Warm Roof vs Cold Roof: Flat Roof Insulation ExplainedOn a flat roof the position of the insulation decides how the roof behaves for decades. This guide compares warm and…Guide · 9 min readFlat Roofing in Crawley
Garage, extension and dormer roofs, repaired or renewed.
Our flat roofing service covers garage, extension, dormer and balcony roofs in Crawley and the towns around it: GRP, EPDM, felt and liquid systems, repaired or replaced after a survey, with a single written price.
- Survey photos of your own roof
- One written price
- Repairs, re-roofs and emergencies
- Tidy site, waste taken away
Signs your flat roof is failing
- A brown stain on the ceiling below
- Water standing on the roof after rain
- Blisters or bubbles in the covering
- Splits or cracks along joints and edges
- A soft or bouncy patch underfoot
What our flat roofing service covers
Garage roof replacementOld covering stripped, decking boards checked and a new roof laid to a fall that drains cleanly into the gutter.
GRP fibreglass roofsResin and glass matting laid over sound boards, trimmed at every edge and finished with a topcoat.
EPDM rubber roofsOne rubber sheet cut to the roof, stuck down and detailed around upstands, trims and rooflights.
Felt flat roofsLayered bitumen felt, fused with a flame or applied cold, with the method chosen to suit the walls and timber beside it.
Liquid coatingsA cleaned and repaired roof recoated in a liquid membrane, where the existing surface and deck are sound enough.
Flat roof repairsSplits, blisters, trims, upstands and outlets put right, with the cause traced before anything is patched.
Repair, overlay or replace?
- RepairA dry, solid deck, and one clear fault in one spot on the roof.
- OverlayThe deck is sound, but the old covering is worn out across the surface.
- ReplaceThe deck is soft or wet, or the covering has failed across the whole roof.
How a flat roof job runs
- 1Describe the roof and faultYou say whether it is a garage, extension or dormer, and where the ceiling is marked.
- 2A survey from above and belowWe check the covering, outlet and deck from the roof, then the ceiling underneath, and photograph what we find.
- 3One written priceA single total in writing for the repair or new roof, with what we found set out plainly.
- 4Work, check and sign-offWe complete the job, confirm that water drains to the outlet, and share finished-roof photos with you.
The flat roof diary
Inside a flat roof job
Six stages of a flat roof job around Crawley, from the first look at a ceiling stain to the final check on the finished roof.
Stage 01 · First look
Flat roof repairs and replacements in Crawley
A flat roof fails where water meets an edge, a joint or a low spot, and our first job is to find which one.
Our flat roofing Crawley service covers the roofs that sit on garages, rear extensions, porches, bay windows, dormers, balconies and shop units. A flat roof is never quite flat. It is built with a slight fall so that rain runs towards a gutter or an outlet, and a single waterproof covering, laid in sheets or as a coating, keeps the weather out of the room below. There are no overlapping tiles to shed water if one part gives up, which is why a small split can drip into a bedroom during a single storm. It is also why the first job is to find where the water gets in, which can be a long way from where it shows.
- Membrane across the deck
- Upstand at the wall
- Outlet and gutter
- Edge trim
What do flat roofers in Crawley actually do?
The flat roofers Crawley households call take on three kinds of work. The first is repair: patching a split, refixing a loose edge trim, resealing an upstand (the short upturn of membrane where the roof meets a wall) or clearing a blocked outlet. The second is an overlay, where a new waterproof layer goes onto a deck that is still sound. The third is replacement: stripping the old covering, renewing any decking boards that have gone soft and laying the new roof to a fall that drains. Which of the three you need depends on what the survey finds, so we look before we price. Our guide to repairing or replacing a flat roof sets out how that decision is made.
Where do flat roofs fail first?
Edges and joins give up before the open middle of a roof does. The usual weak points are the junction with a house wall, the drip edge over the gutter, the collar around a pipe or rooflight, and any lap where one sheet meets another. Flat roof repairs Crawley homeowners ask for therefore tend to begin with a stain on a ceiling or a damp patch beside a wall. Once we are on the roof we check the whole surface, because a leak that shows at one corner can travel a long way along the deck before it drips.
