Upstands and warm roof build ups

Warm roof

The insulation sits above the structural deck, with a vapour control layer below it and the waterproofing above. The deck and the structure stay warm, on the room side of the insulation.

This is the modern default and it is what almost every new extension uses. It performs better, it puts no cold surfaces where condensation can form inside the build up, and it needs no ventilation void.

Cold roof

The insulation sits between the joists, below the deck, so the deck itself is cold. That means a ventilated void above the insulation is essential, because any moisture that gets in has to be carried away.

Common on older extensions and generally not used for new work now.

What that means for the kerb

In a warm roof, the kerb interrupts the insulation layer. Unless its outer face is insulated, it is a cold bridge running the entire perimeter of the rooflight, losing more heat than the glass and forming condensation on the inside of the frame in winter.

Insulating the outer face of the kerb is cheap while it is being built and impossible afterwards without opening the roof.

In a cold roof, the kerb passes through a ventilated void, and the ventilation path must not be blocked by it. A kerb built tight across the void interrupts the airflow that keeps the deck dry.

Vapour control

In both cases the vapour control layer has to be carried up the inside of the kerb and sealed to the rooflight frame. Where it is not, warm moist room air enters the build up at the perimeter.

That produces damp in the ceiling around the opening rather than water on the glass, and it is invariably reported as a leaking rooflight.

Establishing which you have

On an existing roof it is not always obvious from below. Part of the survey is determining the build up before anything is specified, because the detailing follows from it.

What a warm roof is

A flat roof where the insulation sits above the structural deck rather than between the joists.

From the bottom: ceiling, joists, deck, vapour control layer, insulation, waterproof covering. Everything below the insulation, including the joists and the deck, is inside the warm envelope and stays at close to room temperature.

It is the modern standard, and for good reasons. The structure stays warm and dry. There is no cold void to ventilate. Thermal bridging at the joists disappears, because the insulation is continuous over them. And it is generally easier to achieve good airtightness.

The alternative, a cold roof with insulation between the joists and a ventilated void above, is older practice and still found on a great many extensions.

Why the upstand is taller on a warm roof

Because the insulation layer sits on top of the deck, so the finished roof surface is considerably higher than the deck itself.

The kerb has to be built from the deck, up through the insulation, and then a further 150mm above the finished surface. On a build up with 150mm of insulation, that is a 300mm kerb from the deck even though only half of it is visible.

Getting that wrong is a common error. A kerb built 150mm from the deck disappears almost entirely once the insulation and covering go on, leaving the rooflight sitting essentially at surface level with no margin against standing water.

The three things the kerb must do

Lift the seal clear of water. At least 150mm above the finished surface, with the deck falling away from it. Every flat roof holds some water and a perimeter seal designed to shed it is not designed to be submerged.

Carry the insulation line up and around. This is the detail specific to a warm roof and the one most often done badly.

The insulation on the deck runs up to the kerb and stops. Unless insulation continues up the outer face of the kerb, the timber punches straight through the insulation line around the whole perimeter of the opening, presenting a substantial area of poorly performing envelope.

Rigid insulation on the outer face, continuous around all four sides, lapped properly into the deck insulation so there is no gap at the base, and taken up to meet the underside of the rooflight flange.

Carry the vapour control layer up and seal it. On a warm roof the vapour control layer sits below the insulation and above the deck. At the opening it has to be dressed up the inside face of the kerb and sealed to the rooflight frame.

Where it is cut and left, moist room air passes into the insulation layer. Wet insulation is expensive, slow to discover and difficult to remedy without stripping the roof.

Cold roofs, where it differs

The insulation is between the joists, and there is a ventilated void above it.

The kerb is therefore shorter from the deck, because the deck is already close to the finished surface. But the insulation line is at ceiling level, so a rooflight opening cuts through it there and the insulation has to be dressed back up to the frame at that level.

The vapour control layer sits at ceiling level too, below the insulation, and is sealed around the opening there.

The additional consideration on a cold roof is the ventilation. The void is ventilated deliberately to keep it dry, and a rooflight opening must not obstruct that airflow. Insulation pushed into a ventilation path while working around an opening is a common and avoidable error, and it produces a damp void rather than solving anything.

