Guide · Flat Roof Rooflight Installation
Flat rooflights on a cold roof build-up
A cold roof puts the insulation between the joists, below the deck, so the deck and the timber above the insulation sit outside the warm envelope and stay cold. It was the standard way of building a flat roof in this country for decades, which means most existing flat roofs in Essex are cold roofs, and most flat rooflights being fitted into existing roofs are going into one.
That matters because a kerb driven down through a cold roof interrupts the one thing a cold roof depends on: a ventilated air path above the insulation. This page covers how to identify a cold roof, what a rooflight does to it, and the three honest ways forward. It sits under flat roof rooflight installation.
What a cold roof is, and how to tell you have one
The definition is positional. In a cold roof the insulation sits between and level with the joists, with a void above it and the deck on top. In a warm roof the insulation sits above the deck and everything structural is on the warm side. There is nothing else to it, but the consequences are entirely different.
Identifying one from below is usually possible without opening anything up. A flat roof whose total build-up is only 220 to 250mm from ceiling to finished surface is almost certainly cold, because a warm roof needs the joist depth plus 120mm or more of insulation on top. Ventilation openings along a fascia or a soffit are another sign, and so is a build date before about 2005. A single core drilling or lifting a small area at the edge settles it.
Why cold roofs are so common on older Essex flat roofs
The county has more flat roof stock than most. Harlow and Basildon were built out with flat roofed terraces, garages and outbuildings on a scale that is unusual, and the rear addition flat roof is close to universal on the Victorian and Edwardian terraces in Chelmsford, Colchester and Maldon. Post war extensions across the commuter towns added another layer of it.
Practically all of that stock is cold roof construction, and a good deal of it has been overlaid at least once. So the common Essex job is not a new roof with a rooflight designed into it. It is a thirty or forty year old cold roof with an ageing covering, into which somebody now wants a flush glazed unit.
The ventilation path, and the 50mm figure
A cold roof works because moist air that gets past the ceiling into the void above the insulation is carried away by ventilation before it can condense on the cold underside of the deck. The convention is a clear continuous air gap of at least 50mm above the insulation, ventilated at opposite ends of the roof, with openings equivalent to a 25mm continuous strip at each end.
Two properties of that path matter here. It must be continuous, meaning air can travel from one end of each joist bay to the other. And it must be cross ventilated, meaning there are openings at both ends and not just one. A path that is open at one end only does not ventilate, it just breathes in and out.
What a kerb does to cross ventilation
A rooflight opening blocks joist bays. The trimmers that frame the opening run across the bays, and the kerb sits over them. Any bay that runs into that opening is now closed at that point, so air entering at the eaves reaches the trimmer and stops.
On a small roof with the rooflight set near one edge, two or three bays are affected and the rest of the roof still ventilates. On a rooflight set in the middle of a four metre deep roof, every bay it crosses is cut in half, and both halves are now dead ended. That is the condition that produces damp deck timber above the ceiling within a few winters, and it is entirely predictable from the plan before anything is cut.
Interstitial condensation, and where it shows up
The moisture is not coming from outside. It is coming from inside the house: cooking, washing, drying clothes, breathing. Warm air holds it, warm air rises, and any gap in the ceiling lets it into the void. When it meets the underside of a deck that is at outdoor temperature on a January night, it condenses.
The signature is a delayed one. Nothing appears for a year or two, then a brown ring on the ceiling in late winter that fades in summer and returns bigger the following February. Timber above the ceiling darkens and softens, insulation slumps where it has taken up water, and the ceiling fixings start to stain through the paint. It is not a water ingress problem, and looking for a hole in the covering will not find anything.
If a ceiling mark appears in February and dries out by May, the roof is condensing rather than letting water in.
The vapour control layer, and why it is rarely continuous
Every cold roof is supposed to have a vapour control layer on the warm side of the insulation, immediately above the plasterboard, to stop moist air reaching the void in the first place. In practice, on a roof built in 1978 or 1994, it is either absent or it is a polythene sheet that has been penetrated by every downlight, cable and ceiling fixing installed since.
That is why ventilation carries the load on cold roofs rather than the vapour barrier. It is also why cutting new openings in the ceiling for a rooflight reveal is a moment to improve things: the reveal lining can be detailed with a proper vapour layer, taped at its junctions, and that is a real gain even if the rest of the ceiling stays as it is.
Fitting a rooflight without changing the build-up
This is the minimum intervention route and it works within limits. The opening is formed, the joist bays that are interrupted are given their own ventilation route, and the roof continues as a cold roof.
Giving the interrupted bays a route means one of two things. Either a cross ventilation path is created above the trimmers, using counter battens or a proprietary ventilation channel running around the opening and connecting to the bays either side. Or the affected bays are vented directly through the roof surface with roof mounted vents. Both are legitimate. Both need the kerb build-up designed to accommodate them, which is why the decision comes before the timber is cut and not during.
Converting to a warm roof at the same time
The better answer where the covering is due for renewal anyway. The existing covering and deck come off, insulation between the joists is removed, a vapour control layer goes over the deck, rigid insulation goes on top, an overlay board goes over that, and the covering is laid over the lot. The joists are then warm, the ventilation problem disappears entirely, and the rooflight kerb has a straightforward warm roof detail as described on upstands, kerbs and warm roof build-ups.
