Fitting a flat roof covering comes down to four steps: get the deck dry and flat, check the fall (the Dutch trade guidance sets its minimum at 16 millimetres per metre towards the outlet, and BS 6229 asks for more), lay the covering, then finish the details properly. For a DIYer, EPDM is far and away the best choice: one rubber sheet with no seams, no open flame, and a service life of 40 to 50 years. Bitumen performs just as well, but then you are working with a gas torch on a timber roof, and that is exactly where self-builders come unstuck.
Let me be honest straight away: laying the covering is not the hard part. Any reasonably handy person learns to roll out and bond a sheet of EPDM in an hour. The hard part is the edges, the corners, the penetrations and the fall. That is where a flat roof leaks. Never in the middle.
Everything you need is below: how to choose between EPDM and bitumen, how to check whether your roof deck is good enough, what the requirements are for fall and upstand height, when to insulate at the same time (and what help is available towards the cost), the full step plan for both materials, the details that really matter, what it all costs, and the seven mistakes that have your roof leaking within five years.
Which flat roof covering should you choose?
There are three types you will come across on a house. Briefly, because the choice is actually made quite quickly.
| EPDM (rubber) | Bitumen (2-layer) | PVC / TPO (plastic) | |
|---|---|---|---|
| Service life | 40 to 50 years | APP around 20 to 25 years, SBS around 25 to 30 years | 20 to 30 years |
| Seams in the roof field | Often none (one sheet cut to size) | Overlaps at every sheet | Welded at every sheet |
| Open flame needed | No | Yes when torching, no with self-adhesive | No, but you need a hot air welder |
| DIY friendly | Yes, very doable | Self-adhesive only, and with great care | No, needs a welder and experience |
| Material cost | Medium | Low to medium | Medium to high |
| Weak point | The bonded edges and corners | The overlaps and the abutments | The welded seams |
EPDM is one big rubber mat. You order it cut to size, including extra length for the upstands. So for an extension or dormer of 4 by 5 metres you literally get one sheet with no seam in the roof field. That is the big win: where there is no seam, no seam can open up. The standard thickness for roofing is 1.14 millimetres. If you walk on the roof regularly, if a roof terrace is going on top, or if you are fitting solar panels, go for 1.52 millimetres.
Bitumen is the classic roofing felt, in two layers: an underlayer and a mineral-finished cap sheet. Laid correctly it makes a perfectly good roof. SBS bitumen (rubber modified) stays more flexible in the cold and lasts longer than APP bitumen (plastic modified). The problem for the self-builder is the application: APP essentially has to be torched, while SBS can also be cold bonded or self-adhesive.
PVC and TPO you leave to the roofer. Those sheets have to be hot air welded together, and that takes tools and routine you are not going to buy for one roof.
In short: if you are doing it yourself it will be EPDM, or self-adhesive bitumen if your roof has a lot of penetrations and awkward corners (getting rubber neatly around a chimney is trickier than working with bitumen strips).
When to do it yourself and when not to
Let us be level headed here, because a leaking roof costs you more than the roofer would have.
Doing it yourself works fine on:
- A shed, garden room, garage, carport or store
- A dormer or small extension up to roughly 25 square metres
- A roof that is straightforward: one outlet, one penetration at most, no rooflights
- A roof you can reach safely, with a tower scaffold or a properly footed ladder
Get it done for you on:
- Roofs over roughly 30 square metres, certainly if insulation is going in as well
- A roof with lots of penetrations, roof lanterns, rooflights or abutments against a higher wall
- Anything where you would have to work with a gas torch on or against timber
- A roof that abuts the wall of your house, where a leak does damage inside straight away
- A roof where you cannot stand safely
That last point is not a formality. The heaviest part of this job is not the bonding, it is lifting rolls and boards up there and working safely once you are on top. Hire a tower scaffold. It costs a few tens of pounds for a weekend and saves you a broken ankle.
Step 1: check and repair the deck
This is the step everyone wants to skip and the one that decides everything. A new covering over a rotten deck is money thrown away.
Walk your roof. If it feels springy or spongy anywhere, the deck is damaged there and it has to come out. Can you see brown staining or a white bloom on the timber from the loft or inside the extension? Same story.
