Flat rooflights on a cold roof build up

The ventilation path

A cold roof depends on a continuous air path from one eaves to the other, above the insulation. That path is what keeps the deck dry.

A rooflight kerb dropped into a cold roof interrupts it. A kerb built tight down between the joists blocks the flow across the roof, and the area beyond it stops being ventilated. Damp then appears on the deck downstream, some distance from the rooflight, which makes it difficult to attribute.

The detailing has to maintain the path around the opening, which usually means the kerb sits on top of the deck rather than being built down into the void, with cross ventilation preserved.

Vapour control

In a cold roof the vapour control layer sits below the insulation, at ceiling level, and it must be continuous. A rooflight puts a hole in the ceiling.

Unless that layer is carried up around the opening and sealed to the kerb, warm moist air from the room goes straight into the ventilated void, and there is a great deal of it in a kitchen or bathroom.

The misdiagnosis

This is the classic. Condensation forms on the underside of a cold deck, runs, drips onto the ceiling, and appears as a damp patch near the rooflight. Everybody concludes the rooflight leaks.

The distinguishing test is the same as always: does it happen in cold dry weather? Rain leaks follow rain; condensation follows cold.

Converting to warm

Where a cold roof is due for recovering anyway, converting it to a warm roof at the same time is usually the better answer. It removes the ventilation dependency entirely, performs better, and makes the rooflight detailing simpler and more robust.

Doing it as part of a recovering is a fraction of the cost of doing it on its own.

Cold roof and warm roof, in one paragraph each

A cold roof puts the insulation at ceiling level, between and under the joists. The structural deck above it sits outside the insulation, so the void between the insulation and the deck is cold. That void has to be ventilated, because any moisture reaching it must be carried away rather than condensing on the cold deck.

A warm roof puts the insulation above the deck, so the deck and the joists sit inside the thermal envelope and stay warm. There is no cold void, so there is nothing to ventilate.

Almost every flat roof built in this country before the last twenty years or so is a cold roof. Almost every new one is a warm roof. Which one you have determines how a flat rooflight can be detailed, and it is the first thing to establish.

Why a rooflight is a problem in a cold roof

The rooflight opening interrupts the ventilated void.

A cold roof works because air moves through the void from one side to the other, entering at one eaves and leaving at the other, carrying moisture with it. That cross flow depends on the void being continuous.

Cut a hole in the deck and build a kerb through it, and you have placed an obstruction across the flow path. Air arriving at the kerb has to go around it, and if the joists run the wrong way relative to the kerb, or the trimmers box the opening in, it may not be able to go anywhere at all.

The result is a stagnant pocket beside the rooflight where moisture accumulates and condenses on the underside of the deck. Over a few winters that rots the deck and the joists, silently, above a ceiling nobody has opened.

What the ventilation path needs

Cross ventilation maintained around the opening, which means an air route past the kerb on every side.

Where joists run perpendicular to the flow, the trimmers around the opening block individual joist bays. Those bays need cross noggins drilled or notched, or a continuous ventilation route formed above the joists, so air can pass from one bay to the next around the obstruction.

The void depth above the insulation has to be maintained too. Squeezing insulation up tight against the deck to make room for the kerb closes the path in exactly the place it is most needed.

This is design work, not something to resolve on site with the deck already cut.

The kerb, and the cold bridge it creates

In a cold roof the kerb passes from the warm side to the cold side through the insulation layer, which makes it a direct conductive path by construction.

Insulating the kerb is therefore not optional. It needs insulation on its outer face, continuous with the ceiling level insulation, so the thermal line is unbroken where it turns up to meet the rooflight frame.

Where that is not done, the internal face of the kerb runs cold, and because the kerb is at the top of a warm room where humid air collects, it produces condensation, staining and eventually mould around the rooflight reveal.

The overwhelming majority of flat rooflight damp reported as a leak is this, and the tell is that it appears in cold weather rather than after rain.

The vapour control layer

In a cold roof the vapour control layer sits at ceiling level, on the warm side of the insulation, and its job is to keep moist internal air out of the cold void.

At a rooflight it has to be carried up the inside of the kerb and sealed to the rooflight frame with tape, continuously, all the way round.

An unsealed vapour control layer at the kerb is worse than no rooflight at all, because it opens a direct route for warm humid air from the room into the coldest part of the roof. Kitchens and bathrooms are the worst cases, and a rooflight over either deserves particular attention here.

