Fitting a roof window

Summary: Learn how to fit a roof window, prepare an opening and fit a window.

A roof window - or skylight - can be an essential part of a loft conversion, transforming an ordinary attic space into a bright, attractive room. There are two types of roof window: curb mounted and framed-in-place. A curb mounted roof window stands vertical to the roof plane. A frame-in-place roof window fits flush with the plane of the roof.


When planning a roof window there are several important factors to consider.

  • Read the manufacturer's instructions.
  • The window must be big enough to provide adequate lighting for the room. Generally speaking, the smaller the angle of the roof pitch the taller the window needs to be.
  • Position the roof window on the side of the house that receives the most light.
  • The window should be roughly located in the centre of the room.
  • The window should be positioned at a height so that you can see out of the window whether you are sitting or standing.
  • The top of the window should be within easy reach.
  • It is unlikely you will need planning permission to install a roof window, but to avoid any problems it is advisable to check especially if you are planning to work on a listed building or if the building is in a conservation area.

The framed-in-place roof window is the most common type and the easiest to fit. They are sold in complete kits consisting of the window and the surrounding flashing, which will be made from either galvanised steel or aluminium. The instructions below will give you a guide to installing a framed-in-place roof window into a slate roof.


Preparing the opening

  • Make sure the skylight is positioned so that one side of the window is aligned with a rafter. Check that none of the roof rafters are in the way, for these must not be cut without professional advice.
  • Mark the dimensions of the window given by the manufacturer on the inside of the roof felt.
  • Hammer a nail up through the roof at each corner of the window.
  • Using a roof ladder climb onto the roof and remove slates from inside the area marked out by the protruding nails. Take extreme care when working from height.
  • Then cut away the roof battens and roofing felt in the area marked out by the four nails.
  • Next cut through any rafters that are obstructing the window space. This can be either done from the roof or from inside the loft.
  • Using 2 x 4 timber - 100mm (4in) wide and 50mm (2in) thick - cut two horizontal trimmers and nail them between the rafters to set the height of the window.
  • Cut one vertical trimmer and nail in position to set the width of the window. Remember that one side of the window will be aligned to a rafter.

Fitting the window

The method of installation will vary according to the model of window you have bought. To install the window you will need suitable flashing. Many roof windows come with their own flashing kits. Fitting the window will require two people.

  • Before fitting the frame remove the glazed sash section.
  • Position the frame and fasten it in place using the brackets provided. Around the frame there will be a line marked, which indicates the level of the surrounding roof battens when the frame is in place.
  • Measure the diagonals of the frame to ensure it is square. If the measurements are equal then the frame is square.
  • Moving up on to the roof fit the flashing around the frame according to the manufacturer's instructions. Secure the flashing as instructed with nails or roof cement. Start by fitting the sill flashing at the bottom of the frame. Then move up the sides of the frame. Finally, fit the flashing above the frame.
  • Replace the slates around the frame. It may be necessary to cut some of the slates so they overlap the flashing to meet the frame.
  • Replace the glazed sash section into the window.


Some roof window kits come complete with a prefabricated lining that when fitted provides a neat, finished surround to the window. If this is not the case you will need to line the opening with plasterboard.

  • Cut the plasterboard to the required dimensions and nail in position to the timber around the frame.
  • Finish the joints with filler or jointing tape.
  • The entire loft can now be insulated. Plasterboard can be fitted to the rafters and then plastered.