Fit a Velux Blackout Blind With DKL Fabric in 6 Steps for 90% Light Block

Read the code before you buy anything

Every Velux window carries a type and size code on a plate near the top of the sash, visible when you open the window fully. It reads something like GGL MK04 or GGU SK06. The two letters and two digits after the model name are what a blackout blind is sold against. Buy a DKL or DKU blind that matches that exact code and the side channels will line up with the runners already built into the frame.

Guessing the size from a tape measure alone is where most failed orders start. The fabric width is narrower than the glass, because the blind sits inside the channels rather than across the full opening. A blind ordered one size out will not clip in at all.

What is in the box

A genuine Velux blackout kit contains the blind cassette with the rolled fabric, two aluminium side channels, four mounting brackets, a bag of screws, and a thin paper template. The fabric runs in the channels, which is what blocks the light leak down the edges. Cheaper universal blinds skip the side channels entirely, and that single omission is why they glow around the border at dawn.

The brackets are the part people lose. They are small black or white plastic clips, two per side, and they hook into slots that the manufacturer pre-drilled into the timber or polyurethane frame at the factory. Open your window and look at the top corners of the sash. You should see two pairs of small holes or pre-formed recesses. Those are your anchor points.

The actual fitting sequence

Work with the window tilted fully open and held on its catch so both hands are free.

Start with the top cassette. Push the two top brackets into the upper pre-drilled holes until they click. They only seat one way round, with the open jaw facing down toward you. Offer the cassette up so its end caps drop into the open jaws of the brackets, then press until you feel it snap home. Give it a gentle tug. A cassette that drops out at this stage was never seated; it was resting.

Next come the side channels. Each channel has a hooked top end that slides up under the cassette and a foot that fixes to the lower frame. Slide the hook in first, swing the channel flat against the frame, and check that the fabric guide groove faces inward toward the glass. Mark the lower screw position through the bracket hole with a pencil.

Here is the step that catches people out. Pull the blind fabric down about ten centimetres and feed the stiffened side edges of the fabric into the channel grooves before you screw the channel foot down. If you fix both channel feet first and then try to thread the fabric, the fabric edges fight you the whole way and usually pop out of one groove halfway down.

With the fabric edges captured in both grooves, screw the lower channel feet into the marked points. The supplied screws are short and self-tapping; a manual screwdriver gives you more feel than a drill and far less chance of stripping the soft frame timber.

A quick sanity check

Pull the blind down to full extension. The bottom bar should meet the lower frame square, with no light visible at either edge.

When the blind will not pull down straight

A blind that racks to one side, where one corner of the bottom bar reaches the sill before the other, almost always points to channels that are not parallel. The blind mechanism assumes both channels sit the same distance apart top and bottom. If the lower screws went into holes that were a few millimetres off, the channels splay slightly and the bottom bar binds against the tighter side.

The fix is tedious but reliable. Back out both lower screws, pull the blind to half height, and use a tape to measure the gap between the two channels at the cassette and again at the feet. Those two figures should match. Shim the splayed channel inward with the screw holes you have, or fill the original pilot hole with a sliver of matchstick and wood glue, let it set, and re-drill a fresh pilot a couple of millimetres over. Soft loft timber takes a second pilot hole without complaint.

A second cause of a crooked pull is fabric that jumped its groove during fitting. Run a finger up each channel and feel for the point where the fabric edge has popped out and folded back on itself. Ease it back into the groove with a blunt plastic spudger or the back of a teaspoon. Metal screwdrivers scratch the anodised channel and snag the fabric.

If the blind pulls down square but creeps back up on its own, the spring tension in the cassette is set too high for the drop. Velux cassettes have an adjustment, though reaching it means partially unclipping the cassette. Most installers leave a slightly over-tensioned blind alone, because a blind that holds anywhere on the runner is the normal behaviour and a blind that drifts down is the fault, not the reverse.

Loft windows sit at an angle, and that matters

Gravity helps you on a vertical window and works against you on a sloped roof window. On a pitched Velux the blackout blind relies on the side channels to hold the fabric flat against the glass, because nothing else stops it from sagging away from the pane at a shallow angle. This is the whole reason the side channels exist on a roof blind. Skip them or fit them loose and the fabric bellies outward in the middle of its travel, and the light leak follows.

A blind on a window that pitches close to flat, near horizontal, carries more fabric weight pulling down on the cassette spring. If you are fitting to a low-pitch roof, expect to live with the spring tension a touch higher than feels necessary on a steep window.

The teaser was right about the seam

The single most common complaint after a self-fit is a thin vertical line of daylight down one or both edges. Trace it back and you nearly always find a channel screwed down a hair too far from the glass, leaving the fabric edge sitting at the lip of the groove instead of buried in it. The cure is to slacken the channel feet, push each channel firmly toward the glass while the screws are loose, and retighten. Two millimetres of inward travel kills the seam.

Tools you actually need

A cross-head manual screwdriver, a pencil, a steel tape, and a stepladder cover almost every domestic fit. A cordless drill helps only for the pilot holes if you have replaced a stripped fixing; on the supplied self-tappers it does more harm than good in soft frames. Keep a plastic spudger or an old store card nearby for coaxing fabric back into a groove.

The one item people forget is a head torch. You are working above eye level, often into a shaded recess, and the difference between a bracket that has clicked home and one that is merely resting is invisible in poor light.

A point the instructions skip

Velux print their fitting leaflets for a window in good condition, freshly installed, with every pre-drilled hole clean and square. Older windows that have been painted over, re-glazed, or knocked about during a roof repair often have brackets holes clogged with filler or paint, and the factory recesses no longer accept the clips cleanly. Nobody tells you this in the box.

So the question that hangs over an older frame is not whether the blind fits the code on the plate. It is whether the frame still holds the geometry the blind was engineered around, or whether years of decorating have quietly moved the goalposts.