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7 tips for specifiers when installing roof windows and skylights

The benefits of installing roof windows and skylights are clear: they provide high levels of natural light, they can be wonderful sources of ventilation, and they contribute to the thermal comfort of a building. More than that, roof windows and skylights provide an aesthetically pleasing addition to the external appearance of a property.

But what should you consider to ensure the installation runs smoothly once it gets to site? While generally easy to install, knowing what to expect can help guarantee things go smoothly.

Glazing Vision recently spoke to Clarkson Builders about how to properly install them and keep work on-site progressing as expected.

Here are our top recommendations:

1. Window size

Roof windows and skylights come in an array of shapes and sizes, and choosing the right dimensions for your project is critical.

The ‘standard’ for roof windows is 34.4″ x 23.62″, and these are installed between rafters at 23.62 inches. Other common sizes are 35.4″ x 38.5″ and 35.4″ x 47.2″ – the latter is designed to fit in a double-rafter space. They can also be provided in much larger sizes – for example, Glazing Vision custom skylight windows can measure up to 118″ x 39″.

2. Structural support

The most effective way of supporting a roof window is to use double trimmers for horizontal support and double rafters for vertical support. Double hangers are placed in each corner, and single hangers are located around connected rafters.

The double supports comprise two pieces of rafter timber with 23.62″ (either C16 or C24 between 3.9″ x 1.9″ or 8.8″ x 1.9″, depending on specification and structural calculations). The rafters should be fixed together with bulldog timber connectors and M12 bolts.

When fixing the timber, the head of the bolt should be recessed on the side of the wood to which the skylight will be fixed. Otherwise, it could protrude into the area beneath the glazing.

As with all structural requirements, relevant calculations should be carried out by a qualified structural engineer.

3. Window Aperture

Remember that the size of the window aperture may not be the size of the window itself. The actual requirements will depend on the individual roof window, so be sure to consult the installation guide early in the planning stages.

When preparing the rafters, consider how elements like the insulation sarking and the ends of the battens will be supported. The roofing underlay should be folded back and fixed appropriately.

Other elements will also need to be added to the roof, such as tilting fillets, battens and flashing. Out-of-plane skylights will also require the construction of a window curb.

4. Site delivery and storage

Take time to consider how skylights and roof windows will get to site – and once there, how will the units be stored to ensure they aren’t damaged.

Some manufacturers, such as, offer free delivery through an online shop and by choosing next-day delivery, you can minimise the on-site storage requirements.

More importantly, don’t underestimate the weight of a roof window. A small, 29.5″ x 29.5″ laminated skylight weighs just over 68 pounds and the units can reach more than 330 pounds depending on the size and type of glazing. You may need to rent lifting equipment for bulky units, or for hard-to-access locations

Heavier units should also be assessed in terms of Health and Safety. In some cases, mechanical assistance may be required for moving the roof windows around site safely.

5. Getting the roof window or skylight onto the roof

In most cases, construction materials are delivered ‘to the kerb’, this is the case for skylights and roof windows. Unless you have a particularly obliging driver, you will need to work out how to get the unit onto the roof.

6. Instructions and tools

Many roof windows come with an installation guide that actually covers multiple products – and those products can look very similar while having slightly different fixings and flashings. If these instructions aren’t followed correctly, it could impact the installation process and how the roof window or skylight performs.

Be sure to check the instructions to ensure that the installation team has the right tools. Depending on the manufacturer, special drill bits may be required.

7. Tiling around roof windows

Roof windows are usually found on pitched roofs (unlike skylights, which are typically found on flat roofs).

When tiling around roof windows, it’s important to ensure that the bottom tile is not too close to the window, as the flashing cannot be installed correctly. The best way to plan the tile gauge is to measure from the bottom of the roof, as this allows for the best finish and helps to ensure the roof window performance isn’t compromised.

When using pantiles (these are usually made from concrete but may also be clay in heritage projects), be sure to chamfer the top of the bold roll beneath the window.

This should all be explained in the instruction manual, which should outline the distances to install your battens to the sides, top and bottom of the window.

Phew – remember all that?

With so much to think about, it’s easy to see why it’s so important to look beyond appearance and performance of a building element at the specification stage. If you have any questions or concerns about a product you’re specifying, it’s a good idea to get answers and advice as soon as possible from the manufacturer.

Here at Glazing Vision, our technical support team is on-hand to provide assistance, and you can order next-day skylights from We also offer a variety of resources to help you learn more about our range of products, including our AIA approved Lunch and Learn presentation.