What makes a good deck? For a flat roof, 18 millimetres is the standard thickness for board material. Underlayment board and OSB are both used a lot; underlayment is stronger and holds its shape better, OSB is cheaper. Plywood decking usually runs between 18 and 22 millimetres. More important than the brand of board is this:
- The boards sit tight and there is no step between two boards (a 3 millimetre lip will telegraph through your EPDM and become a wear point)
- All screw and nail heads are countersunk, nothing is proud
- There is a 2 to 3 millimetre gap between boards so the timber can move without buckling
- The timber is bone dry. So not: rain on Saturday, bonded down on Sunday
Clean the deck. Every twig, leaf, patch of moss, bit of gravel and loose mineral chipping off. Old nails out. Sand off rough spots and proud splinters. One screw left behind will work its way through your membrane within two winters.
Step 2: falls and drainage, the bit everyone gets wrong
A flat roof is never really flat. It has to run towards the outlet, and that goes wrong more often than you would think.
The standard: BS 6229 works on designing to a fall of 1:40, so that after deflection and building tolerances you still end up with the minimum finished fall of 1:80, which is 12.5 millimetres per metre. The Dutch trade guidance for fully supported roof coverings puts its minimum at 1.6 per cent, so 16 millimetres per metre. Take the design fall as your target and treat 16 millimetres per metre as an absolute floor. On a roof 5 metres deep, 1.6 per cent means 8 centimetres of height difference between the back and the roof edge with the outlet.
Why it matters. Standing water is not just ugly. It collects dirt and leaves, it grows a green film, it freezes in winter and it puts the roof under permanent load. A puddle 2 centimetres deep over 20 square metres is already 400 kilos on your structure. And exactly where water sits, every type of roof covering ages fastest.
Measuring: lay a long straightedge or batten from the back to the roof edge and put a spirit level on it. Or simpler still: on a dry day, tip a bucket of water onto the roof and watch where it goes and whether it clears. If a puddle stays behind, you have a problem to solve now, not later.
Not enough fall? Three solutions:
- Tapered insulation. Insulation boards cut to a fall. That is the neatest solution going, because you insulate and correct the fall in one move.
- Firrings or battens on the deck, increasing in depth, with a new layer of board material on top.
- Resetting the roof by raising the joists. Serious work, but on a roof that is completely out it is the only real answer.
Emergency overflow. This is the cheapest insurance policy on your whole roof. If your outlet blocks with leaves, the roof fills up until water spills over the edge somewhere or, far nastier, until the structure gives way. An emergency overflow is nothing more than a short pipe through the roof edge, set slightly higher than the normal outlet. If water comes out of it, you know immediately that your outlet is blocked. In the UK, rainwater drainage is designed to BS EN 12056-3 and covered by Approved Document H, and on larger roofs you have the capacity calculated; the Dutch equivalent for that capacity sits in NEN 3215. On an extension of a few dozen square metres the rule of thumb is simple: one good outlet plus an emergency overflow, and if in doubt two outlets rather than one.
Step 3: insulation and vapour control, warm roof or cold roof
If you are renewing the covering anyway, this is the moment to insulate. Opening a roof up a second time is a waste of good work.
Warm roof. The insulation sits on the deck, under the covering. Underneath it, on the warm side, goes a vapour control layer. This is the technically best and most widely used build-up for a flat roof: the whole structure stays warm, so condensation gets no chance, provided that vapour control layer is properly sealed.
Cold roof. The insulation sits under the deck, between the joists. The structure stays cold. That is asking for condensation in the build-up, rot and mould. Do not do this on a flat roof. If you have a ceiling inside with insulation between the joists and you are now going to insulate from above, get someone who understands building physics to look at it, because two layers of insulation with an air gap between them can go badly wrong.
Inverted roof. The insulation goes on top of the covering, with ballast over it. Used on roof terraces and roofs that get walked on a lot. Needs compression resistant insulation (XPS) and ballast, so only where your structure can carry that weight.
The vapour control layer is not a detail. Warm, moist indoor air that gets into your roof build-up cools down there and comes out as water. That water has nowhere to go, because above the insulation sits a completely watertight layer. A vapour control layer you fitted beautifully but later drilled a light fitting through is no longer a vapour control layer. Lap the joints, tape the penetrations, finish the connections at the walls.
The rules when you renovate. In England, Approved Document L treats a flat roof you are re-covering as a thermal element being renovated, with a target U-value of 0.18 W/(m²·K) for the finished roof. That is a minimum requirement, not an ambition. In the Netherlands the same thing is set out in article 5.20 paragraph 2 of the Besluit bouwwerken leefomgeving: renew the insulation layer completely and you need a thermal resistance of at least 2.1 m²·K/W; renew a separating construction only partly and the requirement is an average of at least 1.4 m²·K/W, unless the existing level is already higher. Floors, not targets. In practice you reach numbers like that with a thin layer of insulation, and then you are leaving a lot on the table.
