Velux have dominated the niche in loft and attic windows for decades and their New Generation designs are no exception.

As with anything new, installing your first Velux window can be daunting despite the care and attention the company spends on clear instructions. This guide is a supplemental guide to installing a centre-pivot window (specifically the GGL MK04 design with EDW flashing).

There will be slight variations in this method when installing other models and using other types of flashing.

Equipment and Disclaimer

The good news is that almost everything you will need, including screws, should be included in your Velux pack.

However, you will need a saw for cutting the roof batons and a screwdriver (or drill with screwdriver attachment). You will also need a Stanley knife for cutting felt and various tools for removing, re-laying and possibly chamfering tiles or slates. A crowbar is useful if you need to adjust the frame.

In writing this guide we assume you are either a roofer or a DIY enthusiast who is experienced and confident in working on their own roof. In all other cases we advise you to call a professional installer. Working on a roof is always risky and you are solely responsible for that decision. If you are unsure of the condition of the deck you should not step on to any roof. If you do decide to step on to the roof, a sheet of OSB3 board or plywood can help to distribute your weight evenly.

Preliminary tasks:

First, check you have all you need for the installation. You should have:

  • The Velux window itself
  • BDX insulation and under felt collar
  • Appropriate flashing kit
  • BDX vapour collar

The modern Velux windows come in cardboard boxes that can be flattened by simply depressing the tabs and pulling the corners out. Don’t use a Stanley knife. Velux have changed the way they pack the boxes so if you think there are pieces missing, chances are they are in one of the other boxes to avoid confusion.
Note that there will be a red square on the cover of the box in this model/flashing combination. This is to do with the type of flashing you are installing and its importance will be revealed later.

You should find that the sash and frame are already connected in the main box. After removing all other polystyrene protection, use the control bar at the top of the sash to open it up. You can then remove the polystyrene blocks from the bar, position them on the top (bridge) of the sash and flip the whole sash over so that it stands on its new polystyrene ‘feet.’

You can then depress the buttons in the hinge and separate the sash from the frame.

Fit your Velux window in 7 steps:

Step 1: Create your roof opening
For this model, the width of the opening must be between 40-60mm wider than the window itself. After you have cut this space, install the installation baton so that its top edge is at least 80mm above the first course of tiles beneath the window. The lower edge of the upper baton should then be added a further 45mm higher than the window height. These batons both need to extend horizontally by the required distance (usually 100mm at the top and 250mm at the bottom but check the instructions).

Trim the felt and wrap the excess back out of the way, fixing it around timbers for extra waterproofing. You can now remove the tiles beneath the window.

Step 2: Fixing the BDX insulation collar

Remove the BDX insulation collar and clip it together at the corners. This can then be lifted into place inside the window opening.

Step 3: Attaching the window frame brackets

Here is where the colour of the square on the box comes in handy. You will notice that there are two coloured lines along each edge of the window frame. For this model/flashing combination you need to line the window brackets up with the red line and attach them to the top and bottom of the frame (in blue line installation, the brackets would go on the sides).

First, attach the brackets to the lower baton using the screws provided. The 30mm screws go inside (use the pre-drilled holes to help you position them) while the longer 80mm screws go through the baton, any counter-baton and the rafter on the outside.

At the top, partly screw a 30mm screw into one of the slotted holes in the hinge. Leave 10mm proud for now. You will usually find that one part of the flashing kit (Part 5) has already been attached to the frame. You may prefer to remove it before attaching the sash as it can get in the way.

Step 4: Attaching and aligning the sash

Attaching the sash is simply a case of slotting the curved part of its hinge into the corresponding slot.

Once in place, check that the space between frame and sash is even all round. If the sides are uneven, gently use a crowbar to adjust the frame. If the bottom is not in line, you can slot the wedge included behind the appropriate top bracket (this is why you left the screws proud!)

Once satisfied, depress the hinge buttons and remove the sash again. Fully tighten the 30mm screws at the top and add 80mm screws on the outside as you did below.

Step 5: Installing the under felt collar

First, mark the position where the drainage gutter will go. This should be at an angle so that any runoff from above will be directed away from the window. Remove any sections of counter-baton that may be in the way.

Cut a flap in the felt and use the sealant included at the sides of the flap. Place the BDX collar around the window with the bulk of the material at the top. Remove the plastic backing so that the strips of sealant can be used to attach the collar to the frame. Dress the corners as per the instructions. Staple the excess collar around batons, making cuts to wrap it around counter-batons as necessary.

Finally, attach the gutter and fold the collar inside the channel. Trim the excess collar, fold in the flap of felt and secure with the supplied clips.

Step 6: Installing the flashing kit

The flashing kit comes in seven parts. Part 5 is usually pre-attached to the frame (see Step 3).

To allow the flashing apron to sit neatly beneath the window, you may need to chamfer any profiled tiles (making sure the highest points, when viewed in profile, are no more than 130mm from the bottom of the window).

Place Part 1 (the bottom apron), shaping it around the tiles. Flip over and bend the apron back by no more than 45 degrees, before replacing into position and securing to the bottom and sides of the frame using Part 2 of the kit.

Part 3 is simply slid into place and fixed with tabs. If you have not done so already, remove the pre-installed Part 5 pieces and slide on the side covers (Part 4), clipping them over Part 3. Part 5 can then be reinstalled by clipping to the housing at the top of the frame and then slotting into the hinges. Screw in the hood system (Part 6), remembering to discard any polystyrene backing. Finally, position the top flashing (Part 7) and secure with the fold-down tab.

You can now re-tile around the window, following the specific instructions for the type of tiles you have. Cut the foam gasket to match the profile of the tiles and leave a gap of 60 to 150mm between the top of the window and the tiles above.

Once you’ve given the bottom apron a final dressing down you can clip in the sash for the final time.

Step 7: Installing the BDX vapour barrier

The BDX vapour barrier is installed by pushing inside the internal lining rebate of the window using the tool provided. Fix into the corners using the screws supplied before taping to the existing vapour barrier.