Reusing an existing kerb, and when not to

What we check

Square. Both diagonals measured; they must match. A parallelogram kerb will not take a rectangular unit, and the unit cannot be adjusted.

Level along the top. The roof beneath falls, but the top of the kerb should be level. A kerb built at constant height on a falling roof sits at an angle.

Height. Around 150mm above the finished surface at the lowest point. Too low and the covering never had anywhere to terminate, which is usually why the old unit leaked.

Condition. Probe the corners. Water sits there and rot starts there.

Insulation. Whether the outer face was insulated. Often it was not, and a replacement is the one opportunity to correct it.

Vapour control. Whether the layer was carried up and sealed to the old frame, which decides whether the damp everyone blamed on the unit was actually condensation.

When it has to be rebuilt

Any rot. Out of square by more than a few millimetres. Too low. Or where it was built for a system with a different kerb dimension convention and the new unit needs a different size.

What rebuilding involves

The covering is cut back around the opening, the kerb removed and rebuilt, insulation and vapour control detailed properly, then the covering dressed up the new kerb and terminated. On a single ply or felt roof that joint can be made well. On GRP it is harder, and sometimes the honest answer is that the roof wants recovering.

Why it is worth doing properly

Because a new unit on a bad kerb fails the same way the old one did, and you will have paid twice to learn that.

The most valuable thing on a flat roof

The expensive part of a flat roof rooflight is not the unit. It is forming the kerb, correcting the fall around it, insulating it and dressing the covering up it.

So where an existing kerb is sound, square and correctly built, a replacement becomes one of the quicker jobs we do: lift the old unit off, prepare the kerb, set the new one on. One to two days.

That is a genuine saving and it is worth checking for before assuming a replacement means starting again.

The five checks

1. Height. At least 150mm above the finished roof surface. Every flat roof holds some water, and a unit bedded onto a low upstand sits in it after every shower. A perimeter seal designed to shed water is not designed to be submerged.

This is the most common failure. A great many older domes sit on 50mm or 75mm upstands, or straight on the deck, and that is frequently why the old unit failed.

2. Fall. Does the deck fall away from the kerb, or toward it? Standing water against the base is the commonest cause of a failed flat roof rooflight, and no amount of kerb height fixes a roof that drains the wrong way.

Look at the roof after rain. Where the water goes, and where it does not, answers this in thirty seconds.

3. Square and level. A rooflight is manufactured to a tolerance. A kerb that has twisted, sagged or was never square will not take a new unit evenly, which leaves the frame stressed and the seal loaded unevenly from day one.

4. Sound timber. A kerb that has been sitting in water for years may itself be decayed. Press it. Timber that gives is coming out.

5. Insulation. Is the outer face insulated? Frequently not on older work, and this is the item that matters most for how the room feels.

When to reuse

Where all five checks pass, reuse it. Clean it up, check the covering termination at the top, and set the new unit.

Where four pass and the fifth is the insulation, reuse the structure and add the insulation while the covering is open around it. That is a small addition to work already in hand and it is far cheaper than doing it as a separate exercise later.

When not to

Where the height is inadequate. Rebuilding or raising the kerb is the whole point of doing the job properly, and reusing a 75mm upstand means repeating the history of the unit you are replacing.

Where the fall is wrong. This has to be corrected locally around the kerb, using tapered insulation or firrings under a new deck section, and that means opening up around the kerb anyway.

Where the timber is decayed. Not a judgement call. It comes out.

Where the covering is at the end of its life. If the membrane around the kerb is brittle, blistered or lifting, disturbing it will make it worse, and dressing a new termination into failing material is not a repair.

Where the new unit is a different size. Obvious, and worth stating, because dome sizes and modern rooflight sizes come from different traditions and rarely match.

The size question, which is an opportunity

Where the kerb has to be rebuilt anyway, there is latitude about size and position that would not exist otherwise.

That is worth using rather than replicating a dome position chosen thirty years ago for reasons nobody remembers.