Hybrid and inverted build ups

Worth mentioning because both occur.

A hybrid has insulation both above the deck and between the joists. That can work and it needs designing rather than assembling, because the ratio between the two layers determines where the dew point falls within the construction.

Getting that wrong produces condensation inside the build up, and a rooflight opening through a hybrid roof needs the detail to be thought about rather than improvised.

An inverted roof has the insulation above the waterproofing, protected by ballast or paving. Kerb heights there have to clear the insulation and the ballast, so they are taller again, and the drainage detail at the opening is specific to the system.

The covering, and the sequence

The membrane is dressed up the full height of the kerb and terminated properly, before the rooflight sits on it. The unit then sits on top with its own flange overlapping that termination.

Water landing on the glass runs onto membrane rather than into a joint. Reversing that order, or stopping the membrane short, is where a great many flat roof rooflights leak.

Compatibility matters. Bituminous membranes are not compatible with every material, and a rooflight system detailed for single ply may need a different arrangement on felt. That is a specification question worth confirming.

Insulated composite kerbs

Some manufacturers supply kerbs with the insulation already integrated, arriving as a unit rather than being built up on site.

They cost more. They are neat, they are square, and they remove the risk of the insulation being omitted by somebody who did not know it was meant to be there.

On a job where several trades are involved and the kerb falls between the roofer and the rooflight installer, that is worth considering.

What to establish before ordering

Whether the roof is warm, cold or hybrid, because the kerb height and the detailing follow from it.

The insulation thickness, because that determines how tall the kerb has to be from the deck.

Which way the deck falls and where the water goes.

And whether the kerb insulation and the vapour control layer sealing are named lines on the quotation or assumptions.

None of it is visible when the job is finished, and all of it decides how the room performs for the next twenty five years.

Why a warm roof makes the upstand easier

In a warm roof the insulation sits above the deck, so the deck and the joists are inside the thermal envelope. There is no cold ventilated void, so there is nothing for the upstand to interrupt.

That removes an entire category of problem. In a cold roof the upstand blocks the ventilation path and air has to be routed around it, or a stagnant damp pocket forms beside the rooflight and rots the deck silently. In a warm roof that question does not arise.

Where the insulation goes at the upstand

This is the detail that decides whether the installation performs.

The roof insulation runs across the deck and then turns up the outer face of the upstand, continuing until it meets the underside of the rooflight frame. No break, no gap, no thinning at the corner.

What that achieves is an unbroken thermal line: the upstand is inside the insulation rather than passing through it, so its internal face stays warm.

Where the insulation stops at the base of the upstand, the timber box becomes a conductive collar around the whole unit, standing into cold air on four sides. The internal face runs cold, sits at the top of a warm humid room, and produces condensation, staining and eventually mould around the reveal.

That single omission is the most common defect on flat rooflight installations, and it is reported as a leak every winter.

Insulated kerb systems

Manufactured units with the insulation built in, so the detail is resolved in the product rather than on site.

They are worth specifying because they remove the possibility of the insulation being forgotten, they are dimensionally accurate, and they arrive square. On a job where the kerb is being built anyway, the cost difference against timber plus insulation is modest.

The tapered insulation opportunity

A warm roof build up is where the fall is normally created, using tapered insulation boards.

That is worth planning around the rooflight rather than in spite of it. The falls should carry water past the upstand to an outlet, not into a dead corner on the upslope side where it will stand permanently.

Arranging that is a layout decision made on a drawing, and it is far easier to get right before the boards are cut than after.

Height, with the build up in mind

The upstand has to clear the finished surface, not the deck. On a warm roof the insulation and the covering add real depth, and on a ballasted or green roof considerably more.

Measuring the required height from the deck rather than from the finished level is how kerbs end up too low, and it is not correctable afterwards.

The vapour control layer

In a warm roof it sits below the insulation, on the deck, and it has to be carried up the upstand and sealed to the rooflight frame continuously.

Photographs before anything covers it. It is the only record that will exist.