Can you replace a rooflight on a flat roof?

If the kerb is sound

Square, level along the top, adequate height, no rot. Then the old unit comes off, the new one goes on, the covering termination is made good, and it is typically a day.

The new unit is made to the measured kerb, so this is normally a made to measure order with a two to four week lead time before the fitting day.

If the kerb is not

Rotten at the corners, out of square, or too low for the covering to have terminated properly, which is usually why the old one leaked.

Then the kerb is rebuilt, and that means cutting back the roof covering around the opening, rebuilding, detailing insulation and vapour control, and dressing the covering up the new kerb. Two to three days, and the joint into the existing covering has to be made well.

The covering matters

Single ply welds cleanly. Felt bonds well if it is sound. GRP is harder to make a durable new joint into, and on an older GRP roof the honest answer is sometimes that it wants recovering rather than patching.

Why we survey first

None of the above is visible from a photograph, and a price given over the phone for a flat roof replacement is a guess about the kerb.

Worth doing at the same time

If the kerb is being touched at all, insulating its outer face and sealing the vapour control layer to the new frame are cheap while it is open and impossible afterwards. On an old installation neither was likely done.

That is often why the room was cold and the perimeter misted, and the replacement is the one chance to fix it.

Yes, and it is one of the quicker jobs we do

Where the existing kerb is sound, square and the right height, a replacement is largely a matter of lifting the old unit off, checking and preparing the kerb, and setting the new one on. One to two days.

The expensive part of a flat roof rooflight is forming and insulating the kerb, and on a replacement that work is already done.

What we check before quoting

The kerb height. At least 150mm above the finished surface. Where the original was bedded straight onto the deck or on a low upstand, that is the reason the old unit failed and it has to be corrected rather than repeated.

The fall. Whether the deck falls away from the kerb. Standing water against a perimeter seal is the commonest cause of a failed flat rooflight, and correcting it locally is part of the job.

Insulation on the outer face. Frequently absent on older work, and worth adding while the covering is open around the kerb.

The covering. How the membrane is dressed up the kerb and terminated. Where that was done wrongly, or where the covering is near the end of its life, it needs addressing.

The timber. A kerb that has been sitting in water for years may itself be decayed, and that is cut out and replaced.

Matching the size

Flat roof rooflights are made to a range of standard sizes, and a replacement is chosen to suit the existing kerb rather than the other way round.

Where the original was a bespoke size, or where the kerb has moved out of square over the years, a made to measure unit may be the answer, with several weeks of lead time.

The upgrade opportunity

While the unit is off, adding kerb insulation and correcting the fall costs far less than doing either as a separate exercise. So does upgrading the glazing: solar control on a south or west facing roof is a modest premium at this point and a great deal later.

If the old unit was a polycarbonate dome, replacing it with a modern glass unit improves security, thermal performance, appearance and safety in one job.

Why we survey

Whether the kerb exists, how high it is, whether it is insulated and which way the deck falls are the four things that decide the cost, and none is reliably visible in a photograph from the ground.

What the existing kerb decides

Everything, on a straightforward job.

Where the kerb is sound, square and level, and the new unit is available to suit its dimensions, a replacement is a comparatively simple exchange: old unit off, kerb prepared, new unit bedded and sealed, waterproofing dressed in.

Where the kerb is out of square, rotten, or an unusual size, the kerb is rebuilt first and the job becomes considerably larger.

Kerb condition is therefore the first thing a survey establishes, and it is not visible until the old unit is lifted, so a sensible quotation carries a provisional allowance for it.

The chance to fix what was wrong

A replacement is the one moment the detailing is accessible, and it is worth using.

Insulate the kerb on its outer face, continuous with the roof insulation, if it was not before. This is the single most common omission on older flat rooflights, and it is the cause of the condensation and staining that owners report as a leak.

Seal the vapour control layer to the new frame with tape.

And check the fall. If water has been ponding against the old kerb, it will pond against the new one unless the fall is corrected, and correcting it later means lifting the covering again.

The covering

The waterproofing has to be dressed into the new unit properly, which on a felt or single ply roof means a roofer competent in that system, and on a roof near the end of its life means asking whether the covering should be replaced at the same time.

Fitting a new rooflight into a membrane with three years left in it is money spent twice.