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How to write a quote for a flat roof replacement (with a real worked example)

Adam Wójcik 19 October 2026 10 min read

A roofer I know priced a flat roof on a rear extension from the back garden, with a pair of binoculars and a tape measure. Twenty-odd square metres, new membrane, job done. When his lads stripped the old felt, the deck underneath was black and soft along one edge, two joists had gone at the wall plate, and the outlet was sitting higher than the surrounding roof. The membrane was the easy part. The extra timber, the extra day and the skip for rotten boards came out of his own pocket, roughly £700 in all.

That's a quote that priced the visible bit and ignored what holds it up. This guide covers how to write a flat roof replacement quote so the deck, the falls and the weather are in writing before you set up the ladders.

TL;DR

Measure the roof, then measure what it needs

Flat roofs are priced by the square metre, but the square metres on the quote are not just the length times the width. Measure the plan area first. Then add the upstands where the roof meets a wall, the edge trims, any kerbs around rooflights or vents, and the outlet details. On a 6m by 4m roof the plan area is 24 square metres. The membrane you order will be nearer 30 once you've allowed for the turn-ups and the cut-offs.

Write the plan area on the quote. It gives the customer something to check. Also note:

Pick the system before you price it

The customer asked for a new flat roof. You need to know which one before you can quote it. The common options on a domestic job are these (single-ply PVC or TPO suits bigger roofs but needs approved-installer training):

EPDM rubber membrane

A single sheet, usually bonded to the deck, with few or no joints on a small roof. It's quick to lay, tolerant of movement, and a manufacturer's guarantee is available if you're a registered installer. Material cost is moderate and labour is low, which is why it's a popular choice for extensions and garages.

GRP (fibreglass)

Resin and matting laid on the deck, cured in place. It's seamless and hard-wearing, and good for walk-on roofs and awkward shapes. It's more weather-dependent, because the resin needs the right temperature and a dry deck, so build the contingency into your dates.

Built-up felt or torch-on

Still common, and often the cheapest on materials. A torch-on system is hot work, so check your insurer's conditions before you quote it and make sure the quote says how fire risk is managed. Many insurers want a fire watch after you've finished and specific precautions on site.

Whatever you choose, quote what you'd actually fit. A customer comparing a £2,400 felt roof with a £3,700 EPDM roof is comparing two different jobs, and the quote should say so.

The deck, the insulation and the falls are the real quote

The membrane is the bit everybody sees, and it's the smallest part of the risk. Most of the cost and most of the callbacks come from what sits underneath.

The deck. You can't judge it from the top. Stripping the old covering usually shows the truth. Wet or delaminated boards need replacing, and damp at the edges often means rotted fixings or rotten joist ends. Quote a new deck as the base case if the old one is suspect, and use a provisional sum for joist repairs, which you can't size until you've lifted the boards. Say in writing that the sum will be adjusted to actual cost.

The insulation. A replacement roof is a chance to bring the thermal performance up, and in some cases it's a requirement. If you're replacing more than half of the roof surface, the thermal upgrade rules in Approved Document L can apply, so check with the local building control body or the competent person scheme you work under before you quote. A warm roof build-up with PIR boards over the deck is common and keeps the structure on the warm side. Put the thickness and the type on the quote.

The falls. Water has to leave the roof. BS 6229 recommends a design fall of 1:40, which leaves you a finished fall of at least 1:80 once deflection and tolerances are allowed for. If the old roof ponds, the cause is usually falls, not felt, and a new membrane over a flat deck will pond again and be the one with your name on it. If you need firrings to build the falls, that's a materials and labour line of its own.

Ponding is your problem once you've covered it: if the existing roof holds water and you're not changing the falls, put that on the quote as an exclusion and tell the customer in person. If you are changing them, price the firrings.

A worked example: 24 m² EPDM warm roof on a rear extension

Here's the quote from Adam Wójcik trading as Wójcik Roofing, for a single-storey rear extension on a semi in Leeds. Existing roof is old felt on a ply deck, plan area 6m by 4m, one rooflight to be retained, brick wall on one side. Customer is a homeowner, so this is a consumer job.

Price build-up:

Check the arithmetic: 1,555 + 1,320 + 280 + 240 + 300 = 3,695. That works out at about £154 a square metre all in. The roofer isn't VAT-registered, so there's no VAT line, though that changes once turnover goes past the registration threshold (£90,000 at the time of writing). If you are registered, the quote must say whether the figure includes or excludes VAT.