Keep reading
On an older roof it can be hard to know what the covering is. A grey gritty surface suggests bitumen felt, a smooth black sheet with few joins suggests rubber, a hard glossy finish suggests fibreglass, and a brushed coating could be a liquid system laid over something else. We identify it before suggesting anything, because a repair that suits one material can fail on another. On a garage or extension we also look at the gutter and walls the roof drains into, since a perfect membrane still leaks if its water has nowhere to go.
Stage 02 · Materials
GRP, EPDM, felt and liquid roofs compared
Four waterproofing systems cover the flat roofs we are asked about, and each one ages and fails in its own way.
The flat roofs we repair or renew carry one of four systems: GRP, EPDM, felt or a liquid coating. GRP is fibreglass matting set in resin and finished with a topcoat, so it dries into a hard, jointless shell. EPDM is a synthetic rubber sheet, usually laid over the roof in a single piece. Felt, in flat roof work, means layers of bitumen sheet, either fused with a flame or applied cold. A liquid system is a coating, brushed or rolled on, that cures into a waterproof skin over a surface that is sound enough to take it. Each behaves differently when the building moves, when water finds a fault and when someone walks across it, so the choice should come from the roof, not from habit.
Flat roof system words
- GRP
- Glass-fibre matting set in resin
- EPDM
- Synthetic rubber sheet laid in one piece
- Torch-on felt
- Bitumen felt fused with a flame
- Liquid system
- Coating that cures into a waterproof skin
- Deck
- The boards the covering sits on
Which flat roof covering lasts longest?
Published lifespans are ranges rather than promises, and they depend on the deck, the fall and how the roof is looked after. EPDM is expected to serve for 20 to 30 years, according to GreenSpec. The Less flat roof guide, updated in March 2026, puts GRP at 25 to 40 years and felt at 10 to 20 years. On those figures a GRP fibreglass roof Crawley garages and extensions carry should outlast a felt flat roof Crawley neighbours had fitted at the same time, though a poorly supported deck can crack either one. A warranty is a separate thing from a lifespan, and no covering lasts if its water cannot drain.
Whatever the system, the covering is only as good as what lies under it. A rubber sheet on a sagging deck will pond, fibreglass on boards that flex will crack at its joints, and a coating over wet old felt will blister. So we judge the deck and the fall first and the covering second, and we read each product's data sheet to see what it may be laid over before we promise a method.
Choose the covering for the roof it has to protect, not for the one you saw next door.
Why fit a rubber membrane instead of felt?
An EPDM rubber roof Crawley extensions and garages carry is easy to picture: one sheet across an even rectangle, with the work concentrated at the edges and the upstands. Fewer joins means fewer places for a leak to start. Felt costs less per square metre in the published guides, and a well-laid felt roof does its job, but it has more laps to keep sound and a shorter typical life. Flame-applied felt is also hot work, so we assess the timber, the walls and the ground below before choosing it, and a cold-applied or non-flame system is often the better call beside timber fascias.
Is liquid flat roofing right near Gatwick?
Liquid flat roofing Gatwick way, or anywhere in our area, suits a deck that is sound and dry. The coating goes over the existing surface after cleaning and repair, so nothing is stripped, and it can be worked round awkward details. Our page on liquid-applied flat roofing shows where it fits. Near the airport, ask about lifting gear: Gatwick requires a permit for cranes within 6 km, with exceptions for low or screened equipment. A garage roof rarely needs one.
- 20 to 30 yearsEPDM expected life (GreenSpec)
- 25 to 40 yearsGRP typical life (Less, March 2026)
- 10 to 20 yearsfelt typical life (Less, March 2026)
Stage 03 · Where it sits
Garage, extension, dormer and balcony roofs
The building under the roof decides most of the detail, so we sort flat roofs by what they sit on.
