Insulation upgrade

Warm Roof Conversion and Flat Roof Insulation Upgrades

Most people arrive at this needing a roof re-covered and finding out that the insulation is now part of the conversation. That is not an upsell. Renewing enough of a flat roof brings its thermal performance into scope, and the moment the covering is off is the one moment the upgrade is cheap. This page is about the job itself: what changes on site, what the extra depth does to the edges, and when it is not worth doing.

Book a survey

Or call us on 07593 196505

Symptoms

What an old cold roof feels like from underneath

Where the insulation sits, and why that decides how the roof handles moisture, is explained in the guide to warm and cold build-ups, meaning the order of the layers. What brings people to this page is not the theory. It is one of these, usually blamed on something else first.

A room that never warms up. The extension, the dormer bedroom or the office over the garage that costs a fortune to heat and goes cold within the hour. Old insulation between joists was thin to begin with and has usually slumped, leaving gaps nobody can see.
Damp patches nobody can find a leak for. Marks that appear in cold weather rather than wet weather, often near the wall or in a corner. That pattern is condensation forming inside the construction, not water arriving from outside, and chasing it as a leak wastes money.
Black spotting at the ceiling edges. Mould grows where the surface is coldest, which on a poorly insulated flat roof is the perimeter and anywhere a joist bridges the insulation.
The deck, quietly rotted. The expensive one. The deck is the boarding the covering sits on, and where ventilation above the insulation was blocked or never built, it spends every winter damp. The first anyone knows is a soft patch underfoot when the covering comes off.

The work

What a conversion involves on site

The covering comes off and the deck is exposed. What happens to that deck if it turns out to be damaged is the replacement page's subject; assume for a moment it is sound and dry. A sealed vapour control layer then goes down across it, insulation boards go above that, and a new waterproofing system goes over the top. Three additions, in that order, and the order is not negotiable: the sealed layer has to be under the insulation or the whole point is lost.

The interesting part is not the middle of the roof. It is everywhere the roof stops.

Upstands and edges

Every upstand, which is just the covering turned up at an edge, has to rise by the depth that has been added, so they are rebuilt rather than reused. Flashings come out and go back higher. Trims, drips and verge details are renewed to the new level, and any rooflight kerb goes up with them, which is why rooflight work so often lands in the same programme. An upstand left at its old height with a thicker roof beside it is a leak with a date on it.

Thresholds and doorways

The one that stops jobs. A door onto a terrace or a balcony has a fixed sill, and the finished roof surface has to stay a safe drop below it or water walks straight indoors. Adding depth eats that drop. Sometimes the answer is a tapered arrangement that thins towards the door, sometimes it is a drainage channel across the threshold, and sometimes the honest answer is that the door has to move or the conversion does not happen. It is measured on the survey, before anybody is committed to anything.

The trigger

Why the upgrade comes up at all

Because renewing a flat roof is not treated the same as repairing one. Once a significant proportion of a covering is being replaced rather than patched, current Building Regulations bring the thermal performance of that roof into scope, and a like-for-like membrane swap stops being the whole job.

You will find no numbers on this page, and you should be wary of any roofer who quotes them off the top of their head. What applies depends on the building, what is already in the construction and which approved details the project is being designed against, and those are settled by the specification for your roof rather than by a figure on a website. What we will do is say plainly whether your job crosses that line, put the specification in writing with the price, and tell you where building control sits in it.

Limits

When a conversion is not on

Height is the usual reason, and it is not negotiable by enthusiasm. Where a parapet (the low wall at a roof edge) is too shallow to take a taller upstand and still keep the covering below the capping on top of it, or a threshold has no drop left to give, the roof physically cannot carry the depth. Sometimes a thinner, better-performing board buys enough room. Sometimes it does not.

There are also roofs where it is possible and still not worth it. A cold roof with a genuinely working ventilation path, a covering with years left in it, or a small area under a big one are all better left alone and revisited when the covering is due. That is a judgement the condition report should make for you in writing, and it is one we will make against our own interest, because a conversion nobody needed is the kind of job that comes back.

Money

How it is priced, and why not on its own

As a line inside a re-covering, never as a job of its own. Almost everything a conversion costs is shared with the work already happening: the access, the strip, the labour on the roof and the new waterproofing are being paid for regardless. What the conversion adds on top of that is the boards, the sealed layer under them, and the rebuilding of every edge and upstand to the new height.

Which is why the same insulation costs one thing on a roof that is coming off anyway and something entirely different on a roof that is not. If somebody has quoted you for a conversion as a standalone exercise on a sound covering, the question worth asking is what the covering has to do with it. As with everything else here, the figure comes after the roof has been seen and is written down before anyone starts.

Questions

Frequently asked questions

Can a roof be converted without stripping the old covering?

Sometimes, and it is worth understanding why the answer is not simply yes. Where the existing membrane is sound and continuous, it can act as the vapour control layer for a new build-up laid over it, which saves the strip. But it only works if the survey shows the old covering is dry and firmly held, because anything trapped underneath is then sealed in for good, and it adds the whole depth on top of a roof that already has some. On a roof due re-covering anyway, taking it off is usually both cleaner and cheaper.

Will the roof get noticeably higher?

Yes, by the thickness of the insulation and the new covering, and that is the constraint the whole job turns on. It is measured before anything is agreed, because the height has to go somewhere: up the upstands, past a door threshold, and under whatever the parapet allows.

Do I have to upgrade the insulation if I only want the leak fixed?

No. Repairs and patches are repairs, and nothing about fixing a fault obliges you to rebuild the roof around it. The requirement is tied to re-covering a large enough part of the roof, so it is renewal that raises the question, not maintenance.

Can the insulation go under the ceiling instead?

It can, and on some buildings it is the only route left when the height above is spoken for. It is a different job with different risks: the ceiling comes down, the ventilation path above the insulation has to be right and stay right, and you lose room height rather than gaining roof height. Where that is the honest answer we will say so, but it is joinery and plastering as much as roofing.

Is a conversion worth it on a roof with years left in it?

Usually not on its own. The saving is in doing it while the covering is off for other reasons, so on a sound roof the sensible plan is to note it and do it when the covering comes to the end of its life. A survey that records the build-up now makes that decision easy in five years.

Cold room under a flat roof, or a re-covering coming up?

Tell us what the room does in winter and roughly when the covering last went on. That is usually enough to say whether a conversion belongs in the same job.