Sanity check on time. Three days for two roofers on 24 m² is a realistic spread: strip and clear on day one, deck and insulation on day two, membrane and trims on day three. Labour comes to £440 a day for the pair before materials and costs.

The line the customer will question is the provisional sum. Explain it before they ask: you can't see the joist ends until the old deck is up.

Write down the weather, the access and the exclusions

Flat roof arguments are rarely about the price. They're about the rainy days and the bit nobody covered.

Under "Included", cover:

Under "Not included", cover:

Also say how you'll leave the roof at the end of each day. A half-stripped roof and a forecast of rain is the situation that costs the most, so tell the customer you won't strip more than you can make watertight before you leave.

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Quote forms, an invoice, T&Cs that cover late payment interest, a variation order for the extras you find under the old deck, plus 22 more pages of UK trade templates. No card needed.

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Payment, guarantees and what you owe

A flat roof job puts real money into materials before the first day. Insulation boards and membrane aren't cheap, so the payment plan should reflect that.

For the £3,695 job above, a sensible split is:

That adds up to £3,695. Any adjustment to the provisional sum goes on the final invoice with a note of what you found, ideally with photos of the rotten timber before it was removed. If you're working for a business customer rather than a homeowner, the same plan works and you have the Late Payment Act behind you if the last stage drifts. If it does, the claim route in England is Money Claim Online, and in Scotland it's the Simple Procedure. There's more on structuring the stages in the stage payments guide.

If you're offering a guarantee, put the terms on the quote and on the final invoice. State who is giving it (you or the manufacturer), how long it lasts, and what voids it, such as other trades walking on the membrane or fitting aerials through it. If the manufacturer's guarantee needs a registered installer and a completed form, say that you'll register the roof and when the customer will get the certificate.

You also owe the customer work done with reasonable care and skill under the Consumer Rights Act 2015. That's the standard they can hold you to, and Citizens Advice has plain guidance for homeowners.

What NOT to do

  1. Don't quote from the garden. Get on the roof, check the edges, press the deck, and look at the ceiling underneath for stains. Twenty minutes up there saves a day of unpaid repairs.
  2. Don't price the membrane and call it a roof. The deck, the insulation, the falls and the edge details are part of the job. If they aren't on the quote, you're paying for them.
  3. Don't cover ponding and hope. New membrane over a roof that already holds water will hold water. Either build the falls or write the exclusion and tell the customer.
  4. Don't strip more than you can close. The first wet night on an open roof can cost more than the job. Strip in sections you can make watertight by the end of the day.
  5. Don't agree extras on the ladder. When you find rot, photograph it, write it down, price it, get a yes by text or email and then fix it. A variation order takes two minutes and stops the argument at the end.

The short version

A flat roof quote works when it's built like the job: measure the plan area and the edges, choose the system, price the deck and the falls, allow for what you can't see, and write down what's in and out. Do that and the customer knows what they're paying for and you know what you're being paid for. If you want the labour-rate side of the sum in more detail, the pricing guide goes through how to work out a day rate that covers your costs.

Frequently asked questions

How much does a flat roof replacement cost in the UK?

It depends on the system, the access and the condition of the deck. The worked example above is a 24 m² EPDM warm roof at £3,695, about £154 a square metre all in, with a provisional sum for repairs. A roof that needs a lot of joist work, a scaffold or complex flashing will cost more.

Which flat roof system is best for a small extension?

EPDM and GRP are the most common on small domestic roofs because they can be laid with few or no joints. The right choice depends on access, the weather window and the guarantee you can offer. Quote what you'd fit and say why.

Do I need building regulations approval to replace a flat roof?

A like-for-like replacement often doesn't need an application, but if you replace more than half of the roof surface the thermal upgrade rules in Approved Document L can apply. Check with building control or your competent person scheme before you quote.

What fall does a flat roof need?

BS 6229 recommends a design fall of 1:40, which gives a finished fall of at least 1:80 after deflection and tolerances. If the old roof ponds, the cause is usually the falls, so price firrings or write an exclusion.

Can I charge extra if I find rotten timber under the old deck?

You can if the quote says you can. A provisional sum with a note that it will be adjusted to actual cost covers you, and a written variation covers anything beyond that. Photos of the damage before you remove it help.

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Adam

Adam is a builder who's spent years in the UK construction field, on everything from bathroom refits to full rewires. These guides come from what he's picked up on the tools and off them: quoting, getting paid, and keeping the paperwork from eating your evenings.