Kerb heights when the surface must stay flush

The upstand still exists

It is not omitted. It is buried within the terrace build up.

The roof waterproofing is still turned up an upstand around the opening, to the same effective height as any other rooflight. The difference is that the paving, decking or pedestal system sits alongside that upstand rather than below it, so the finished walking surface arrives level with the top of the glass.

The waterproofing is therefore protected to the same height. It is simply not visible.

The build up decides the frame

Because the finished level is set by the deck, the frame height has to be established from that. A pedestal paving system might be 100mm; a timber deck on joists 150mm; a bonded system 60mm.

So the deck build up is decided before the unit is ordered, not after. Changing the paving thickness later changes whether the panel sits flush.

Drainage at two levels

Water reaches two places: the surface, where it drains through joints or gaps, and the waterproofing layer below, where it runs to the outlet.

Both need a route past the unit. The perimeter channel handles the surface; the falls in the waterproofing handle the layer below. A design that considers only one of them will pond at the other.

Access for maintenance

Pedestal paving lifts, which is a genuine advantage: the waterproofing and the drainage around the unit can be inspected without dismantling anything. A bonded or fixed deck cannot, which is worth weighing at design stage.

What not to do

Reduce the upstand height to make the levels work. That is the detail keeping water out of the building, and the deck build up is what should be adjusted instead.

The conflict at the heart of walk on glazing

Every principle of flat roof rooflight detailing says lift the unit clear of standing water: at least 150mm of upstand, with the deck falling away from it.

Every principle of terrace design says do not put a 150mm step in the middle of a walking surface.

Walk on glazing has to satisfy both, and the way it does it is by moving the defence from height to drainage. Understanding that is the whole of this subject, because it explains why perimeter drainage on a flush unit is not a refinement but the primary protection.

What a kerb normally does

Three things.

It lifts the glazing and its perimeter seal above whatever water the roof holds, which is always some, and considerably more after a downpour or with a blocked outlet.

It gives the roof covering something to be dressed up and terminated against, so that water landing on the glass runs onto membrane rather than into a joint.

And it provides a place for insulation, so the perimeter of the opening is not a cold bridge punching through the roof insulation.

A flush installation has to achieve all three by other means.

How a flush unit is protected instead

The surrounding surface falls away from the unit. Not toward it, and not level. This is the first line of defence and it is designed into the terrace rather than added afterwards.

A perimeter drainage channel. A continuous slot or channel around the panel, collecting whatever water reaches it and carrying it to an outlet. On a flush unit this is doing the job the 150mm upstand does elsewhere.

An outlet that actually runs. A channel discharging into a blocked gulley is a pond, and a pond against a seal is exactly what the whole arrangement exists to prevent.

An upstand within the build up. Even where the finished surface is flush, the waterproofing layer is frequently turned up at the opening within the construction, so the membrane termination is above the drainage level even though nothing is visible above the paving.

Why maintenance matters more here than anywhere

Because the protection is dynamic rather than static.

A 150mm kerb keeps working while you ignore it. A drainage channel does not: leaves, silt, moss and grit collect in it, and a blocked channel produces exactly the standing water the design was built to avoid.

That is why the commonest cause of trouble on flush walk on installations is not a defect at all. It is a channel nobody has cleared for three years.

Twice a year on a plot with trees, annually otherwise, and check the outlet at the same time. It takes minutes and it is the single most valuable thing an owner does for these units.

The thermal problem, which is the second half

A flush unit over heated space is a cold bridge at the perimeter unless it is dealt with deliberately.

On a raised kerb the insulation goes on the outer face and disappears under the covering. On a flush installation there is no outer face in the same sense, so the insulation has to be worked into the surrounding construction and taken right up to the frame.

Get that wrong and the symptom is condensation on the ceiling below, around the edge of the panel, in cold weather. It reads as a leak and it is not one, and it is awkward to correct after the terrace surface is laid.

The vapour control layer has to be continuous and sealed to the frame for the same reason, so warm moist air from the room below cannot reach the cold side of the construction and condense inside it.

