When a flat roof is replaced, the covering on top is only half of the decision. The other half is what lies beneath it: the deck, the insulation, and the layer that stops warm, damp air from the house reaching cold timber. Roofers describe the two main arrangements as warm and cold, and the names tell you most of what you need to know. In a warm roof the insulation is above the structure, so the timber stays on the warm side of it. In a cold roof the insulation is below the structure, so the timber sits in the cold.
This guide explains each build-up in plain terms, shows where the vapour control layer belongs, and sets out what the Building Regulations ask when you renew a flat roof, including the Part L figures of 0.35 and 0.16 and the 15-year payback rule. It also covers condensation, the main reason these roofs fail from the inside, and it gives the questions to ask so that your quote describes the build-up and not just the top sheet. It sits within our flat roofing work across Taunton and the surrounding villages.
The two build-ups, side by side
A flat roof is a stack of layers. Looking from the ceiling upwards, a typical cold roof goes: plasterboard, a vapour control layer, joists with insulation packed between them, an air gap, the deck, and the waterproof covering. The gap above the insulation is meant to be ventilated, so that any moisture reaching it is carried away.
A warm roof reverses the order of insulation and structure: plasterboard, joists, the deck, a vapour control layer laid on the deck, rigid insulation boards over that, and the waterproof covering on top. The deck and joists sit below the insulation, in the heated side of the building, and there is no void to ventilate. That is the point of the layout: the structure never reaches the temperature at which damp air condenses on it.
What about an inverted roof?
A third arrangement, the inverted roof, puts the insulation above the waterproof layer, held down by ballast such as gravel or paving. It protects the membrane from sun and temperature swings, and it is used where a roof is a terrace or walkway. It needs insulation that tolerates water and a structure strong enough for the weight, so it is a specialist design rather than the usual choice for a house extension.
You can often tell which you have from outside. A cold roof usually has small vents, called eaves vents, at the front and back edges of the roof, or a strip of vented soffit, which is the board under the overhanging edge. A warm roof has none. If you are unsure, the survey will include a look at the edges and, where needed, a small opening to read the layers.
Cold roofs: how they work and fail
A cold roof keeps its insulation between the joists. The ceiling below is lined, and the gap above is vented along opposite edges so air can pass through. Done well, it works, and plenty of cold roofs have served for years. Warm air from the house is stopped at the ceiling by the vapour control layer, and anything that does get through is carried out by the airflow.
The trouble is that a cold roof relies on three things at once: an airtight ceiling, a clear air path, and no gaps in the insulation. If any one fails, vapour from cooking, bathing and breathing meets the chilly underside of the deck and condenses into liquid. The timber then stays damp, and over time the deck softens, the covering above lifts, and the ceiling stains.
Why do cold flat roofs struggle with ventilation?
On a flat roof the air has to travel a long way sideways, with no height to help it move. Wide roofs, boxed-in edges, a fascia added later, or insulation pushed hard against the deck can all close the path. BS 5250:2021, which covers moisture management in buildings, is the document designers use for these details.
Keeping an existing cold roof is possible, and it is sometimes the right call when the structure is sound and the ventilation can be restored. But when a roof is stripped to the deck anyway, the choice is wider. Our answer on condensation under a flat roof covers the signs to look for.
How a warm roof works
In a warm roof, the insulation boards lie on top of the deck, and the deck stays at room temperature. Because the timber never gets cold, vapour reaching it has nowhere to condense. There is no void to vent and no airflow to depend on. The rigid boards are laid with tight joints, and the waterproof covering is applied over the top.
The vapour control layer matters here too, though its job is different. It sits on the deck, below the insulation, and stops damp air from the house reaching the cold underside of the boards. Where it is torn, lapped badly or left open around pipes and rooflights, moisture finds a way in. That is why it is checked as the layers go down, not after.
Why is a warm roof the usual choice when replacing?
It can be done from above, so the ceiling, wiring and decoration below are left alone. It also works well with a new membrane, because the roofer builds the whole stack in one go. And it keeps the structure dry, and damp timber is where cold roof failures begin. Tapered boards, cut to a slope, can also restore a proper fall to a roof that has lost one, which helps water run to the outlets instead of settling in a dip. Our answer on tapered insulation explains how they work.
A warm roof raises the finished height of the roof by the thickness of the boards. That affects upstands, door thresholds and the flashings along the wall edge, so the survey checks the details before the quote is written.