A garage roof is usually a plain rectangle with a single run to a gutter. An extension roof sits against the house wall and often carries insulation, a rooflight or an overhang. A dormer roof is small but full of edges, with a cheek (the side wall) on both sides and a join where it meets the main slope. A balcony roof is a roof that people walk on, so it needs a surface that takes footfall without puncturing the waterproofing under it. All four are flat roofs, and all four fail at their details before they fail in the middle.
How does garage roof replacement work?
Garage roof replacement Crawley wide follows one sequence. We survey the roof and the ceiling below, then strip the old covering and look at the decking boards. Soft or stained boards come out and new ones go in, the fall is checked with a level, and only then does the new covering go on, with fresh edge trims and flashings. A single garage is a modest area, so the job is short beside a house roof, and a ladder is often all the access it needs.
Which roof, which check
- Garage
- Identify the sheet material first
- Extension
- Insulation and wall junction details
- Dormer
- Cheeks, flashings and small outlets
- Balcony
- Drained, walkable waterproof finish
What if the garage roof is asbestos cement?
HSE advises assuming that a garage built or refurbished before the year 2000 contains asbestos, and corrugated cement sheets are a familiar roof of that age. Its guidance calls asbestos cement roofs fragile in every case, so keep off the sheets and never rest a ladder on them. HSE counts the material as non-licensed work when whole sheets are lifted out with care and kept intact, but once it crumbles and throws up significant dust the job turns notifiable. Waste also has strict wrapping and labelling rules, and the household exemption for hazardous waste leaves asbestos out. So the first step is to have the material identified before anyone drills, saws or lifts a sheet. Our post on asbestos garage roofs and Crawley garage blocks goes further.
Which flat roofing services suit a dormer or balcony?
Dormer and balcony roofs are small, so access and detail drive the work more than area does. A dormer needs its cheeks, its flashing up to the main roof and its guttering sorted, and our guide to dormer flat roof replacement walks through it. Lead is the traditional answer for the joins, and our lead work service covers it. A balcony needs a drained, walkable finish, and whether it needs consent depends on what is being built, so ask the council's planning team. Shop units and larger roofs sit with our commercial roofing service.
Stage 04 · Diagnosis
Repair or replace: signs of failure
A flat roof tells you when it is failing, if you know what to look for and where to look.
Water still standing on the surface long after rain has stopped is the first sign anyone can spot from an upstairs window. Other signs are blisters, hairline cracks, splits along laps and trims, a covering that has lifted at an edge, and a patch of deck that gives underfoot. Inside, a brown ring spreading on a ceiling points to a leak. Not every stain is a leak, though. A damp patch that comes in cold weather and dries in warm weather points to condensation, so we look in the room and the roof void before blaming the covering. Reading these signs well is most of the work of deciding between a repair and a new roof, so we read the ceiling and the roof as a pair.
What to look at
- Blister in the surface
- Ponding at a low spot
- Split along a joint
Why is water standing on my flat roof?
Ponding means the water has no easy way off. The usual causes are a fall that was too shallow to begin with, a deck that has sagged between its joists, or an outlet that is blocked with leaves and moss. Under BS 6229:2025 the finished fall must still be at least 1:80 on the main roof area and in internal gutters, and IKO's advice, drawing on the 2018 edition, is to design for 1:40 so the finished roof lands above that minimum. A roof that holds water also ages faster, because every puddle sits on the covering and finds any weakness. Our answer on ponding water on a flat roof goes through the causes and fixes.
When does a flat roof repair stop making sense?
A repair makes sense when the deck is dry and sound and the fault is one split, one failed upstand or one blocked outlet. A single blister can be opened, dried and sealed. It stops making sense when blisters, splits or cracks cover the surface, when the boards are soft, when water has got between layers of old covering, or when the roof is already past the typical life for its material and keeps leaking in new places. A patch on a tired surface only moves the leak along, and you pay for the same job again next winter.
Keep reading
We give a straight view after the survey. If a repair will hold, we price it. If the roof is beyond repair, we explain what we saw, why a patch would fail and what the alternatives are, which may be an overlay rather than a full strip. You decide with the facts in front of you and nobody is pushed into a bigger job than the roof needs.
