Does a roof window in a tiled roof need a kerb?

A roof window in a pitched tiled or slate roof does not sit on a kerb. It is fixed to the rafters with brackets and surrounded by a flashing kit made for the roof covering. The kerb question applies to flat rooflights and lanterns.

Who builds a rooflight kerb, the builder or the installer?

In brief

Either can, and it is agreed job by job before work starts. On a new flat roof the builder often forms the kerb from the sizes we send in writing, because the carpenters are already on the roof. On other jobs we build it ourselves. In both cases the exact kerb size for the chosen unit is fixed before the deck is boarded, so the rooflight fits the first time.

A kerb is the raised timber upstand that a flat rooflight or lantern is seated on
Builders often form it on a new roof; we build it on other jobs
The size comes from the unit and goes to the builder in writing
Whoever builds it, the size is fixed before the deck goes on
The roofer, then the rooflight fitter, follow the kerb in that order

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What a kerb is and why the size matters

A kerb is a low timber frame, usually built up from the roof joists, that surrounds the opening and lifts the rooflight clear of the roof surface. The roofing membrane is dressed up its outer face to form the upstand, and the rooflight frame sits on top. On a flat roof it is the part that keeps water running past the opening and not into it.

The size is set by the unit. A flat rooflight or roof lantern has outer dimensions, and the kerb has to match them to within the maker's tolerance. If the kerb is out, the frame can overhang, sit unevenly or leave a gap needing packing out. Height follows the maker's instructions. A common aim is an upstand that stands 150mm or more proud of the finished roof, which gives the covering room to turn up behind the frame.

When the builder forms the kerb

On an extension or a new flat roof, the carpenter is already cutting joists, boarding the deck and fitting firrings to give the roof its fall. Forming a kerb at the same time is a small addition to that work, and it means one trade sets out the whole roof without a second visit.

In that case the builder or architect receives the kerb dimensions from us in writing, for your unit, with the overall outside size, the internal opening and the height. They build to those figures, the roofer then covers the deck and turns the membrane up the kerb, and we arrive to fit the rooflight onto a base that is ready for it.

The builder's price should name the kerb, and our quote states whether the kerb sits in our price or theirs. Settling this at the start means it is neither paid for twice nor missed by both parties.

When we build the kerb ourselves

If a flat roof is already covered and you want a new rooflight in it, there is no carpenter on site. We trim the joists for the opening, form the kerb from treated timber, and leave it ready for the roofer to dress the covering around it. Where the roof covering is a type that the roofer handles, such as a warm roof membrane, we agree with them who does the cutting back and making good.

We also build the kerb where a builder would rather not take on the risk of getting the size wrong. A kerb we have formed for a unit we ordered is certain to match that unit, because the same person measured both. The cost of the kerb then appears as a line in our quote.

For help with what moves the price of a retrofit, our answer on what it costs to fit a skylight sets out the factors.

Why the size is fixed before the deck goes on

Once the deck is boarded and the membrane laid, moving a kerb brings the roofer back as well as the carpenter, and the covering has to be cut back and made good around the new position. It is far cheaper to settle the size in advance than to adjust afterwards.

A made-to-measure unit is also ordered from the kerb size. If the size changes after the order, the unit may need to be remade, and the delay that follows is the sort of thing our guidance on rooflight lead times describes. For that reason we send the kerb sizes as soon as the unit is chosen, and ask the builder to confirm them back.

If the drawings change, tell us. A different rafter spacing, a thicker insulation layer or a change in roof pitch can alter the kerb, and a revised sheet is quicker than a correction on site.

The order of trades around the kerb

The carpenter, or we, form the opening and the kerb to the written sizes.
The roofer lays the covering across the deck and turns it up the kerb.
The electrician runs first-fix cables for an electric unit or reveal lighting.
We fit the rooflight and check it against the kerb and the covering.
The plasterer closes up the reveals after the unit is in and weathertight.

Doing the work in that order means nobody undoes anyone else's work. The roofer is not asked to cut a finished membrane to fit a late rooflight, and the plasterer does not return to make good a reveal around a unit that was installed afterwards.

Putting it in the contract

The simplest way to avoid doubt is to name the kerb in the paperwork. Ask your builder whether the kerb is in their quote, and check our quote to see whether it is in ours. The line that matters is the one that says who forms it, to which dimensions, and by what date.

Our written quote sets out the unit, plus the points to settle with the builder, and the kerb sizes follow separately in writing. For the whole sequence, from survey to the finishing trades, see our rooflight installation service in Southampton.