How To Read A Flat Roofing Quote (And Spot The One That Will Cost You Twice)

Three quotes are sitting on your kitchen table. One says £1,900. One says £3,100. One says £4,400. All three are for the same garage roof, all three came from firms who seemed perfectly reasonable on the phone, and there is nothing on any of the three pieces of paper that explains why the top one is more than double the bottom one.

Most people, at this point, do the only thing they can do. They pick the middle one and hope.

Here is the awkward truth about that spread: it is usually not three prices for one job. It is three different jobs, written up to three different standards of detail, and the low one is usually low because a whole element never made it onto the page.

The missing item does not stop existing when it comes off the quote. It just gets discovered on day two, when your roof is open and your negotiating position has evaporated.

Flat roofing is unusually prone to this, for a reason that is worth understanding. The two things that decide what your roof costs are both hidden underneath the covering, where nobody can see them until the work starts. That gives an optimistic quoter a lot of room, and it gives you almost no way to check. So rather than comparing the totals, it is worth learning to read the documents.

The boards underneath are the whole ballgame

If a quote does not mention the deck, it is not finished.

The deck is the timber layer your covering sits on. On older roofs it is very often chipboard, and chipboard that has spent a decade quietly damp behaves nothing like the stuff that went down new. It does not look dramatic. It does not sag or announce itself. It stays quietly structural right up until somebody kneels on it and finds out otherwise.

This matters commercially because replacing decking is real work with real materials, and it is the single biggest reason two honest quotes for the same roof can be a thousand pounds apart. One firm has allowed for it. The other is hoping.

What you want to see is either a stated allowance for deck replacement, or a clear statement that the deck will be inspected once the covering is off and priced separately at an agreed rate. Both are honest.

What is not honest is silence, and what is definitely not honest is a proposal to lay a new covering straight over the existing one without anybody having lifted a corner to look.

That approach is fast, it is cheap, and on the wrong roof it means sealing damp timber under a waterproof layer with no route out. The rot carries on in the dark, and you have bought a thirty year membrane sitting on boards that will not see ten.

There are genuine situations where overlaying is the right call, mostly on larger commercial roofs where the existing build-up is dry and stripping would be hugely disruptive. The distinction is whether somebody checked or somebody assumed.

Ask where the water is supposed to go

The second hidden variable almost never appears on a quote at all, which is remarkable given how much it decides.

A flat roof is not flat. It is built with a slight slope so that water runs to an outlet and leaves. Over several decades, as timbers settle and take up loads, a lot of these roofs quietly flatten out. Once that happens the roof stops being a roof and starts being a shallow tray.

Standing water is patient and it is relentless. It leans on every joint until one of them gives. It gathers silt and grows moss, which keeps that area permanently damp and eventually chokes the outlet, which makes the puddle bigger, which makes everything worse. Left alone through a cold snap it freezes, expands, and levers the joint open a fraction more each time.

None of that is solved by a new covering. Lay the best membrane made by man over a deck with no fall and you have bought yourself the same problem with a longer warranty attached. Restoring the slope means firrings or tapered insulation boards, and it is a line item.

So when you are comparing flat roof repair quotes, look for any acknowledgement that the falls have been assessed. A firm that has walked the roof after rain and noted where water was still lying knows something the other two do not. If nobody mentions drainage, ask directly, and take a vague answer as information.

The insulation question that quietly moves the total

Here is a rule most homeowners have never heard of and which changes the price of a lot of jobs.

Replace more than 25 per cent of a roof’s surface and the work counts as renovating a thermal element. Once it does, building regulations apply and the roof has to be brought up to the current insulation standard rather than the one it was built to in 1974.

The figures are specific. Under Approved Document L, an entirely new roof – the one over an extension you are building – has to reach a U-value of 0.15 W/m²K. Re-cover or renovate a roof that is already there and the target eases slightly to 0.18 W/m²K, applied wherever it can sensibly be met.

Translated into something you can picture, that tends to work out at roughly 120 to 150mm of rigid board, give or take, depending which product goes on.

Boards cost money. Two quotes, one of which has priced insulation and one of which has ignored the fact that the job triggers it, are not comparable in any useful sense. And the one that ignored it has not saved you anything, because the requirement does not go away.

There is a practical consequence worth knowing about too. Adding 120mm or more on top of your deck makes the roof taller, and that height has to go somewhere.

Door and window heads immediately below the roof line, thresholds, existing upstands where the roof meets a wall – these all need working out before the boards arrive rather than after. A quote that mentions height has been produced by somebody who has done this before.

“Guaranteed” is doing a great deal of work in that sentence

There is a wrinkle in how this trade uses the word, and it is worth thirty seconds of your attention.

Manufacturers offer long, impressive warranties on their membranes. Twenty years, thirty, sometimes more. Those numbers get repeated in sales conversations because they sound like a commitment to the standard of the fitting.

They are not. What they actually insure is the material itself against a factory fault, and factory faults are vanishingly rare. A roll of EPDM will sit in a warehouse outliving everyone reading this. What goes wrong is the fitting.

A corner closed up in a hurry towards the end of a Friday. An upstand stopped short of where it needed to reach. A trim screwed down with whichever fixings were left in the box.

So treat them as two separate questions. What does the manufacturer cover? And what does this specific firm guarantee about its own workmanship, for how long, and will they put it on paper before starting?

Only the second answer is any use to you if a stain appears in year six. Watch what happens to the room when you ask for it.

Insurance works the same way. “Fully insured” in an email is a phrase, not a document, and asking for the certificate itself takes one line of reply. Nobody in this country needs a permit to start fitting flat roofs tomorrow morning, which is precisely what gives the optional credentials their value.

Belonging to a trade body such as the National Federation of Roofing Contractors, or sitting on a vetting scheme like TrustMark, does not by itself make anyone good at the job. It does mean an outside party has checked them over at some stage, which is more than can be said for a mobile number and a van.

Access should be its own line, not a rounding error

Getting people and materials safely onto a roof is a genuine cost, and it varies enormously. A single storey garage you can reach off a ladder is one thing. A second storey roof, a balcony over a public path, or anything above a shop that needs edge protection and out-of-hours working is a different exercise entirely.

None of that is unreasonable to charge for. What is unreasonable is folding it silently into a total so it cannot be compared, or leaving it out and raising it once the job is underway. Access on its own line is a small sign of a firm that expects to be checked.

What the document tells you about the people

Step back from the numbers and read the quotes as writing.

One of them probably describes the covering in detail and says almost nothing about anything else. That firm has told you which part of the job interests them. The parts they skipped – the deck, the drainage, the perimeter detailing – are precisely where flat roofs actually fail, and it is not a coincidence that they are also the boring parts.

Another might be a single line and a number. It is quick to produce, impossible to compare, and gives you nothing to hold anyone to later.

The one you want is the one you can take apart. Deck, insulation, covering, trims and access, each carrying its own figure, so you can weigh the parts up individually and put off anything that can safely wait. That format is more effort to write. It is also the only version that lets you make an informed decision, which is presumably why it is the least common.

One last thing, and it applies regardless of what any quote says. Nothing about roofing should be agreed at your front door. After any bad night of weather somebody will knock and offer to nip up and check things over, since they happen to be on the road anyway.

Some of those callers are entirely legitimate local firms. Enough of them are not that a blanket policy is the safe one: if you want your roof looked at, ring somebody you chose yourself.