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What is the minimum roof pitch for a skylight or a roof window?

Skylights and roof windows are both popular choices for bringing natural daylight into a property, and skylights can also provide a variety of other benefits, such as natural ventilation and access to roof terraces.

When selecting a skylight or roof window, there are several things to consider – one factor that could play an important part in your decision is the pitch of the roof.

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 rooflights and roof windows; this should help you to decide which product is most appropriate for your project.

Installing a skylight on a flat roof

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.

The recommended pitch for skylight installation on a flat roof

For many skylight applications on flat roofs, there should be a minimum pitch or fall of 0.6 in 12. This is enough to ensure that water will drain off the surface of the glass and avoid pooling.

The higher the pitch, the more effective it is at draining off the water, so although 0.6 in 12 is often the recommended minimum pitch, installing the unit slightly higher – for example at 1 in 12 will prove more efficient.

To achieve the required pitch, a curb is usually constructed around the aperture in the roof, which will accommodate the slope into it for the skylight to be structurally fixed to. If you are unsure, the skylight manufacturer should provide drawings to indicate minimum curb heights, which will allow your builder to calculate how high the top of the slope needs to be to achieve the desired pitch.

However, walkable skylights require level pitch for safety reasons. A fall of 0.2 in 12 will help water run off the unit without adding a significant incline.

Steeper pitches for skylight installation on flat roof

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.

The correct pitch for installing a roof window on a flat roof

Unlike skylights mounted onto a curb, roof windows are installed in-plane, meaning they follow the pitch of the roof and are mounted flush with the surface.

Answering the question: “What’s the minimum roof pitch for a roof window?” is also much easier to answer: because the units follow the existing roof pitch they do not require any additional height at one end to allow for drainage, and roof windows should be installed on roofs with a pitch of at least 3 in 12.
Please note that specific requirements may vary depending on the manufacturer. Always check that the specification meets your roof design.

To learn more about skylights and roof windows – from benefits and specification considerations, through to installation – request one of our AIA approved presentations.