What the job involves
A flat roof replacement means taking off the old covering, checking the structure underneath, and fitting a new waterproof system. The roof might sit on an extension, a garage or a dormer. A proper job is more than laying a new membrane over the top. It usually covers these steps:
- Stripping the old felt or other covering back to the deck.
- Inspecting the timber deck and replacing any boards that are damp, soft or delaminated.
- Fitting insulation where it is needed or required.
- Laying a vapour control layer if the build-up calls for one.
- Installing the new covering with edge trims, upstands, flashings and outlets.
- Reconnecting or replacing gutters and fascias where they are affected.
Flat roofs are not truly flat. They need a slight fall so that water drains off rather than sitting in puddles. If the existing roof ponds water, a good roofer will talk about correcting the falls rather than just covering the problem.
The three usual coverings are:
- Felt: layers of reinforced bitumen felt, often torch-on or cold-applied. Modern high-performance systems are a long way from the cheap single-layer felt of the past.
- EPDM rubber: a single sheet of synthetic rubber, usually bonded to the deck. It has very few joins, which suits simple shapes well.
- GRP fibreglass: liquid resin laid over glass matting to form a seamless, hard-wearing surface. It suits awkward shapes and areas with lots of detail.
What affects the price
No two flat roofs cost the same, even at similar sizes. These are the things that move the price most:
- Roof size: a larger area means more material, more labour and often more scaffolding. Small roofs still carry fixed costs such as setting up, waste removal and travel, so they cost more per square metre than big ones.
- Choice of covering: felt, EPDM and GRP each have different material costs and need different skills. The cheapest material is not always the cheapest finished job once labour and details are counted.
- Condition of the deck: if water has got in, the boards and sometimes the joists will need replacing. This is the most common reason a final bill rises, and it often cannot be fully judged until the old covering is off.
- Insulation: adding or upgrading insulation increases material and labour. Replacing a roof may trigger building regulations requirements on thermal performance, so ask your roofer or local building control what applies to your project.
- Access and scaffolding: a single-storey garage with side access is simple. A dormer or a first-floor roof usually needs scaffolding or a tower, and tight access slows the work and the removal of waste.
- Details and penetrations: rooflights, vents, pipes, chimneys, and the junctions where the roof meets walls all take time to waterproof properly. A roof with many details costs more than a plain rectangle of the same size.
- Dormers: the cheeks and sometimes the front face may need re-covering or repairing as well as the flat top, which adds to the work and the height risk.
- Edges, fascias and gutters: if the fascias are rotten or the gutters are failing, replacing them while access is in place makes sense but adds to the total.
- Waste and old layers: several layers of old covering take longer to strip and cost more to dispose of. Older garage roofs may contain asbestos-based materials, which need specialist handling, so get this checked if there is any doubt.
- Guarantee and workmanship: a roofer who offers a proper written guarantee, and who has the training and approvals for the system, may charge more than one working without them.
- Location and demand: labour rates differ across the UK, and good roofers are often busier in settled weather.
Ways to keep the cost down
- Get several itemised quotes for the same specification so you can compare like with like.
- Replace before it fails. A roof that has only just started to leak is far cheaper to deal with than one that has rotted the deck and ceilings.
- Combine jobs. If the gutters, fascias or rooflights also need doing, having them done together saves on scaffolding and repeat visits.
- Keep the design simple. Moving or adding rooflights and outlets increases cost.
- Clear access for the roofer and for waste removal, such as moving cars, plants and garden furniture, saves time.
- Ask about a repair if the problem is local. A full replacement is not always needed, though be wary if the roof is clearly at the end of its life.
- Agree what happens if the deck is bad before work starts, so you have a fair price per board rather than an open-ended extra.
- Do not choose on price alone. A cheap roof that fails early costs more in the end.
What should be included in a quote
A good quote is written, itemised and specific. Check that it covers:
- [ ] The covering system named, including the manufacturer and product line.
- [ ] Full strip of the old covering and disposal of waste.
- [ ] Inspection of the deck, and what is allowed for repairs.
- [ ] A stated price for replacing damaged boards or timbers if found.
- [ ] Insulation type and thickness, and whether it is a warm or cold roof build-up.
- [ ] Vapour control layer, if one is needed.
- [ ] Edge trims, drip details, flashings and upstands.
- [ ] Outlets, gutters and fascias, and whether they are replaced or reused.
- [ ] Rooflights, vents and other penetrations.
- [ ] Scaffolding or access equipment.
- [ ] Building regulations position, and who arranges any sign-off.
- [ ] Start date, expected duration and payment terms.
- [ ] Guarantee details, in writing, with what it covers and for how long.
- [ ] Insurance cover held by the roofer.
- [ ] VAT shown clearly.
Questions to ask a roofer
- Which system do you recommend for my roof and why?
- Have you fitted this system many times, and are you approved by the manufacturer?
- What will you do if the deck is rotten once the old roof is off?
- Will the roof have proper falls, and how will you deal with ponding?
- Does the work need building regulations approval, and who deals with it?
- Who will be on site, and will you use subcontractors?
- What is the guarantee, and is it backed by you or the manufacturer?
- How do you protect the house and garden if the weather turns?
- Can I see recent similar jobs or speak to a past customer?
How long it takes and what to expect
A small garage or extension roof can often be finished in a few days, weather permitting. Larger roofs, dormers, or jobs needing extensive deck repairs take longer. Scaffolding, if needed, is usually put up before work starts and taken down after.
Expect noise from stripping, and the smell of bitumen or resin during the work. Torch-on felt involves naked flame, so ask how the roofer manages fire risk. GRP and some adhesives need dry conditions to cure properly, so a good roofer will postpone rather than lay on a wet deck. Keep pets and children away from the working area, and make sure the roofer is happy for you to look at progress photos, especially of the deck and insulation, which are hidden once covered.
After completion, ask for your guarantee paperwork, details of the materials used, and any building control certificate if one applies.
Signs a quote is too cheap or too expensive
Too cheap
- No itemisation, just a single figure with a vague description.
- No mention of deck inspection, insulation or edge details.
- The roofer proposes laying new covering over the old without explaining why that is acceptable.
- No written guarantee, no insurance, or reluctance to give an address or references.
- Pressure to pay a large sum upfront or to start immediately.
- A price far below the others, which often means corners will be cut or extras added later.
Too expensive
- Costs that are not explained, such as large allowances for scaffolding on an easily reached roof.
- A premium system proposed without a reason, where a simpler one would do.
- Extensive extras listed as certain when they cannot be known until the deck is exposed.
- No difference in specification from cheaper quotes that are otherwise detailed.
Compare the prices on this page with your quotes. If yours sits well outside the range of what other homeowners paid for a similar job, ask the roofer to explain the difference line by line.