Usually yes. If the existing kerb is sound and at a workable height, and the covering around it has life left in it, a flat rooflight can be swapped without touching the rest of the roof. What always changes is a band of covering immediately around the kerb, because the old unit was sitting on the old termination and the new one has to sit on a new one. That band is typically 300mm to 500mm wide. Anything beyond that is a different job, and this page sets out what pushes it there.
The three things that decide it
We look at the same three items on every survey of this kind, in this order.
- The kerb. Is the timber sound, square, at a sensible height, and insulated on its outer face?
- The covering. Is it still flexible and well bonded, and will it accept new material welded or lapped into it?
- The opening size. Does a current unit exist that matches what is already there, or close enough that the kerb can be adapted?
Three yeses means a straightforward replacement, usually inside a day. One no means the scope grows, and it is better to know which no you are dealing with before the price is agreed.
Matching the unit to the hole you have
Older rooflights were made to sizes that no longer exist in current ranges, and the way a unit sits on the kerb has changed too.
Some older units sat inside the kerb opening. Most modern flat units sit over the outside of it, on the top face, which means the kerb has to be the right external size rather than the right internal size. A difference of 30mm in either direction is manageable by packing or by adding a sub-frame. A difference of 150mm is not, and at that point the kerb comes off. Made-to-measure is always available and removes the problem entirely, at the cost of lead time, which is set out on flat rooflight standard sizes and made-to-measure.
Where the old covering stops
This is the detail that decides whether the work stays local, and it is invisible until the old unit is lifted.
On a well-built installation the covering comes up the kerb, over the top, and is held on the top face. Lift the unit off and the termination is intact and reusable, and the new frame closes over it with a new bed. On a poorly built one, the covering stops halfway up the kerb, or was never taken over, or is bonded only with adhesive that has let go. Then there is nothing to close over and the covering has to be renewed up the kerb before the new unit can go on.
What a local band of new covering means
New material is dressed up the kerb, over the top and out onto the roof, then joined to the existing covering. How that joint is made depends entirely on the material.
Single ply is the easy case: new membrane hot-air welds to old membrane of the same type, giving a joint as good as the original. Torch-applied felt laps well onto sound felt, less well onto felt that has gone brittle and lost its mineral surface. GRP needs the existing laminate abraded back to give a mechanical key, and the area worked is wider as a result. Lead is dressed and clipped rather than bonded, and reuses much of the old material where it is sound. The material behaviour is covered on single-ply, felt and GRP roofs around a rooflight.
Same-size swaps, and why they are the clean job
Where the new unit matches the old external kerb size, the job runs to a predictable shape. Old unit off, old bedding cleaned back, termination checked, new bed laid, new unit set and fixed, new flashing or trim closed down, inside cleaned up.
No structural work, no plastering, no scaffolding beyond access, and the roof is open for a matter of hours rather than days. It is also the job where a temporary cover is simple to arrange, because the opening is a known size. We do not leave a roof open overnight in any circumstances, and on a same-size swap the question rarely arises.
Going bigger, and what that forces
Enlarging the opening is a different project wearing the same name, and it is worth being clear about that at the outset.
A larger opening means cutting further into the joists, new or upsized trimmers, and possibly a structural calculation. The kerb is rebuilt from scratch. The insulation around it is re-cut, the vapour control layer is re-formed, and the covering is renewed over a much wider area because the new kerb footprint does not sit where the old one did. Inside, the ceiling is opened up and the reveal is rebuilt and re-plastered. It is a good thing to do if the room needs it, and it is not a replacement.
When the kerb has to come off
Four findings mean the kerb is rebuilt regardless of what you hoped for.
Rot or softness in the timber, most often at the upslope corners where water has been sitting. A height that leaves the covering terminating within 75mm of the roof surface. No insulation on the outer face, where the ceiling below has been showing water in cold weather. Or a kerb built off the deck rather than off trimmers, which will not carry a modern unit. In each case rebuilding is the cheaper outcome over any sensible timescale, and we would rather lose a job than fit a new unit onto a kerb that is going to undo it. What good looks like is on how high a rooflight kerb should be.
When the covering is near the end anyway
Sometimes the honest advice is to do the roof first. If the covering is 25 years old, brittle, blistered across the field and losing its surface, welding new material into it will not hold well and you will be back on the roof within a few years with a new rooflight already in place.
In that situation we will say so and suggest you get the covering renewed by a flat roofing contractor, then bring us in to install the rooflight into the new build-up. That sequence is cheaper overall, gives a better detail at the kerb, and means the new unit is guaranteed against a roof that will outlast it. It also means we are not the ones doing the roof, and we are content with that.
What we establish at survey
Kerb dimensions inside and out, kerb height above finished surface, timber condition, covering type and condition, where the existing termination sits, insulation position, and whether the ceiling below shows any history of water.
From that we quote one fixed price with the likely outcomes stated, including what happens if lifting the old unit reveals something we could not see. You get told on the day, with the options, rather than after the event. We handle installations and replacements only, across Oxford and up to twenty five miles by road, and the workmanship carries a ten-year guarantee. Call 01865 704245 or email info@heritageskylightsoxford.co.uk.
Helpful reading on this
Costs, comparisons and the questions we are asked most.
Tell us about your roof
We start with the building, the covering and the slope. Then we tell you what will suit it, and what it will cost, as a fixed written number.
- Surveyed before it is priced
- 10-year workmanship guarantee
- Building Control notification handled
- New installations and replacements