Part L and a renewed flat roof
Approved Document L (Volume 1, dwellings) is the part of the Building Regulations that deals with saving fuel and power. It lists renovation work, and one item it names is a new waterproof membrane for a flat roof. So a fresh covering on a house is more than a waterproofing job, because it can bring an insulation requirement with it.
What U-values does a renovated flat roof have to meet?
A U-value measures heat loss, in watts per square metre per degree, so a lower figure means a better roof. For existing roofs that are being renovated, Table 4.3 of the 2021 edition sets a threshold of 0.35 and an improved value of 0.16. In plain terms, if the existing roof loses heat at a rate worse than 0.35, it should be brought up to 0.16.
Paragraph 4.13 allows some give. Where the target cannot be reached in practice, or the saving would take longer than 15 years to repay the cost, the upgrade goes as far as the 15-year payback allows. The same paragraph sets a ceiling: after an upgrade, a thermal element should generally not be worse than a U-value of 0.7. A new or replacement roof element is held to a stricter 0.15 under Table 4.2.
The 2021 edition took effect on 15 June 2022 and remains the one to cite today. A 2026 edition takes effect on 24 March 2027 for work that is not in connection with higher-risk building work, and its tables are renumbered 3.2 and 3.3.
Ask your building control body whether your job is notifiable. This guide describes the rules and is not legal advice.
Do you need building control for the new roof?
The national test comes from the Planning Portal. No building control application is normally needed for a re-roof when less than 25 per cent of the total building envelope is affected and less than 50 per cent of the roof is affected. The envelope is everything that wraps the building: walls, floor, roof and the openings in them. On a house with a flat roof extension, renewing the whole extension roof can be a large share of the roof, so check the figures before assuming the answer.
There is no re-roofing percentage on Somerset Building Control's own pages. Its fee calculator does list a re-roof or insulation upgrade to a single dwelling under thermal element renovation, so that is how a notifiable job is charged. Its telephone number is 0300 303 7790 and it covers the former districts.
Approval is also needed if the new covering makes the roof covering 15 per cent or more heavier, or if it will perform differently in a fire. The Planning Portal suggests an engineer or surveyor should check the weight.
Is planning permission needed to change the roof?
Swapping one covering for another of matching type is normally Class C permitted development for a house. Listed houses keep Class B and Class C permitted development but also need listed building consent where the work changes their character. Our answer on insulating a flat roof when replacing it goes into the choice in more detail.
Choosing between them and what moves the price
For most replacements the practical question is not which is better in theory but which suits your roof as it stands.
Which suits my flat roof?
- Choose a warm roof when the deck is being replaced or can be reached from above, when the ceiling below should stay untouched, or when the old cold roof has damp timber.
- A cold roof may suit when the existing structure is sound, the ventilation path is clear, and the height at the edges cannot rise.
- Ask for a survey first when you are unsure what is in the roof now, because the answer sits behind the covering.
Cost depends on what is underneath, so the survey drives the price more than the label does, and two roofs of the same size can differ a good deal. What moves the total is the area of the roof, the thickness of insulation needed to reach the U-value, whether the deck is sound, the covering you choose, scaffold and access, and the details at walls and outlets. For context, the typical UK range for a whole flat roof replacement is £1,500 to £6,000, averaging £2,400 (MyJobQuote, Oct 2026), and insulation is part of that spread.
Ask any roofer to say in writing which build-up they propose, how the vapour control layer is detailed, and what U-value the roof will reach. That turns a vague quote into a clear one, and it lets you compare two quotes that describe the same roof. The written price follows the survey. You also see photos of the deck and layers as they come off, so the build-up you pay for is one you have seen.
Build-up queries
Can I turn a cold flat roof into a warm one?
Yes, and the moment to do it is when the covering is stripped for replacement, because the deck is open to inspection and the new layers can be planned together. Rigid insulation goes over the deck with a vapour control layer under the boards. The finished height rises, so upstands and flashings are checked first.
Will a warm roof end condensation problems?
It removes the usual cause, which is vapour reaching cold timber, but only if the layers are detailed properly. A torn vapour control layer or gaps at penetrations can still let damp air in. Condensation is a design and workmanship matter, not a promise of any one build-up.
How thick does flat roof insulation need to be?
The thickness depends on the boards chosen, because each product has its own conductivity. The target is the U-value, 0.16 for a renovated roof under Table 4.3 unless feasibility or the 15-year payback rule relaxes it. A roofer or designer works the thickness out from that figure.
More on this subject: Flat roofing in Taunton
