A Brief Guide to Roof Windows

If you’re re-roofing a property or constructing a new roof, one consideration is to install windows into the roof.

 

A Velux Centre Pivot Roof Window being installed at JJ Roofing Supplies

If the roof area is to be used as a living space then it is almost essential to add a window to allow light into the area, and extra ventilation as necessary. There are a wide range of windows to choose from, and this guide will discuss some of the most common styles of roof windows, and how to fit them.

Planning Permission

In general terms you do not require planning permission to fit a roof window, providing the work meets the following criteria:

  • Any alterations must project no more than 150mm from the existing roof plane.
  • No alteration should be higher than the highest point of the roof.
  • Side-facing windows should be obscure-glazed.
  • Any opening must be at least 1.7 metres above the floor.

If you live in a conservation area, or plan to incorporate solar panels, extra conditions may apply, so it is always best to check with your local planning department if you are unsure.

http://www.planningportal.gov.uk/permission/commonprojects/roof/

Types of Roof Windows

tile and slate flashing kits

There are many different types and styles of window available, and some of the most common types are detailed below:

Centre Pivot Roof Windows

The centre-pivot window is the most common type used for pitched roofs, as it can be hung in a variety of situations and is particularly suited to loft extensions. The windows are suitable for roofs with a pitch from 15° to 90°, and the handle positioned at the bottom makes it easy to open, even when the window is positioned at high level.

Top Hung Roof Windows

Top hung windows are ideal for restricted living spaces and can also provide a means of escape as an emergency exit. The windows pivot at the top or at the three-quarter point of the frame, to allow a wide opening. The windows are also suitable for roofs with a pitch from 15° to 90°, and incorporate a trickle vent to provide ventilation even when the window is closed.

uPVC Pitched Roof Windows

uPVC windows are ideal for pitched roofs, in particular for rooms such as bathrooms and kitchens which create a lot of condensation, steam or smoke. uPVC frames are strong and durable, and require very little maintenance. The windows are also suitable for roofs with a pitch from 15° to 90°.

Ventilated Roof Windows

Ventilated roof windows are similar to the standard centre pivot windows, but incorporate a trickle vent to provide extra ventilation, even when the window is closed. The windows are also suitable for roofs with a pitch from 15° to 90°.

Conservation Roof Windows

There are over 8000 conservation areas in the UK, and special planning permission is required for certain building projects, including roof windows.

Velux Conservation Window for Slate roofs

The conservation roof window is a variant of the centre pivot window, but the external profiles are painted black and a glazing bar runs down the centre of the window pane to meet conservation building regulations.

Installation

The first and most important decision to make before fitting a roof window is whether any rafters need to be cut to allow the installation. This should be avoided if at all possible as it will weaken the roof structure, and most windows on the market are designed to fit in-between the rafters. Fitting two windows side by side or one above the other is preferential to fitting one large window that requires the cutting of rafters.

The higher up the roof the window is installed the more light it will allow into the room. You should consider the finished room at the planning stage, to determine the position of the window in terms of the light it will allow in, while also considering ease of opening and closing. From a practical perspective it is not recommended to fit a window higher than 6 courses of tiles from the ridge as this could result in ridge tiles becoming loose.

  • First measure the dimensions of the base plate and mark the approximate position of the window on the rafters.
  • Remove tiles, battens and underlay as necessary to create the opening for the window.
  • Complete the timber framework which will support the window by fitting timber trimmers to the top and the bottom of the opening between the rafters.
  • Cut a strip of 300mm wide lead sheet to form an ‘apron’ below the window at the sill. The lead sheet should extend 150mm either side of the base plate.
  • To smooth the path of the lead sheet, a small triangular ‘tilting fillet’ (sawn treated timber) should be fitted along the top of the row of tiles beneath the opening. Then lay a thick line of silicone roof sealant along the surface of the lead apron to ensure water tightness to the joint when the base plate is fitted.
  • Dress the lead sheet down so that it overlaps the tiles below.
  • Carefully position the base plate into the framework, and check that it is level and square.
  • Secure the base plate to the rafters with coach bolts.
  • The sides of the window should be made watertight with lead soakers fitted under the adjacent tiles.
  • To achieve an even edge to the tiles on each side of the window, ‘tile and a half’ sized tiles can be used on every alternate tile course.
  • To make the top of the base plate watertight, a head flashing is fitted. This is similar to the lead apron used at the sill.
  • The top of the window can be finished with rows of tiles nailed to the battens. The lower row should be shorter eaves tiles, which will cover the head flashing.
  • Carefully re-attach the glazed window casement to the base plate.
  • To the inside, the window reveals should be lined with strips of insulation. The bracket for the handle or winder will need to be firmly screwed in place, so a strip of timber should be fixed to the centre of the lower reveal before insulating.
  • Finally, cut plasterboard strips to cover the window reveals to the sides, top and bottom.

The above is a general guide to fitting roof windows, but each manufacturer will supply different instructions with their windows. Always follow the manufacturer’s specific instructions in detail to ensure a perfectly-fitted window.

By Eddie Dunk

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