Stage 05 · Pricing
What a flat roof costs near Crawley
Five things move the price of a flat roof: access, area, system, insulation, and whatever sits under the old covering.
A flat roof price comes from the roof in front of us, not from a rate card. Five questions settle it each time: how we reach the roof, how many square metres it covers, which system goes on, what insulation goes under it, and what we find once the old covering is off. Our guide price for flat roofing is £50 to £100 per m², and the national guides below show why that spread is wide. Whatever the figure, you receive one total in writing after the survey.
What do national price guides show for flat roofs?
Checkatrade's September 2026 rubber roofing guide gives £80 to £100 per m² for EPDM, with an average of £90, and £50 to £80 per m² for felt. Its fibreglass guide from the same month starts GRP at £36 per m² for the covering alone and gives around £210 per m² fully insulated. For repairs, its June 2026 table gives £130 to £180 per m² for GRP and £120 to £170 for EPDM, excluding VAT, for up to 3 m². A whole flat roof is listed at £2,800 to £7,500 in a table last updated in December 2024, so treat that one as dated. As arithmetic only, a 15 m² garage roof comes to £750 to £1,200 in felt or £1,200 to £1,500 in EPDM on those per square metre ranges, before decking repairs, access or VAT. These are national guide figures that describe the market, and they cannot price your roof.
What sets a flat roof price
- Access
- Ladder, hoist, tower or scaffold
- Area
- Square metres of roof to cover
- System
- GRP, EPDM, felt or liquid
- Insulation
- Warm roof layers add cost
- Findings
- Condition of the deck once uncovered
Choosing a warm roof adds insulation boards and a vapour control layer to the job, and the insulation sets how the edges and rooflights must be built. Fitting it while the old covering is already off avoids opening the roof twice.
Why does access change a flat roof price?
A garage roof can often be reached with a ladder alone. A dormer, or a roof over a two-storey extension, may need a tower or scaffold, and Checkatrade's fascia and soffit table puts scaffold at £300 to £600 a week for each elevation. If the scaffold stands on the public pavement, the highway authority must grant a licence. West Sussex County Council's fee is £266 for a single month, and we arrange the permit jointly with the scaffolder, so it is not left for you to chase. Scaffolding on private land needs no licence. Check whether any quote you receive includes VAT and scaffolding, because the Checkatrade repair figures exclude VAT and the standard rate is 20%. In law you can normally cancel within 14 days of agreeing a contract at home, unless you asked us to visit for urgent repairs.
The covering is only part of the bill. Reaching the roof and fixing the deck matter more.
Two flat roof quotes only compare if they describe the same roof: the same system, the same insulation, the same decking repairs, edge trims and outlets, and the same answer on VAT and scaffold. Ask each roofer what happens if the deck turns out to be rotten once the old covering is off, because a price that assumes the deck is fine can grow later. Ours records in writing what we could not check from above, and any extra work waits for your say-so.
- £80 to £100EPDM per m², Checkatrade September 2026
- £50 to £80felt per m², Checkatrade September 2026
- £300 to £600scaffold per elevation per week, September 2026
Stage 06 · Sign-off
Insulation, Building Regulations and sign-off
A new flat roof is the natural moment to improve the insulation, and a few rules decide what the job must meet.
Two sets of rules can touch a flat roof job: planning, which is about looks, and the Building Regulations, which are about performance. For a straight swap of covering neither is usually a hurdle, but replacing a roof is also the easiest moment to improve its insulation.
What is a warm roof, and why does it matter?
On a warm roof the insulation sits above the deck, under the waterproof covering, so the timber stays on the warm side of it. On a cold roof the insulation sits between or below the joists, and the space above it needs ventilation to carry moisture away. A cold roof with poor ventilation can condense water inside the structure and rot the timber from within. Our guide to Building Regulations for flat roofs covers the energy side.
Does a new flat roof need Building Regulations approval?
