Flat rooflights on a cold roof build-up

What makes a roof a cold deck, and how to tell from inside

A cold deck carries its insulation between the joists, below the structural deck. The deck, the covering and the joists above the insulation line all sit at close to outside temperature, which is where the name comes from.

From inside you can usually tell without opening anything. If the ceiling of a flat-roofed extension is fixed directly to the underside of the joists and the room feels the same height as the original house, it is very likely a cold deck. A warm deck of the same era carries its insulation above the deck, which raises the finished roof level and usually shows as a taller upstand at the abutment and a deeper fascia at the edge. Ventilation grilles or a continuous slotted soffit vent along the fascia are the giveaway: a warm roof has no reason to have them.

The 50mm void, and the rule that has kept cold roofs standing

A cold deck works only if warm moist air that leaks up into the roof void can be carried away before it condenses on the underside of a cold deck.

That means a clear, continuous air space above the insulation, conventionally at least 50mm deep, running the full length of every joist bay, with openings at both ends. Guidance has for many years called for ventilation at opposite sides equivalent to a continuous 25mm gap, with cross flow between them, and for the ventilated span to be kept modest, in the region of five metres.

Every one of those conditions is about air being able to travel. A cold roof does not manage moisture. It removes it.

What a kerb does to air moving across a cold roof

Now put a rooflight in. A kerb 1200mm wide sitting across joists at 400mm centres blocks three joist bays completely. The air that used to travel the length of those bays reaches the trimmer, stops, and has nowhere to go.

Downwind of the opening, the same three bays now have a dead end at their other side. Two lengths of unventilated void have been created out of one working one, and they are directly beneath a deck that a rooflight has just made colder still by taking the insulation out of the middle of it.

This is the mechanism behind most damp discovered around a rooflight in a cold roof, and it has nothing to do with the unit or the covering.

The kerb timber sits on the cold side of the insulation

In a warm deck the kerb is wrapped in insulation, so its timber runs at something close to room temperature and stays dry.

In a cold deck the insulation stops at the ceiling line, and the kerb rises out of a cold void into cold air with nothing warm anywhere near it. Its inner face is exposed to whatever air is in the void, and the void air on a bad night is close to saturation. The kerb is also the piece of timber carrying the fixings that hold the unit down and the tape that closes the vapour layer.

Insulating the kerb externally is still worth doing on a cold roof, and it helps the reveal below, but it does not change the fundamental position of the timber relative to the insulation line.

Dew point, and where the water actually forms

Air at 20 degrees and 55 per cent relative humidity has a dew point of about 10.7 degrees. Any surface in contact with that air, at or below that temperature, collects water.

In a cold deck on a January night, the underside of the deck board is sitting at perhaps two or three degrees. Room air that has found a route past the ceiling reaches it and gives up its moisture immediately, as droplets on the board or as frost. During the day it thaws, runs down the joist, and soaks into the insulation and the ceiling below. The occupant sees a damp area on the plasterboard near the rooflight reveal in February which dries out by April and comes back the following winter.

The vapour control layer in a cold roof, and why continuity is harder

Both build-ups need a vapour control layer on the warm side of the insulation. In a warm deck it sits on top of the structural deck, one continuous plane, easy to see and easy to tape.

In a cold deck it sits under the joists, above the plasterboard, and it has to be interrupted by every downlight, every cable, every ceiling fixing and now by the rooflight reveal. It is installed from below, in strips between joists or as a sheet across them, and the taping is done overhead. A layer that is 95 per cent continuous performs far worse than the number suggests, because moisture goes through the five per cent under pressure.

Around a rooflight the layer has to be taken up the inside of the reveal and taped to the kerb, which on a cold roof means working in the void from below through a limited opening.

Cross ventilation, and the path that has to run past the opening

Ventilation only works if there is a pressure difference driving it, which is why openings on opposite sides matter more than the total free area.

A rear addition ventilated only along the eaves at the back, with the other three sides built into walls, has no cross flow to speak of even before a rooflight goes in. Add an opening in the middle and the two ends of the roof become separate compartments, each with one vented edge or none. On roofs like that, converting the build-up is usually a shorter conversation than trying to ventilate it.

Why the guidance moved away from cold decks

Cold flat roofs were standard domestic practice into the 1990s and there are a great many of them. Current guidance discourages them for new work, and the reason is not that they cannot work. It is that they only work when several things are simultaneously correct and stay correct.

The void has to remain clear. Nobody may stuff extra insulation into it from the loft hatch. The vapour layer has to stay unpunctured, which downlighters and later cabling routinely defeat. The ventilation openings have to stay open, which paint, expanding foam and a later fascia replacement routinely defeat. A warm deck asks for none of that vigilance.

