Should Rooflights Be Replaced When a Flat Roof Is Re-Covered?

Yes. Replace the rooflights when the flat roof is re-covered. The access, set-up and disposal are already paid for, the new covering can be dressed onto a new insulated upstand rather than a 30-year-old one, and one contractor owns the joint. Split the two jobs and the interface between membrane and kerb belongs to nobody when it leaks.

That is the whole argument, and it holds whether the rooflight has failed yet or not. If a dome on a roof you are about to re-cover is 25 years old, it is going to need replacing inside the guarantee period of the new covering, and the only cheap moment to do it has already arrived.

The shared costs, and what they actually are

People assume the saving comes from a discount. It does not. The unit price of a rooflight is what it is. What you save is everything that gets paid for twice when the jobs are separate.

The real problem is not money. It is the kerb

Here is what happens on a re-cover that leaves the rooflight in place.

The roofer strips the old covering, lays new insulation, and reaches the existing upstand. That upstand is whatever was built with the extension: typically bare untreated or lightly treated timber, no insulation on the outer face, and after two or three decades of a dome frame bedded on mastic, the top edge is often soft. The roofer cannot see much of it, because the rooflight is still bolted down on top and taking it off is not in the scope. So the new membrane gets dressed up around a kerb that has not been assessed, terminated under a frame that has not been lifted, and finished with a bead of sealant where the two meet.

Now you have a 25-year single-ply or EPDM covering, guaranteed and correctly laid, joined to a component that is at the end of its life, with the joint sealed by a material that has a service life of about five years. The weakest element in the assembly is the one nobody priced.

The second problem is level. New insulation raises the roof surface. A 150mm upstand with 40mm of new tapered insulation dressed around it is a 110mm upstand, measured before you account for the fall. Our minimum is 150mm above the finished roof at the lowest point of the fall, 200mm on open-coast sites, and a re-cover is precisely the operation that quietly eats that dimension. If the upstand is not raised to suit, the roof has been improved everywhere except the one place it leaked.

Split responsibility, and what happens in year four

This is the part that costs people the most and gets discussed the least.

When one contractor lays the covering and a different one fits or leaves the rooflight, there are two guarantees and one joint. Water appears on the ceiling reveal in the fourth winter. The roofer says the membrane is sound and the leak is at the unit. The rooflight installer says the unit is sound and the membrane termination has lifted. Both are telling the truth as far as their own work goes, because the failure is at the interface, which neither of them specified and neither of them warranted.

The homeowner then pays a third party to find it. That third party is working blind on somebody else’s detail, and the usual outcome is mastic, which brings you back in eighteen months.

When the same contractor does the covering detail and sets the unit, there is one guarantee covering one assembly and no seam in the responsibility. Ours runs to 10 years on workmanship, and it covers the upstand, the dressing and the unit as one thing, because we built all three.

What it looks like in Suffolk and north Essex

The 1960s to 1980s estate housing at Kesgrave, Rushmere St Andrew, Chantry and Whitton is hitting this now, in volume. Flat-roofed garages and utility extensions, original domes, and oxidised bitumen felt that expired years ago. Those two things reach end of life within a few years of each other because they were installed on the same day, and treating them as one job rather than two is simply admitting that.

Central Ipswich terraces with flat-roofed rear additions are the other common case, often with restricted access down a side alley where getting a tower in is half the effort of the job. Doing it once matters more there than the materials do.

On the coast at Felixstowe or Harwich there is a further reason. A re-cover is the only practical opportunity to change the fixing specification, because the fixings are under the covering. Retrofitting 316L stainless and isolation washers to an existing installation means stripping back to do it, which is the job you were already doing.

When splitting them is the right call

Not always, and we will say so. If the covering is modern polyester-reinforced felt with ten good years left and the dome has cracked, replace the dome now and re-cover later. If the roof is failing and the rooflight is a sound flush glass unit under five years old, it can often be carefully lifted, set aside and re-set onto a rebuilt upstand. That is a specification decision made at survey with the unit in front of us, not a rule.

What we will not do is dress a new covering around an old dome and call it finished. If you want the detail behind how the covering meets the kerb, see rooflight flashing and membrane detailing and kerb construction. Timing questions on the rooflight itself are covered on when to replace a dome rooflight, and if you are replacing more than one, the cost of replacing several at once sets out where the savings sit. The full picture is on the flat roof skylight installation hub.

If a re-cover is on your list this year, request a fixed quote and tell us the covering, the age of the rooflight and roughly where the property is. We will survey both together and price them as one job.