Downlighters cut into the ceiling around a rooflight puncture the same layer, and a ring of them around a rooflight in a cold roof is a genuine problem rather than a theoretical one.

What a survey should establish

Whether it is a cold roof at all. Lifting a ceiling access hatch, or drilling a small inspection hole, settles it in minutes and changes the entire specification.

The joist direction relative to the proposed opening, because that determines what the ventilation path has to do.

Whether the existing ventilation actually works. Many cold roofs were built with inadequate ventilation and have been quietly deteriorating for decades. Adding a rooflight to an already marginal roof is how a slow problem becomes a fast one.

And the condition of the deck. On a roof of age, a rooflight installation is one of the few occasions the deck is exposed, and finding soft timber then is far better than finding it later.

The case for converting to a warm roof

Where the covering is due for replacement anyway, this is usually the right answer.

A warm roof build up puts insulation above the deck, removes the ventilated void entirely, and with it removes the whole class of problems described above. The kerb then sits within the insulation rather than through it, the cold bridge is far easier to control, and there is no ventilation path to maintain.

It also gives a much better thermal result, because the insulation is continuous over the joists rather than interrupted by them.

The cost is height. A warm roof build up raises the roof level by the thickness of the insulation, which affects the upstand at abutments, the flashing into any adjoining wall, and possibly a door threshold or a window sill above. On an extension roof that is usually manageable. Against an existing wall with a window immediately above, it may not be.

The decision point is the covering. Replacing a sound covering purely to convert the build up rarely pays. Converting while the covering is being replaced regardless is close to free.

Where a hybrid goes wrong

Adding insulation above the deck of an existing cold roof, without removing the insulation below, creates a build up with insulation on both sides of a void.

That void is now colder than before, because the new insulation above keeps warmth from reaching it, and its ventilation has usually been compromised in the process. It is the worst of both arrangements and it produces condensation reliably.

If insulation goes above the deck, the insulation below comes out and the void becomes part of the warm zone. Half measures on this specific detail cause damage.

What to ask for on a quotation

Confirmation of which build up the roof is, established by inspection rather than assumption.

How ventilation will be maintained past the kerb, described specifically.

How the kerb will be insulated.

How the vapour control layer will be sealed to the rooflight frame.

Photographs of the vapour control layer taped and the kerb insulated, taken before anything covers them. All of it is buried by the end of the job, and the photograph is the only record that it was done.

How to tell which build up you have

Before anything else, and it changes the entire specification.

Lift a ceiling access hatch if there is one, or drill a small inspection hole and look with a borescope or a torch.

If there is a void above the ceiling with insulation between the joists and a deck above the void, it is a cold roof. If the insulation sits on top of the deck and there is no void, it is a warm roof.

Age is a strong indicator. Almost every flat roof built before the last twenty years or so is a cold roof, and almost every new one is a warm roof. But it is worth confirming rather than assuming, because it takes ten minutes and it determines everything that follows.

Look for evidence the roof is already struggling

Many cold roofs were built with inadequate ventilation and have been quietly deteriorating for decades.

Signs are staining on the underside of the deck, soft or dark timber at the joist ends, a musty smell in the void, and any history of unexplained damp patches in cold weather.

Adding a rooflight to an already marginal roof is how a slow problem becomes a fast one, because the kerb obstructs what little cross flow there was.

Maintaining the air path

Cross ventilation has to continue past the opening on every side.

Where joists run perpendicular to the flow, the trimmers around the opening block individual bays. Those bays need cross noggins drilled or notched, or a continuous ventilation route formed above the joists, so air can pass from one bay to the next around the obstruction.

And the void depth above the insulation has to be maintained. Pushing insulation up tight against the deck to make room for the kerb closes the path exactly where it is most needed.

This is design work, not something to resolve on the day with the deck already cut.

The kerb is a bridge by construction

In a cold roof the kerb passes from the warm side to the cold side through the insulation line. Insulating its outer face is therefore not optional, and where it is missing the internal face runs cold and produces condensation every winter.

The hybrid that must not be built

Adding insulation above the deck without removing the insulation below leaves a void colder than before, with its ventilation compromised. It is the worst of both arrangements and it produces condensation reliably.

If insulation goes above, the insulation below comes out.