their product so that rainwater and debris runs off the glass more effectively, keeping the skylight cleaner for longer.

Skylights can be successfully installed at steeper pitches, but for flat roof applications the height at the top end of the curb will begin to be so great it will compromise aesthetics and potentially contravene planning permissions, which in some cases will limit the height of any structure that can be built above roof level.

Depending on the skylight design, installing on more harshly pitched curbs may result in fouling the framework, so you should always check with manufacturer drawings and recommendations prior to commencing works on site.

Other considerations for roof pitch and specification

Here are a few more things to think about in relation to roof pitch and specifying skylights.

• Deflection – There is a level of flex that will occur in all glazing – and it can be particularly apparent in overhead installations. Wind loads, snow loads and the weight of the glass itself can contribute to the amount of deflection, and this can hinder drainage.
• Orientation – Pay attention to the orientation of the skylight: if a unit has a 16ft width and a 3ft span, it’s better to build the pitch into the span dimension so the water has less distance to travel.
• Capping – Because skylights have minimal pitch, it’s a good idea to check products closely to ensure there is no external capping around the edges. This is because capping can trap moisture and dirt, which can result in unsightly messes and damage to the unit.