How to Fit a Roof Window


Adding a roof window to your attic is a great way to create extra space in your house. Before you begin your installation, it is important to choose the right window for your space to ensure you are making the most of your new room. We have an article on the best roof windows, check this out.

Many people choose to complete their loft conversion on their own whether it be to save money or because they like a challenge. An important thing to remember when installing a roof window yourself, is to choose the right day. Installing a roof window during a rainy or snowy day can cause water damage to your roof, for this reason it is best to wait for a dry and sunny day to complete your installation.

Installing a roof window can seem like a daunting task, especially for first timers. That is why we have put together a helpful guide to make sure your roof window installation runs as smoothly as possible.

Our guide is for a standard roof window therefore certain, more specialised roof windows may require additional steps. It is always important to check the manufacturer instructions before you begin an installation to make note of any specific requirements for that window.

Fitting a roof window

Before you start, you will need:

  • Tape measure
  • Hammer
  • Work gloves
  • Pencils
  • Panel saw
  • Craft knife
  • Staple gun
  • Screwdriver / drill
  • Safety goggles
  • Instructions
  • Nails
  • Flashings – usually included with the window


Step-by-step Guide on how to fit a roof window:

  1. Establish an opening – when creating an opening for a roof window, the gap should be 40-60mm greater than the window itself. If you are installing a window into an existing roof, make sure you mark out the area of the opening before you cut the batons to ensure you are happy with the placement. The bottom of the opening should start from an existing roof baton, from there, measure upwards 45mm more than the height of your window with your tape measure. This is where to install the top baton that your window will be fixed to by drilling into an existing counterbaton. The top baton should overhang the opening by 100mm to ensure it is secure. Excess felting should be folded over the batons and secured down – this acts as additional waterproofing.


  1. Remove the sash – each brand has slightly different methods of removing the sash, so it is important to check the manufacturer instructions at this stage. Usually, to remove the sash, you should partly open the window to access the hinges. Loosen the clips on the hinges to separate the sash from the frame and lift out of the pivot slots – be careful not to drop it.
    Removing Window Sash


  1. Fit the insulation collar – clip the collar together at each corner and push into the opening you have just created. It should be a snug fit.


  1. Fit the frame – this is a very important step as it will ensure your window can be secured into place.
    1. Screw or hammer the brackets onto the side edges of the window at the top and bottom of both sides.
    2. Next, lift the frame into the opening and fix the brackets to the batons. To do this, start with the bottom brackets using a 30mm nail for the inner fixing as this only goes through one baton. For the outer fixing, use an 80mm nail as this needs to go through both the horizontal and vertical batons.
    3. For the top brackets, only screw in the 30mm nails, leaving 10mm sticking out – this will allow for alignment adjustments.


  1. Check the alignment – reattach the sash by lining up the curved hinge piece of the sash with the corresponding part on the frame’s hinge and sliding into place. Next, look at the window from the inside to ensure the window is evenly spaced in the opening, if the alignment is off, a small adjustment using a crowbar should fix this. Then, open the window slightly to check the gap between the sash and the frame is even all the way across. If not, place a small wedge between the top bracket and supporting baton on one side to even out the spacing.


  1. Fix the frame – now you are confident the placing of the window is correct, remove the sash again, in the same way as step 2. Fully screw in the 30mm screws that were partially left out earlier and add 80mm screws to the outer fixing.


  1. Place the drainage gutter – mark out the space needed for the gutter, remember to place it at a slight angle to allow the water to flow easily. Remove any of the batons that would cross the gutter and cut a flap of the underfelt which will later be laid into the gutter remembering to seal it. Do not fix the gutter in place at this stage.


  1. Place the underfelt collar – position the collar around the window. The side with the most material should be at the top of the window. Seal the collar to the edges of the window frame and staple excess material to the batons. Now, fix the gutter into the space you made and fold the underfelt collar into it and secure into place with clips.
    installing the roof underfelt collar


  1. Install flashings – each window requires different flashings therefore it is important to consult the manufacturer instructions at this stage. If your window does not come with flashings, LB Supplies has a wide range of brands available. This step should be done after the tiles are fitted around the bottom section of the window:
    1. Start by placing the apron and bottom flashing before screwing them into place.
    2. Attach soakers to the sides of the window frame, ensuring the top one does not protrude above the window frame.
    3. Next, slide the side frame covers upwards into position until they click into place.
    4. Slot the plastic cover over the topside of the window edge and click into place. Then screw the hood section of the window at the top of the frame.
    5. Finally position the top flashing piece and fix with the fold-down tabs. Once this is done, continue to fit the tiles around the window, ensuring there is room left for water to drain around the window (60-100mm will do the trick).


  1. Reattach the sash –line up the curved hinge piece of the sash with the corresponding part on the frame’s hinge and slide into place.


  1. Fix the vapour barrier – this is done from the interior of the building. Flatten the vapour barrier to the surrounding area, ensuring there are no bubbles. Screw the barrier into place at each corner. Finally, tape to the existing vapour barrier to seal and prevent moisture entering.


Now you have an expertly installed roof window, you may want to look at installing a roof window blind depending on how habitable the space is. However, other than investing in roof accessories it is now time to make the most of your extra space or if you are a tradesman of course, it is time to demand your cash and spend it on a well deserved beer.