Picture a lean-to 3 m wide and 3 m deep on the back of a 1970s semi. The 16 mm polycarbonate has gone the colour of weak tea, one sheet has crept down towards the gutter, and a bucket lives in the corner from November onwards. You want the cheapest fix that actually ends the problem. There are five ways to go, and the cheapest on the day is not always the cheapest after a few winters.
Five ways to replace the roof, priced for one room
Every figure below is for that same room. Our own price comes from our published bands. The others are the ranges shown in 2026 UK cost guides for fitted work, so read them as a guide rather than a quote.
| Option | Typical fitted price | Roof U-value (W/m²K) | Old roof removed? | Building control? |
|---|---|---|---|---|
| New polycarbonate, 16 to 35 mm | £2,000 to £3,500 | 2.3 to 1.4 | Yes | Not normally, if it stays see-through |
| Insulated ceiling under the old roof | £1,050 to £2,750 | Depends on the kit | No | Often, as the roof is no longer see-through |
| Insulated aluminium panels (what we fit) | £3,000 to £3,550 | About 0.43 | Yes, and recycled | Yes, and we apply for you |
| New low-e glass | £3,300 to £5,200 | 1.0 to 1.2 | Yes | Not normally |
| Rebuilt roof on new rafters | More than double the panel price | Designed to about 0.15 | Yes, frame and all | Yes |
Two things stand out. New polycarbonate has the lowest price and also the worst insulation on the list. The insulated ceiling looks cheap too, but it leaves the old roof sitting above it, leaks and all.
What you give up to save money
New polycarbonate sheets
Fresh sheets cure the leak and the yellowing for now. They do very little for warmth. A 16 mm sheet sits at about 2.3 W/m²K, and even 35 mm only gets down to about 1.4. The drumming in heavy rain stays too. If the room is a porch or somewhere to dry washing, this can be the sensible answer.
An insulated ceiling under the old roof
A fitter builds insulation and a new ceiling beneath the existing roof. It is quick and the room does feel warmer. The catch is that everything wrong with the old roof stays up there: tired seals, slipping sheets, dirt in the flutes. You are also hanging extra weight from glazing bars built for a light roof, and the gap between old roof and new ceiling needs careful vapour control or it can collect moisture.
Insulated aluminium panels in your existing frame
This is the job we do. The old roof comes off and goes for recycling. Made-to-measure panels, each a 75 mm slab of solid foam sandwiched between two aluminium sheets, go into the frame you already have. Each panel is about 0.43 W/m²K, white on the inside and any RAL colour on the outside. A small roof is fitted in 1 day. Taking the old roof away, your colour, the building control paperwork and a 10 year guarantee backed by insurance are all in the price, so nothing gets added unless you ask for extras. See how the style affects it on our lean-to roof page.
New glass
Modern low-e glass at 1.0 to 1.2 is a real step up from polycarbonate and keeps every bit of daylight. It costs more than panels for the same room, still lets strong sun bake the room in July, and needs cleaning from outside.
A rebuilt roof on new rafters
The whole roof structure comes off and is rebuilt with new rafters, insulation and a plastered ceiling. It can be designed to the 0.15 W/m²K target for new roofs, and it takes longer on site. For most homes it costs more than twice as much as a panel roof for the same room. If your frame is sound, most of that extra money buys a new structure you did not need. Our guide to what a warm roof actually is explains the difference in build.
The ten year sum
The price on the day is only part of the bill. Heat leaves through the roof every hour the room is warmer than the garden. To count it, multiply the U-value by 10 m² of roof (a shade more than the floor, thanks to the slope), then by 2,150 degree days, the England average, then by 24 hours, and divide by 1,000. The answer is kWh lost in a year. Energy is priced at the October to December 2026 Ofgem cap: 26.32p a kWh for electricity, 7.97p for gas burned in a 90% efficient boiler.
| Same lean-to, 10 m² of roof | New 16 mm polycarbonate | Insulated panels |
|---|---|---|
| Heat lost through the roof each year | About 1,190 kWh | About 220 kWh |
| Yearly cost with electric heaters | About £312 | About £58 |
| Yearly cost with a gas radiator | About £105 | About £20 |
| Price on the day (midpoint) | £2,750 | £3,275 |
| Ten years, electric heating | About £5,870 | About £3,855 |
| Ten years, gas heating | About £3,800 | About £3,475 |
Heat the room with electric heaters through the winter and the panel roof comes out about £2,000 ahead after ten years. On gas the gap narrows to roughly £325. Never heat the room at all and new polycarbonate wins on cost, but then you are paying to keep a room you can barely use from November to March.
Two honest limits on that sum. It only counts the roof, so the glazed walls lose heat either way. And it assumes you keep the room warm all season, which is how most people end up using it once it stops being cold. Price your own shape and size in the roof builder.
Five ways to trim a panel roof quote
- Keep gutters that are sound. If your gutters and fascia are in good order, leave them. New ones add £295.
- Put roof lights where you sit. Each glazed roof light is £395. A small lean-to often needs just one, over the chair or the table.
- Use the lighting you already have. LED spotlights are £345. If the room has wall lights or a lamp you like, you can skip them.
- Choose any colour you want. White, grey or any other RAL shade costs the same, so there is no saving to chase here.
- Let us check the frame first. We look at it at survey. Knowing it is sound before you sign means no surprise repair bill later.
What about fitting panels yourself?
Supply-only foam panels with PVC skins are sold online from about £65 per m², so materials for a 10 m² roof might come to around £650. It looks like a big saving. Before you order, weigh up what you take on: working at height over a fragile roof, sealing the joint where the roof joins the house, and the building control application, which still applies when you do the work yourself. There is no workmanship guarantee either, and a leak at the wall flashing can cost more to put right than you saved.
Our straight answer
If you only want the room in summer and never heat it, new polycarbonate is the cheapest route. If you want a room you use all year, an insulated panel roof normally costs least in the long run, because it avoids paying twice: once for new sheets now and again for a proper fix later. If glare is your only complaint, read roof blinds against a new roof before spending anything.
Next step: see the full bands on our prices page, read how the swap works on replacing a polycarbonate roof, then use get my price to book a survey and a fixed written quote.
Questions
Can I replace just the worst polycarbonate sheets?
You can, and swapping a sheet or two is the cheapest short term fix. The new sheet will not match the faded ones, and the rest of the roof keeps ageing at the same rate.
Is an insulated ceiling under my old roof cheaper than new panels?
On the day, often yes, at roughly £1,050 to £2,750 for a 3 m square room. The old roof stays above it, so leaks and slipping sheets remain your problem.
Does the cheapest panel roof price include building control?
Yes. From £3,000, the price covers taking the old roof away, fitting, any RAL colour, the building control paperwork and a guarantee that runs 10 years, backed by insurance.
Do I pay more for a grey roof than a white one?
No. Any RAL colour on the outside costs the same, and the inside is always white.
