How high should a rooflight kerb be?

Why that number

It gives the covering enough face to be dressed up and terminated properly beneath the frame flange, with a margin above any water that crosses the roof.

Below about 100mm the detail becomes marginal, and below that there is simply nowhere for the covering to finish. That is the single most common cause of a leaking flat roof rooflight.

Measured from the finished surface

Not from the deck, and this trips people up. On a warm roof there is insulation and waterproofing above the deck, so a kerb 150mm above the deck may be only 50mm above the finished surface.

On a green roof or a terrace it is worse: the build up can be 200mm or more, so the kerb has to be considerably taller. That is designed from the actual build up rather than assumed.

At the lowest point

The roof falls, and the top of the kerb is level. So the kerb is deeper on the downslope side, and the 150mm applies where the roof surface is highest relative to it. Measuring at the wrong corner is how a kerb ends up too low on one side.

Too tall

Possible, and mostly a visual matter. A unit sitting well above the roof looks like an afterthought from the garden, and on a shallow extension roof it can affect permitted development where the 150mm projection rule applies.

Checking an existing one

Look at how the covering finishes against the frame. If it stops short, or is held on with sealant rather than terminated under the flange, the kerb was probably too low.

At least 150mm

That is the working minimum above the finished roof surface, and it is the figure to hold any installer to. Some manufacturers state it as a condition of their warranty.

The reason is straightforward. Every flat roof holds some water after rain, and a heavy shower, a blocked outlet or a slight ponding area can raise that level considerably. A 150mm upstand keeps the glazing and its perimeter seal clear of all of it.

Where lower fails

A unit bedded straight onto the deck, or on a kerb of 50mm or 75mm, has no margin at all. It sits in a puddle after every shower, and a seal designed to shed water is not designed to be submerged. Over a few winters it works loose, and that is the commonest cause of a leaking flat rooflight.

On a self built or older extension there frequently is no kerb at all, which is the first thing we look for on any flat roof enquiry.

Where higher is worth it

A roof with poor or no fall, where water genuinely stands. Raising the kerb is not a substitute for correcting the fall, and it buys margin while you do.

A roof that collects debris, under trees, where leaves and silt build up against the upstand.

A green roof, where the growing medium itself is deep and the effective surface level is much higher than the deck. Kerbs there are considerably taller and the figure comes from the build up rather than from a general rule.

A very exposed site, where wind driven rain is pushed up a slope rather than running down it.

What matters as much as the height

Fall away from it. The deck must drain away from the kerb rather than toward it. A tall kerb in a puddle is still a kerb in a puddle.

Insulation on the outer face. The item nobody mentions. An uninsulated timber kerb punches straight through the roof insulation around the whole perimeter of the opening, presenting close to a square metre of poorly performing envelope on a typical unit. It is a sheet of rigid board and an hour, and it disappears under the covering, which is precisely why it gets skipped.

A continuous vapour control layer sealed to the frame, so warm moist air from the room cannot reach the cold side of the construction.

Square and level. A rooflight is manufactured to a tolerance and a kerb that is out of square will not take it properly.

The question to ask

What height, whether the deck falls away from it, and whether the outer face is insulated. Those three answers tell you more about the quality of a flat roof rooflight quotation than anything about the glass does.

What the height has to clear

The kerb lifts the waterproofing termination and the rooflight frame above the level water can reach on the roof surface.

That is not just the depth of a rain shower. It is the depth water can reach when an outlet blocks, when a shallow area ponds, or when a storm overwhelms the drainage for an hour. The kerb is what stops that being a flood into the room.

Guidance sets minimum upstand heights for flat roof abutments and rooflights, and a rooflight kerb is designed to at least that.

What adds to it

A green roof. The kerb has to clear the finished substrate and planting level by a real margin, so wet growing medium is never against the frame. This is where kerbs are most often built too low.

A ballasted or paved roof. Same principle: clear the finished surface, not the deck.

Marginal falls. Where water is likely to stand, more height is cheap insurance.

What limits it

Appearance from the garden, mainly. A rooflight perched on a tall plinth reads as an object on the roof rather than part of it.

And on a lantern, the kerb height combines with the lantern pitch to set the overall ridge height, which on a permitted development extension may be constrained.

The two things that matter as much as height

It must be insulated on its outer face, continuous with the roof insulation. An uninsulated kerb produces condensation on its internal face and it will be reported as a leak.

And it must be square and level, because the unit is made to the finished kerb dimensions and a unit on an out of square kerb does not sit down evenly.