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How to prepare the roof for the new roof window
This series of guides will take you through the process of installing a Site roof window on high profile clay tiles using a Site flashing kit designed for these tiles.
Each section is detailed to ensure you get all the information you’ll need to help you complete the job.
The project has been split in to several articles to take you through each stage of the process in manageable chunks.
The roof window fitting instructions will detail all the measurements and requirements for your new window and to the novice DIYer, these can look complicated and daunting.
If your new roof window fits between existing rafters, it can be a relatively straight-forward project of installing the window. If timbers need to be cut or removed to accommodate the window, you need to make sure that the roof structure remains strong and safe, and this type of installation should only be carried out by a qualified professional.
Ensure the frame is wide enough for the size of the window being installed. For this installation, the width of the roof frame is the width of the external window frame plus 7cm, the width of insulation block.
It’s a good idea to work from the inside edge of a rafter. This not only helps to create a strong support but also minimises the framing work.
With a pen and tape measure, mark on the under felt (or membrane) the area to remove, leaving a minimum of 15cm to the left and right, as shown.
Using a sharp utility knife or scissors, carefully cut away the felt where marked to reveal the tiles.
Carefully remove the tiles from inside of the loft and store inside the loft, do not balance the tiles on the outside of the roof.
With all the surrounding tiles removed, it’s now time to prepare the roof to hold the window. This is achieved by installing a timber support frame (which the window will be screwed onto) and you should refer to the window’s instruction manual for the correct position of these support frame battens.
For example, for this installation, the inner frame dimensions for this window are: Width = B (width of the window, without insulation bloc) + 7cm (to allow for insulation bloc of 3.5cm x2) Height = H (height of the window, without insulation bloc) + 7cm (to allow for insulation bloc of 3.5cm x2)
An additional batten is also fitted 10cm to 12cm above the top framing batten. This will provide a minimum gap of 1cm between the top of the window and the frame above, once the window is fitted.
Nail or screw down all the battens to the roof, ensuring they are set square and level.
With the frame fixed in position, you can now cut back the excess battens within the frame with a panel saw or circular saw.
With a sharp utility knife, mark and cut the excess felt as shown, and remove and dispose of the top and bottom sections of felt, marked with an ‘X’.
It’s really important you achieve a watertight finish around the window frame. To help achieve this, you must add an additional collar of membrane at the top and bottom of the support frame.
Using the utility knife and straight edge or metal ruler, cut two pieces of membrane to the correct size.
With the membrane cut to size, fit the top piece as shown. You might need to loosen the battens or remove nails to allow for the positioning of the membrane.
A strong roofing tape is added across the top membrane and fixed down onto the existing membrane.
Fit the bottom piece of membrane as shown below.