Can you replace a rooflight on a flat roof?
Yes, and it is one of the more predictable jobs in this trade when the kerb underneath is sound. The unit is unbolted from the base frame, lifted clear, and a new one is set onto the same upstand with the covering dressed and made good around it. On a good kerb that is a single day.
What makes flat roof work less predictable than a pitched slope is that the kerb, the deck and the covering are all hidden until the unit comes off, and all three are the same age as the rooflight. This page covers what is found underneath, what can be reused, and how the day runs. It is part of the wider skylight replacement work Sunspire carries out across Chelmsford and the thirty miles around it.
Yes, and the kerb decides how straightforward it is
A flat rooflight does not sit on the roof. It sits on an upstand built off the roof structure, with the covering dressed up the sides of that upstand and over its top edge, and the unit landing on top of a continuous waterproof surface.
So the real question at survey is not about the glass, it is about that box of timber. A kerb that is square, sound, tall enough and properly weathered takes a new unit straight away. A kerb that is fifty millimetres high, uninsulated, built from untreated timber and soft at one corner is rebuilt, and that is a different day. Our page on what an upstand is covers the construction in detail.
Reading the existing kerb before anything comes off
Three measurements settle most of it and all three can be taken with a tape while standing on the roof. The height from the finished covering to the top of the kerb, taken on all four sides because roofs are rarely level. The internal size of the kerb, which is what the new unit is sized against. And whether the top edge is square and out of wind, checked by sighting across the two long edges.
Then the condition test: press the top edge and the outer face with a thumb, and look at where the covering terminates. Sealant bridging a gap between the covering and the kerb is the commonest thing found, and it means the joint has been doing the covering’s job for some years.
Kerbs built to nineteen seventies heights
Domed rooflights of that era were regularly bedded on kerbs of fifty to seventy five millimetres, sometimes on nothing at all beyond an aluminium frame screwed to the deck. Current practice wants one hundred and fifty millimetres from the finished surface to the top of the kerb, for reasons that have to do with ponding depth and wind driven rain rather than fashion.
Where the existing kerb is short, there are two honest routes. Build it up, which means new timber on top of the existing box, the covering taken up and over the new height, and the internal reveal extended to suit. Or accept the existing height with a unit rated for it, on a roof that genuinely drains and sits sheltered. The first is more work and it is usually the right call.
Cutting back a covering that has aged in place
The covering has to come back far enough to expose the kerb and to allow a proper termination to be reformed, which is typically two to three hundred millimetres out from the upstand on every side.
How willingly it comes back depends on what it is. Single ply membrane is cut cleanly and welded to new material. GRP is ground back to a feathered edge and laid up in situ. Torch-on felt is heated, lifted and rebonded in layers. Old built-up felt from the nineteen seventies is the difficult one: it has usually gone brittle, and it cracks rather than lifts. Where a covering is near the end of its own life, setting new glass into it is a short term answer, and the sensible order of work is the covering first.
Insulation found wet once the unit comes out
This is the routine surprise. Cold deck construction, which was standard on Essex flat roofs into the nineteen eighties, puts insulation between the joists with a void above it and a vapour barrier that was often notional. Over forty years, moisture from the room below has collected in that void, and around a cold uninsulated kerb it has collected fastest.
Wet mineral wool weighs several times what it should and has no thermal value at all. It comes out and it is replaced, and the moment to do that is while the opening is open. Where the roof is a warm deck with rigid boards above the deck, the same check applies at the perimeter of the opening, where the boards meet the kerb.
Sizing a new unit onto an old kerb
Flat rooflights are sold against the internal kerb size in most manufacturers’ catalogues, with the unit oversailing the kerb by a stated amount on each side. Older openings were formed to imperial dimensions or to nothing in particular, so an exact catalogue match is uncommon.
There are three usable answers. Take the next standard size up and pack the kerb inwards with new timber to suit. Order a made to measure unit to the actual opening, which adds lead time and cost but avoids touching the structure. Or rebuild the kerb to a standard size while it is open, which is often the cheapest of the three because the timber is inexpensive and the labour is already committed.
Falls around the old unit, and whether they still work
A flat roof is laid to a fall, conventionally a minimum of one in eighty as designed. Two things degrade that over decades: deflection in the joists, particularly where an opening was cut without adequate trimming, and settlement in the build-up.
The check is straightforward. After rain, water that has not cleared within forty eight hours is ponding, and ponding against a kerb is a sustained test that any weakness in the detail eventually fails. Where a survey finds standing water on the upslope side of an existing rooflight, the fall is corrected with tapered insulation or firrings before the new unit goes in. Setting good glass into a roof that dams water beside it simply moves the problem along a couple of years.
How the day runs, strip back to made good
The sequence is consistent. Protect the room below and screen the opening. Strip the covering back around the unit. Release the fixings and lift the old unit clear. Inspect and put right the kerb and the deck. Rebuild or raise the kerb if that is in the job. Dress the covering up the kerb and over the top edge, welding or bonding as the material requires. Set the new unit, fix it down, and make the covering good out to the cut line. Then finish the internal reveal.
The roof is closed and watertight at the end of every working day, which on a two day job means the opening is temporarily covered and weathered rather than sheeted and hoped over.
Harlow, Basildon and the roofs of that period
Essex has an unusually high concentration of flat roofs, and most of them are the same vintage. The new town housing across Harlow and Basildon, the rear extensions added to interwar semis in the commuter towns, and the garage and utility roofs on almost every estate from the nineteen sixties onwards.
What they share is the build-up: chipboard or softwood deck, cold construction, built-up felt, and a domed rooflight on a shallow timber kerb over a bathroom or a stairwell. Knowing that in advance is why a survey on a flat roof spends its time on the kerb and the deck rather than on the glass. The glass is the part that will be fine.
Related questions
- Do rooflights need planning permission?
- Do I need building regulations approval for a rooflight?
- Who notifies building control about a rooflight?
- How much does a rooflight cost?
- Do rooflights add value to a house?
- Is a rooflight cheaper than a dormer?
Everything on skylight & rooflight installation & replacement chelmsford →
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