Converting the build-up while the covering is off

Where the covering has reached the end of its life anyway, converting to a warm deck at the same time as fitting a rooflight is usually the better value of the two available paths.

The sequence is: strip the covering, leave the existing between-joist insulation in place or remove it depending on the moisture risk assessment, lay a vapour control layer over the deck, add 100 to 150mm of PIR insulation above it, build the kerb up off the structural deck through that insulation, then cover. The ventilation problem disappears because there is no longer a void to ventilate, the kerb ends up inside the insulation envelope, and the vapour layer becomes a single continuous plane that can be taped to the kerb properly.

The roof level rises by the thickness of the insulation, which affects the door threshold above, the abutment flashing and the parapet height if there is one. Those are checked before the decision is made.

An overlay warm deck, and the kerb height it produces

The consequence people are least prepared for is the finished kerb height. On a converted roof, the 150mm upstand is measured from the new waterproofing, which now sits 150mm or so higher than the old one.

Measured from the original deck, the kerb head can be 300mm or more above it. That is a deep box seen from inside, and the internal reveal below the unit gets correspondingly deeper. Splaying the reveal outward recovers most of the light and makes the depth read as architecture rather than as a shaft, but it wants deciding at the same time as the build-up rather than at second fix.

Headroom, and the reason the cold roof was built that way

Cold decks exist because they buy height. On a Victorian rear addition where the extension roof has to tuck under a first floor window cill, or on a garage conversion where the roof line was set by the original door opening, the builder had no room above the deck and put the insulation between the joists instead.

Those constraints have not gone away. Where the roof cannot rise, the options are a thinner high performance insulation above the deck, accepting a lower U-value, or keeping the cold deck and ventilating it properly with counter battens. All three are legitimate, and which one applies is a measurement rather than a preference.

Insulating up to the kerb without closing the ventilation path

On a cold roof that is staying a cold roof, there is a detail at the kerb that gets the build-up as right as it can be.

The between-joist insulation is taken tight up to the trimmers around the opening with no gaps at the ends, because a gap at a trimmer is a direct convection loop. The 50mm void above it is maintained right up to and around the trimmer, using proprietary spacers if necessary so that later movement cannot close it. The inner face of the kerb is lined with insulation from the ceiling line to the underside of the frame, which warms the reveal even though it cannot warm the kerb timber. The vapour layer is then taken up that lining and taped to the frame.

Done in that order it is a considerable improvement on the standard detail, and it is one of the invisible items the ten year workmanship guarantee is written around.

The reveal, the plasterboard, and the cold line above the ceiling

Look at the ceiling around an older cold roof rooflight in February and there is often a faint discolouration in a band a few hundred millimetres wide around the opening.

That band marks the area where the ceiling has lost its insulation cover: the trimmers, the noggins and the boxed reveal all interrupt the insulation line. The plasterboard there runs a few degrees cooler than the rest of the ceiling, which is enough to collect dust from the moving air and, in a humid room, enough to support mould growth at the corners. Lining the reveal and the trimmer faces removes the cold line rather than decorating over it.

What a cold deck does to the figure quoted for the unit

A rooflight datasheet gives a whole-unit U-value measured under standard conditions. That number describes the unit, not the hole it sits in.

On a cold deck with an uninsulated kerb and an unlined reveal, the heat loss through the perimeter can be a substantial addition to the loss through the glass, and it is the perimeter rather than the glass that produces the cold surfaces people notice. Fitting an expensive triple glazed unit into an uninsulated cold deck opening spends money at the wrong end of the problem. The relationship between the unit’s numbers and the opening around it is set out on the energy efficient rooflights page.

Cold flat roofs across Gloucestershire, and where they turn up

Rear additions and kitchen extensions built between roughly 1965 and 1995 across Gloucester, Longlevens, Hucclecote and Tuffley are predominantly cold deck, with joists at 400 centres, mineral wool or early polystyrene between them, and a bitumen covering on chipboard or softwood boarding.

Integral garages on the estates at Quedgeley, Abbeydale and Bishops Cleeve are a particular case: converted to habitable rooms years after they were built, frequently with insulation added between the existing joists and no ventilation provision at all, then finished with plasterboard. Those roofs are the ones most likely to be holding moisture before anybody proposes a rooflight, and they are covered on the flat rooflights on a garage conversion page.

In the Severn Vale the outdoor air is damp for long stretches of the winter, which reduces the drying capacity of ventilation air and makes a marginal cold roof perform worse in Tewkesbury than the same roof would on the wolds. In the Forest of Dean, higher rainfall and more shade do much the same thing.