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Roof Window Flashing & Collar Guide


12th July 2018

Every roof window requires both flashing and a collar to ensure it can provide maximum levels of performance. When you purchase a new window, these elements of the product will usually arrive together with the frame and sash. If you require a bespoke window they may have to be purchased separately or created from scratch.

This guide will explain what collars and flashing are, how to install them, what they add to your property and how to keep them in great condition, so they will last for years to come.

What are Collars & Flashing?

Flashing is an essential component required for the correct installation of any roof window. It is used to keep ensure the newly installed window remains watertight. It also fits in with the frame and the surrounding roof structure.

Roof Window Flashing & Collar - Roof Stores
Roof Window Flashing & Collar

On the other hand, collars are fixed into place between the frame and the structural roof opening. This provides insulation by reducing heat loss and removing cold bridges. Cold bridges occur when there is a break in the insulation. If left untreated will lead to condensation and the many problems that can increase over time.

How to Install Flashing & Collars

Installing flashing and collars with roof windows has become a very straightforward process. This is due to the ways in which companies ship the window and all essential extras in one convenient package. The material used on the roof's surface will determine the type of flashing to be installed. However, this will be clearly stated on the product packaging, so you can choose the right one before purchasing.

Installing Collars

  1. Mark the position of the transverse drainage gutter, which should be at an angle to provide adequate run-off for rainwater in the case of any broken tiles that may appear over time.
  2. Once marked, if the roof is counter-battened, remove a section so the gutter can be fixed to the rafter. Then cut a gap in the roofing felt which can later be laid into the gutter.
  3. The felt collar can now be positioned around the frame of the window. The end with more material is positioned at the top and you can remove the covers from the sealant strips on the collar and fix them to the edges of the window.
  4. Staple the collar around the roof battens to fix it snugly to the roof structure. You can make small cuts in the material to help fix it around counter-battens.
  5. Fix the gutter in place at the top of the window, folding the underfelt collar over and into it before cutting off any excess and dressing the flap in the roofing felt into the gutter and using the clips supplied it can be fixed into place.

Roof Window Flashing & Collar - Roof Stores
Roof Window Flashing & Collar

Installing Flashing

  1. If you are using profile tiles on the surface of the roof, these should be chamfered below the window to allow the bottom edge of the flashing to sit correctly in place, so water is able to run off without issue.
  2. When chamfering the tiles, you must ensure that the highest point of the tile, when viewed in profile, is no more than 130mm from the bottom of the window.
  3. Take the bottom apron and put it into position and shape the bottom edge over the ridges of the tiles.
  4. Bend the pleated apron of the flashing back no more than 45° which will help it sit tightly over the tiles. Put the apron back into position and fix into place by screwing down into the frame.
  5. Put the side flashing into place next, which slide into place and are fixed using tabs before nailing through.
  6. The side frame covers are now put into position before being clicked into place.
  7. The hood section at the top is screwed into place before the top flashing piece is positioned and fixed with fold-down tabs either side of the window.
  8. The tiling around the window can now begin with the foam gasket on the side of the flashing cut to match the profile of the tiles where they meet the flashing. The same rule applies to the top of the window, leaving a gap of 60 - 150mm between the tile and the top of the window to allow rainwater to run-off freely.
  9. The bottom apron should be given a final dress down to ensure it fits fully over the tiles.

What are the Benefits of Collars & Flashing for Roof Windows?

Collars and flashing are put in place to make the window fully weatherproof and insulated. The protects the space below from outside elements, especially rainwater, snow and ice, which if allowed to pass through would create condensation and any number of structural issues within the roof. The insulation provided by the collar also means the window remains energy-efficient which will have a positive impact on the entire property. This will allow the homeowner to use the space while reducing heating output as the room will retain the heat without it quickly escaping via any gaps in the window.

Roof Window Flashing & Collar - Roof Stores
Roof Window Flashing & Collar

How to Maintain Your Roof Window Flashing & Collars 

Over time your window will have to be cleaned and maintained to ensure you get full value for money and this also the case for window flashing and collars. An annual check will be sufficient enough to remove any debris such as leaves, twigs or stones that may have built up over the past 12 months. The idea is to remove anything that will prevent rainwater from flowing freely away from the window. This also applies to snow and ice which should be removed as this can create blockage issues.

How Much are Roof Window Collars and Flashing?

For the most up to date pricing on collars and flashing from Roof Stores, click the links in the tables below - our team are on-hand to ensure you make the right decision - call 0330 123 1885 for more information.

There are several different options available, some of which are covered in the price table below. The tiling used on the roof will be the deciding factor on which type of flashing is best suited for the structure. As there are up to 11 different size options for each product we have chosen the mid-range size to give you a rough indication of price.


 Type Size
Velux EDJ 2000 Flashing 78 x 118cm
Velux EDN 2000 Flashing 78 x 118cm
Velux EDJ Flashing for recessed tiles to 90mm 78 x 118cm
Velux New Design EDL Flashing for Slate to 8mm 78 x 118cm
Velux EDN Flashing for recessed slate to 8mm 78 x 118cm
Velux EDP Flashing for plain tiles to 15mm 78 x 118cm
Velux EDW Flashing for tiles to 120mm 78 x 118cm


 Type Size
Velux EDJ 2000 Flashing 78 x 118cm
Velux EDN 2000 Flashing 78 x 118cm
Velux New Design BFX Underfelt Collar 78 x 118cm



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