The Planning Portal's national test says approval is usually unnecessary when under 25 per cent of the building envelope and under 50 per cent of the roof are affected, and a heavier new covering (15 per cent or more) changes that. Crawley Building Control says approval is needed once a roof repair or replacement covers more than 25% of its surface area, so a larger flat roof in the borough starts with a conversation with them. Approved Document L counts replacing the waterproof membrane on a flat roof as renovation, and renovating more than 50% of a roof's surface area brings in the energy upgrade. Table 4.3 sets a threshold of 0.35 and an improved figure of 0.16 W/(m²K), and where that is not feasible or would not pay back within 15 years, the roof goes to the lowest feasible value that does. The 2026 edition applies from 24 March 2027 (Table 3.3).
Does a flat roof replacement need planning permission?
Planning law only treats work as development if it materially affects the external appearance of the building, so a like-for-like recovering of a flat roof does not usually need planning permission. A parapet, lantern or balcony changes the roof, and flats, listed buildings and conservation areas follow different rules, so ask Crawley Borough Council's planning team, or your own council's, before you commit. Bats are fully protected, and GOV.UK names re-roofing as one of the activities that can affect them. It is also an offence to intentionally destroy or damage the nest of a wild bird that is in use. A roost, droppings or an active nest means work pauses until we have spoken with you.
Insulation and rules words
- Warm roof
- Insulation above the deck
- Cold roof
- Insulation below, ventilated air above
- Vapour control layer
- Stops room moisture reaching insulation
- Fall
- The slope that sends water to outlets
When the new covering is down we check the outlet, the trims and the upstands, clear offcuts from the roof and gutter, and send finished photos beside the survey ones. Any fault we noticed but left alone, a worn gutter for instance, is listed in writing for later. To get a price started, tell us what the roof sits on, give us its rough length and width, and say where any ceiling is marked.
Typical UK prices for flat roofing
| Work | Guide price |
|---|---|
| Flat roofing (Crown's own guide) | £50 to £100 per m² |
| EPDM rubber roof, installed | £80 to £100 per m² |
| Felt flat roof, installed | £50 to £80 per m² |
| GRP roof repair, up to 3 m² | £130 to £180 per m² |
| Whole flat roof, older table | £2,800 to £7,500 |
Only the first row is ours; the rest are national guides. The figure for your own roof comes after the survey, as one total in writing.
Crown's own guide for the first row. Checkatrade cost guides: rubber roofing, September 2026; roof repair table, June 2026 (excluding VAT); flat roof replacement table, last updated December 2024.
Guides and answers on flat roofs
Longer guides and quick answers on ponding, warm roofs, GRP, EPDM, felt and the Building Regulations, for anyone weighing up a flat roof job.
Guides and posts
Warm Roof vs Cold Roof: Flat Roof Insulation ExplainedOn a flat roof the position of the insulation decides how the roof behaves for decades. This guide compares warm and…Guide · 9 min read
Building Regulations for Replacing a Flat RoofReplacing a flat roof can bring in Building Regulations in two ways: how much of the building you touch, and how well…Guide · 9 min read
Ponding Water on a Flat Roof: Causes and FixesPuddles on a flat roof are common, and not every one is a problem. This guide covers why water stands, when it starts…Guide · 9 min read
Dormer Flat Roof ReplacementA dormer roof is small, but it carries more joints per square metre than almost any other flat roof. This guide…Guide · 9 min read
Repair or Replace a Flat Roof?A leaking flat roof is not always a finished one. This guide moves from the first signs, blisters, splits and soft…Guide · 10 min read
Liquid-Applied Flat Roofing: When Does It Work?Liquid roofing is poured and rolled on rather than rolled out, which makes it good at awkward shapes and poor at…Guide · 9 min read
Maintaining Flat Roofs: Tips for a Watertight, Durable SurfaceFlat roofs rarely fail without warning. A short check each spring and autumn finds the blocked outlet, the lifting…Blog · 9 min read
Flat Roofing Installation: Professional Techniques for Quality ResultsA flat roof is built in layers, and each one has a job. This is the sequence from bare joists to finished membrane…Blog · 9 min read
Flat Roofing Demystified: Pros, Cons and Best PracticeA flat roof costs less to build and fits where a pitch cannot, but it forgives less. Here are the advantages, the…Blog · 9 min read
Choosing Flat Roofing Materials for Performance and LongevityEPDM, GRP, torch-on felt and liquid coatings all keep water out of an extension or dormer roof. They differ in…Blog · 9 min read
GRP vs EPDM vs Felt: Which Flat Roof for Your Garage?A garage roof is small, plain and easy to forget until it drips onto the car. Here is how the three common coverings…Blog · 9 min read
Asbestos Garage Roofs: Safe Options for Crawley Garage BlocksA garage roof built before 2000 may be asbestos cement, and the way you deal with it depends on the condition of the…Blog · 10 min readQuick answers
- How Long Does a GRP Roof Last?