On a long narrow room, two modest units spaced along the length light it more evenly than one central unit ever did. On a room where the light lands in the wrong place, moving the opening a metre costs almost nothing when the kerb is being formed regardless.

Adapting rather than rebuilding

The middle ground, and frequently the right answer.

A sound kerb of adequate height that is slightly the wrong size can be packed out or trimmed and re dressed. A kerb of adequate footprint but inadequate height can be built up rather than replaced. A kerb missing its insulation can have it added.

Each of those is considerably less work than starting again, and each keeps the structural connection to the roof deck undisturbed, which is worth something on an older roof.

What we do at survey

Look at the fall before anything else, because it decides the shape of the job. Then the kerb height, then square, then the timber, then the insulation.

That is why we will not price a flat roof rooflight from a photograph taken from the ground. Whether a 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 visible from below.

An indicative range by email costs nothing and tells you whether to continue.

The related decision: is the covering due

Worth asking at the same time.

Where a flat roof covering is approaching the end of its life, doing the covering and the rooflight together is considerably cheaper than doing them separately. The access, the setting up and the making good are shared, and the falls, the kerb and the covering can be detailed as one job rather than one retrofitted into the other.

If your flat roof is over fifteen years old and you are considering a new rooflight, ask for both to be priced together before deciding. It is frequently a smaller step than people expect and it removes the awkward situation of a new unit sitting in an old covering that gets replaced three years later.

What good looks like when it is finished

At least 150mm of upstand above the finished surface. The deck falling away from it to an outlet that runs. Rigid insulation on the outer face, continuous around all four sides and lapped into the main roof insulation. A continuous vapour control layer sealed to the frame. The covering dressed up the full height and terminated properly before the unit was set.

None of it visible afterwards, all of it decisive, and all of it worth naming on the quotation so you know it was priced rather than assumed.

What has to be true to reuse it

Four conditions, and all four have to hold.

The timber is sound. Probe the top edge and the corners with a bradawl. A kerb that has had water standing against it, or a waterproofing termination that failed slowly, will be soft where it matters.

It is square and level. Measure both diagonals and check the top edge with a level along each side. A new unit is made to the finished kerb dimensions and the tolerance is small.

It is high enough. Standards and expectations for upstand height have risen, and a kerb built to a lower expectation twenty years ago may not clear the water the roof can hold with a blocked outlet.

It is insulated, or can be. An uninsulated timber kerb is the single most common defect on flat rooflight installations, and reusing one perpetuates it.

The strong argument for rebuilding

The replacement is the only moment the kerb is accessible. Everything about it that is wrong will stay wrong for the next twenty five years if it is not addressed now.

An uninsulated kerb produces condensation on its internal face every winter, staining the reveal and eventually growing mould. Owners report it as a leak, roofers find nothing, and it continues.

Insulating it while the unit is off is straightforward. Insulating it later means taking the new unit out.

The other things to fix at the same time

The fall. If water has been ponding against the old kerb, it will pond against the new unit unless the fall is corrected. That means tapered insulation or firrings, and it happens before the rooflight goes back.

The waterproofing termination. Where the membrane was dressed over the old unit rather than up the kerb and into it, the lap runs the wrong way. Reusing the kerb without correcting that reproduces the fault.

The vapour control layer, taped to the new frame.

Where reuse is genuinely right

A kerb built recently, to current standards, insulated, square and sound, with a roof that drains properly, and a replacement unit available to suit its dimensions.

That situation exists and it makes for a comparatively simple job.

Why it should be a provisional item

Kerb condition is not visible until the old unit is lifted, so a sensible quotation prices the replacement on the assumption of reuse and carries a stated allowance for rebuilding.

A quotation that assumes reuse without qualification is either optimistic or leaving you to negotiate on the day with the roof open.

Checking it before the quotation

Most of the condition can be assessed from inside and from the roof without lifting the unit. Probe the visible timber, check the diagonals across the opening from below, and look at whether water has been standing against the outside.

That is usually enough to tell a quoter whether to price for reuse with an allowance, or for rebuilding outright.