The short answer: patching a flat roof is maintenance and generally sits outside the Building Regulations. Renewing a large enough part of one, building a new roof, or altering the structure brings them into play. The line falls between putting something back as it was and creating something the building did not have before.
Which jobs bring the regulations into play?
Four situations account for nearly all of it on a flat roof, and none of them turns on how expensive the job is.
- Building a roof where there was none. An extension, a garage, a dormer. Everything about it is new work, so everything about it is in scope, from the structure that carries it to how it is finished and drained.
- Renewing a large enough part of the covering. This is the one that surprises people. Once a renewal is substantial rather than local, the roof stops being maintained and starts being rebuilt, and its thermal performance comes into the conversation with it.
- Altering the structure. Cutting an opening for a rooflight, changing joists, replacing a deck with something different. Anything that changes what holds the roof up.
- Changing what is underneath. Turning a garage into a room, or a loft into a bedroom, puts new demands on the roof above it, and the roof is assessed against the new use rather than the old one.
Repairs, patches and clearing drainage sit outside all four. So does replacing a covering over a small area after damage. The distinction is not a loophole, it is the point: nobody wants a building notice filed for a split upstand.
Why will nobody give you a rule of thumb?
Because the honest answer is that the approved details for your project decide, and those are settled against your building rather than against a webpage. Two roofs of the same size, in the same street, built in different decades and being renewed for different reasons, can land differently.
You will find figures quoted freely online, and you should treat every one of them with suspicion for the same reason we publish none here. Requirements change, they are conditional on things a stranger cannot see, and a number remembered from a job three years ago is exactly the sort of confident wrongness that costs somebody a certificate. What should happen instead is that the specification for your roof states what is being built and to what, in writing, before work starts.
Who signs it off?
Building control, either the local authority's own service or an approved inspector working privately. They are the ones who agree what the work has to achieve, look at it at the right moments, and issue the completion certificate at the end. A roofer does not sign off their own work, and a contractor who tells you the job is fine without one is describing a preference rather than a process.
The certificate is the artefact that matters afterwards. It is what a buyer's solicitor asks for, what a lender may want, and what settles the question of whether an extension was built properly. Filing at the time is cheap; reconstructing the position years later is not.
What does this mean in practice for a re-covering?
Mostly that the insulation question arrives whether or not anybody wanted it. Once a covering is being renewed substantially, the roof's thermal performance is in scope, and the sensible reading is that the only cheap moment to improve it is while the covering is off anyway. What that involves as a job, and when the height simply is not available, is set out on warm roof conversion.
On a new roof there is nothing to argue with, because everything is being designed from scratch and the drawings say what is being built. That is the territory of new flat roofs. Between the two sits strip and renew, where what the deck turns out to be determines a good deal of what follows. In all three cases the same principle holds: the specification names what is being built, and building control agrees it before anybody starts rather than after.