Where the surface does not have to stay flush

Worth asking, because a great deal of expense follows from the assumption that it does.

A raised kerb is a trip hazard in a walking route. It is not a trip hazard at the edge of a terrace, against a parapet, or in a corner that nobody crosses.

Where the glazing can sit outside the through route, a conventional 150mm kerb becomes acceptable again, and with it a standard non fragile rooflight rather than a walk on unit. That is a substantially cheaper installation with substantially less to maintain.

It is half an hour with the plan, and it is worth doing before committing to structural glazing.

Green roofs, where the arithmetic reverses

The opposite situation and worth mentioning because the same question arises.

On a green roof the effective surface level is the top of the growing medium, not the deck. A kerb built to the conventional 150mm and then buried in substrate is a rooflight sitting in wet soil.

So the upstand has to clear the substrate depth plus the usual margin: on a sedum roof with 80mm of growing medium, 230mm or more, and considerably more on a deeper build up.

Add a vegetation free margin of gravel or paving around it, typically 300mm or more, to keep roots and growing medium off the upstand, allow the flashing to be inspected, and give somewhere to stand when cleaning the glass.

What to establish before anything is ordered

Whether the surface genuinely has to be flush, or whether the glazing can move out of the walking route.

Which way the existing or proposed surface falls, and where the water goes.

Whether there is an outlet, whether it works, and whether it has capacity for a perimeter channel discharging into it.

What is below: heated space, unheated space, or outside. That decides how much the thermal detail matters.

And whether the structure can carry a walk on unit, which is a separate question and frequently the largest line on the quotation.

Why we survey rather than pricing from a photograph

Because every one of those questions is answered on the roof rather than from the ground, and because getting the flush detail wrong is not a snagging item.

A raised kerb that is slightly too low can be built up. A flush installation with inadequate falls or a channel that does not drain means lifting the surface and starting again, and by then the glazing is in and the terrace is finished.

The contradiction at the heart of it

A kerb keeps water out by height. A flush finish removes the height. So a flush installation deliberately gives up the primary defence and has to replace it with something else.

That something else is drainage, and there is no substitute. Every flush detail that works is a drainage detail; every one that fails, failed at drainage.

The upstand that still exists

Even in a flush installation there is normally an upstand. It is simply below the finished walking surface rather than above it.

The frame sits on it, the waterproofing is dressed up it and terminated into the unit, and the deck build up comes up around the outside. The upstand is doing its job within the construction rather than in view.

Which means the height question becomes: how high above the waterproofing does the termination sit, and can water reach it?

What water can reach it

Not rain landing on the glass, which sheds to the edges. The threat is water moving across the deck.

On a paved terrace, water travels through the joints and along the bedding to the waterproofing below, then moves according to the falls in that layer. If those falls carry it to the rooflight upstand, it will stand against the termination whatever the surface above looks like.

So the falls that matter are in the waterproofing layer, not the paving. That is the distinction most often lost between trades.

The perimeter channel

Where an upstand cannot turn water away above the surface, a channel intercepts it instead.

Three requirements, none optional. A real outfall, with enough fall along the channel to move water to it. A removable grating, or it fills with leaves and grit within two seasons. And sizing for the actual catchment, which is the terrace, not the pane.

The drained and ventilated frame

Good systems assume water will get past the surface seal and design a route back out: a chamber below the glass with weep paths to the outside.

Ask whether the system is drained and ventilated. It is a standard term and it is the difference between a seal failure that produces a damp patch and one that produces water in the room below.

Where a small upstand is the honest answer

On a roof accessed twice a year for maintenance, an upstand costs a fraction as much, weathers far more reliably, and the trip hazard is irrelevant because nobody is walking there.

Flush is worth the cost and the design attention on a terrace people actually use. It is not worth it for appearance alone on a roof nobody crosses.

Asking the drained and ventilated question

Good systems assume some water will pass the surface seal and design a route back out.

It is a standard term. A supplier who cannot answer whether their system is drained and ventilated is not selling a product designed for a flush position.