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

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

When selecting a rooflight or roof window, there are several specification considerations to think about – and 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 rooflight on a flat roof

When installing a rooflight on a flat roof you should ensure that 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 rooflight up slightly at one end, rainwater will accumulate and begin to ‘pond’ 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 rooflight cleaner for longer.

The recommended pitch for rooflight installation on a flat roof

For many rooflight applications on flat roofs, there should be a minimum pitch or fall of three degrees. This is enough to ensure that water will drain off the surface of the glass and avoid ponding.

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

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

In the case of walk on rooflights, which for obvious reasons need to be as level as possible, this pitch is not required. Walk on units will most likely be accessible, so will be easier to access for cleaning. However, a fall of one degree will still help water run off without making the rooflight feel like it’s significantly sloped.

Steeper pitches for rooflight installation on flat roof

Rooflights can be successfully installed at steeper pitches, but for flat roof applications the height at the top end of the upstand 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 rooflight design, installing on more harshly pitched upstands 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 rooflights.

● 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 rooflight so that you can maximise. For example, if a unit has a 5m width and a 1m span, it’s better to build the pitch into the span dimension so the water has less distance to travel.

● Capping – Because rooflights 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 rooflights mounted onto an upstand, 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.

According to EN 14351-1:2006, roof windows should be installed on roofs with a pitch of at least 15 degrees. Roof windows should be CE marked against this standard. Rooflights, however, cannot be CE-marked because they are usually installed ‘out of plane’ on an upstand and are not considered roof windows.

Please note that specific requirements may vary depending on the manufacturer. Always check that the specification meets your roof design.

To learn more about rooflights and roof windows – from benefits and specification considerations, through to installation – request one of our RIBA-approved rooflight CPD seminars.