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6. Further information: roof lights

A rooflight is a window that is installed within a pitched roof or flat roof normally to give more light to rooms or spaces within the home.  Approval under the Building Regulations will generally be needed for the installation of a new rooflight for the following reasons:

  • to install a rooflight, the roof structure will generally need to be altered to create the opening
  • the roof will have to be able to carry the load (weight) of the new rooflight. If the roof can not do this then it will need to be strengthened
  • any rooflight that is installed will need to prove that it has sufficient insulation against heat loss i.e. is energy efficient
  • if the rooflight is in close proximity to a boundary, the fire performance of the rooflight will need to be considered


To install a rooflight in a roof generally entails cutting part of one or more of the roof's rafters or joists away. The cut ends of the rafter/joist will need to have new support introduced - usually achieved by fixing two pieces of timber together which span across the new opening on either side. These double timbers are called 'trimmers'.

The adjacent rafters or joists to which these trimmers are fixed may also need to be strengthened as they will be supporting the load transferred from the cut rafters or joists.  This strengthening can be achieved by fixing a new rafter or joist to them which must also run the full length.

Weather proofing

Once a rooflight is installed the edges (where the rooflight meets the roof) will require weather proofing as well as the glass of the rooflight itself.  This is commonly carried out by using lead flashing or with proprietary kits supplied with the rooflight.  Manufacturers of rooflights may be able to advise on how this can be carried out.


A room that the rooflight is to serve will need to be ventilated.  Ventilation can be achieved by using the rooflight for both rapid and background venting.

Ventilation of the existing roof void(s) will have to be considered as air must still be allowed to flow from one to another.

You may wish to use an installer who is registered with the relevant competent person scheme for replacement windows, doors, roof windows or roof lights.

Energy conservation

Dwellings are required to be energy efficient. A method of achieving greater energy efficiency is to take steps to reduce the amount of heat that is lost through the glazing in both windows and doors. 

If you are to install windows and doors you should be aware that they need to comply with the requirements of the Building Regulations in relation to the amount of heat that can pass through the door or window, including the frame, which is measured as a U-Value.  This U-value should not be exceeded.  For information on the maximum U-Value allowed please refer to Approved Document L1B, Table 1 and 2.