The cost is not as far apart as it sounds, because a large share of it is the covering, the deck and the access, all of which are being paid for in either case. Where the roof covering has less than five years of service left in it, the conversion is usually the sensible arithmetic.
Deck levels, and the step a conversion creates
A warm roof conversion raises the finished surface by 120 to 175mm. That has knock-on effects that need checking before it is agreed.
The upstand against the house wall grows by the same amount, so the abutment flashing and any existing chase move up, and there has to be enough brickwork below the damp proof course line or a window cill to accommodate it. Gutter and outlet levels change. On a rear addition with a first floor window above, the sill height above the new roof can become tight. None of this is unusual, but it is measured at survey rather than assumed, because a roof that gains 150mm under a window that is 200mm above it is a design problem, not a detail.
Insulating the kerb on a cold roof
Where the roof stays cold, the kerb is a timber box surrounded by cold ventilated air on the outside and open to a heated reveal on the inside. It is therefore working as an insulated wall, and it needs to be treated like one.
The practical detail is insulation on the internal face of the kerb or within the reveal lining, taken up to meet the frame, with the vapour control layer of the reveal continuous with it. That differs from a warm roof, where the insulation goes on the outside face. Getting the side wrong puts a cold surface where the warm air is, and that is where the condensation appears.
Part L, and what applies to which case
Fitting a new rooflight into an existing dwelling engages Part L, and the standard applied is to the rooflight itself: a U-value limit for the unit, measured in the vertical or in the horizontal plane depending on the product and its declared basis. That applies whether the roof is warm or cold.
Where a warm roof conversion is carried out, the roof itself becomes a renovated thermal element, and a target U-value applies to the whole build-up. In practice a modern warm roof scheme achieves it comfortably with the insulation thickness the manufacturer’s calculation specifies. Sunspire handles the Building Control notification as part of the job in both cases, which matters years later when a buyer’s solicitor asks for a completion certificate.
| Cold roof, retained | Converted to warm roof | |
|---|---|---|
| Ventilation required | Yes, 50mm path both sides of the opening | None |
| Kerb insulation | Internal face and reveal | External face, continuous with the roof |
| Finished level change | None | Up 120 to 175mm |
| Covering | Cut back and made good locally | Renewed in full |
| Best suited to | Sound covering with years left | Covering near the end of its life |
Signs the existing cold roof is already struggling
Worth knowing before the job is priced, because they change what is sensible. Ceiling staining that comes and goes seasonally. A soft or springy feel underfoot at one part of the roof. Blocked or painted over eaves ventilation. Downlights in the ceiling below, which are effectively chimneys for moist air. Insulation visible at an edge that is packed tight against the deck with no air gap above it.
That last one is extremely common. Somebody has topped up the insulation at some point, filled the void to the underside of the deck, and closed the ventilation path completely. The roof then behaves as an unventilated cold roof, which is the worst of both arrangements, and the deck timber pays for it.
Cold bridging at the perimeter, and the line on the ceiling
On a cold roof the trimmers around a rooflight opening are solid timber running from the warm side to the cold side, and unless the reveal is insulated they carry heat straight out. The result is a defined grey line on the plaster where the reveal meets the ceiling.
It is the same mechanism as on any cold bridge: a colder surface raises local humidity, dust adheres, mould follows. On a cold roof it appears faster than on a warm roof because there is more cold timber in contact with the opening. Insulating the reveal properly at first fix removes it, and it is one of the invisible details behind the ten year workmanship guarantee.
Ventilating the reveal, and what not to do
Sometimes the suggestion is made that the rooflight reveal itself should be vented into the roof void. It should not be. That connects the heated, humid air of the room directly to the coldest timber in the building, which is the exact path the vapour control layer exists to close.
Ventilation belongs in the void, drawn from outside air at the eaves or through roof mounted vents, and the reveal belongs sealed. Where the room below is a kitchen or has high moisture output, the answer is extract ventilation from the room to outside, not a route into the roof.
What a survey checks before quoting on a cold roof
Six things, and they take about half an hour. The build-up depth, measured, to confirm cold or warm. Whether an air gap exists above the insulation and how deep it is. Where the existing ventilation openings are and whether they are clear. The condition and age of the covering, which drives the warm roof conversion decision. Moisture in the deck timber, taken with a meter at several positions. And the ceiling below, checked for staining, downlights and any existing vapour layer.
Those answers decide which of the three routes is right: fit into the cold roof with a designed ventilation solution, convert to warm roof and fit into that, or address the roof first and set the glass into a build-up that is going to last as long as the unit does. Two figures follow, a range by email once the basics are known, then a fixed price once the roof has actually been looked at.
Keep reading
More on flat roof rooflight installation
Flat rooflight falls, drainage and ponding
Read the guide →Walk-on rooflights and where they make sense
Read the guide →Single-ply, felt and GRP roofs around a rooflight
Read the guide →How many rooflights a flat roof extension needs
Read the guide →Flat rooflight standard sizes and made to measure
Read the guide →
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