- How Long Does an EPDM Rubber Roof Last?
- How Much Does a Garage Flat Roof Cost?
- How Much Does a Flat Roof Cost per m2?
- Why Is My Flat Roof Blistering?
- How Do You Find a Leak in a Flat Roof?
- Can You Put GRP Over an Old Felt Roof?
- Is Torch-On Felt Safe?
- Can You Walk on a Flat Roof?
- Does a Flat Roof Extension Need Planning Permission?
- Can You Turn a Flat Roof Into a Balcony?
- Can a Flat Roof Be Replaced in Winter?
- Why Is My GRP Roof Cracking?
- Blocked Flat Roof Outlet: What To Do
- Which Deck Board Is Best for a Flat Roof?
- How Long Does a Felt Flat Roof Last?
- Why Is There Condensation Under My Flat Roof?
- Why Is My Flat Roof Leaking at the Edge Trim?
- What Is the Minimum Fall for a Flat Roof?
- Why Is My Flat Roof Sagging?
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Which covering suits a house extension best?
It depends on the shape and the details. An even rectangle suits a single rubber sheet. A roof with many corners, pipes and rooflights suits fibreglass or a liquid coating that can be formed round them. Felt suits a tighter budget. Ask each roofer why they chose their system, and ask to see its data sheet. A survey of your roof settles the question before any price is set.
What life should I expect from a rubber or fibreglass roof?
For EPDM, GreenSpec's expected service life is 20 to 30 years, while the Less cost guide puts GRP at 25 to 40 years and felt at 10 to 20 years. Treat these as ranges, not promises. The deck, the fall, trapped moisture and how often the outlets are cleared all move the result, and a warranty is a different thing from a lifespan.
Can an old asbestos cement garage roof be replaced?
Yes, but the material has to be identified first. HSE says a building from before 2000 should be treated as containing asbestos, and it calls asbestos cement roofs fragile in every case, so nobody should walk on one. Careful removal of intact sheets is non-licensed work, but crumbling sheets that make significant dust become notifiable. The waste has its own wrapping and labelling rules.
Does a new flat roof covering need Building Regulations sign-off?
It can. The Planning Portal sets a national test of less than 25 per cent of the building envelope and less than 50 per cent of the roof, and Approved Document L treats replacing a flat roof's waterproof membrane as renovation. Crawley Building Control has its own wording, so we check with it on larger roofs. Your building control body has the final say.
Is a patch ever the right answer for a leaking flat roof?
Yes, when the deck is dry and sound and the fault is one split, one failed upstand or one blocked outlet. A patch on a tired surface only moves the leak along, so we look at the whole roof first. If the survey shows wide damage, we explain plainly why a repair would not hold, and what the options are.
Can a flat roof be replaced in cold or wet weather?
Coverings need a dry deck, so rain stops the work, and some adhesives and coatings have temperature limits set by their makers. We check the forecast, keep the opened area small and seal it against rain before we leave each evening. Ask any roofer which products they plan to use and what the manufacturer's data sheet says about conditions.
Tell us about your flat roof
Send the roof's rough length and width, what it is made of if you know, and where the ceiling is marked.