Help with the cost. In the UK there is no general grant for DIY flat roof insulation. The schemes that do exist, ECO4 and the Great British Insulation Scheme, are based on your household and property qualifying, and the work has to be done by a certified installer working to the recognised standards, so doing it yourself rules you out. Check what you actually qualify for before you order anything, because the schemes and their conditions change.
If you are in the Netherlands it works differently: there the ISDE scheme requires at least Rd 3.5 m²·K/W for the new insulation material, with a minimum of 20 and a maximum of 200 square metres of eligible area. The amount is 16.25 euros per square metre for a single measure and 32.50 euros per square metre if you combine two or more measures (for example roof insulation plus floor insulation or a heat pump). Bio-based insulation material adds another 5 euros per square metre. You have 24 months after the work to apply. Always check the current conditions on rvo.nl before you order, because those requirements get adjusted.
Note this: Rd 3.5 takes roughly 12 to 13 centimetres of PIR or phenolic foam, or thicker layers with less insulating materials. To hit a 0.18 U-value you need a noticeably thicker layer than that. So factor in from the start that your roof edge and your upstands have to rise with it.
Step 4a: laying EPDM, step by step
What you need:
- EPDM membrane cut to size, 1.14 mm (or 1.52 mm on roofs that get walked on and roofs with solar panels)
- Deck adhesive (which type depends on the EPDM: for American EPDM on timber usually a deck bonding adhesive, for European EPDM a PU adhesive)
- Contact adhesive for all edges, upstands and details
- Primer for seams and details, certainly with American EPDM
- Seam sealant and mounting adhesive
- Prefabricated internal and external corners and prefabricated pipe collars
- Aluminium roof edge trim
- Adhesive roller, wide brush, pressure roller, trimming knife, broom
Always order generously. Measure your roof field and add, for every upstand, the height of that upstand plus at least 10 centimetres of overlap. Measure it tight and you will come up short at the roof edge, and that is the end of your seamless roof.
Step 1. Roll out and let it relax. Put the membrane on the roof, roll it out and leave it for at least half an hour. The creases from the packaging will pull out. Never stretch the membrane tight, because rubber under tension will pull on your edges later.
Step 2. Position it. Lay the membrane exactly right, with the overlap you need on all sides. This is your only chance.
Step 3. Fold back half. Fold the membrane back on itself over half its length, so half the roof field is clear.
Step 4. Adhesive. Roll the deck adhesive evenly onto the timber. Not too thin (poor adhesion) and not too thick (blistering, and adhesive that never cures). Stick to the waiting time on the tin: with contact adhesive it has to flash off first, with other adhesives you lay the membrane straight into it.
Step 5. Lay it back and press it down. Lay the membrane back down calmly, working from the middle outwards, and push the air out with a broom or pressure roller. Bubbles you leave in now will stay in.
Step 6. The other half. Same story.
Step 7. Edges and upstands. Here you use contact adhesive, always. Coat both surfaces, let them flash off until they no longer stick to your finger, then press together. Work from the bottom up and roller down every centimetre.
Step 8. Corners and penetrations. Use prefabricated corner pieces and prefabricated collars for penetrations. Folding your own corner out of rubber is possible, but the prefabricated parts are the reason this roof lasts 40 years instead of 4.
Step 9. Finish and check. Fit the edge trim, tool off the sealant lines, go over the lot. Walk the whole roof one more time and press down every edge.
Working conditions: dry roof, dry day, no rain on the way. Bring adhesive, sealant and membrane indoors to room temperature the day before, because cold adhesive bonds worse and is miserable to work with. Do not start in frost.
Step 4b: laying bitumen, and why you leave the torch alone
If you want or need bitumen, read this first.
The most important safety rule: you never torch directly onto timber. The first layer on a timber deck goes down mechanically (nailed or screwed with washers) or you use a self-adhesive underlayer. Timber worked over with a torch can smoulder, and it carries on invisibly under the covering you have just laid over it. Then you are up there hours later putting a fire out. In the professional roofing trade, working safely with heat is set out in guidance like the Safe2Torch rules, and flame-free first layers have become the standard; in the Netherlands the same thing is written into NEN 6050.
Practical step plan for self-adhesive bitumen:
- Deck clean, dry, flat, nail heads countersunk
- Apply bitumen primer and let it dry (this is not an optional step, primer is what makes timber or concrete bondable)
- Lay the underlayer: mechanically fixed along the edges and overlaps, or self-adhesive
- Keep to the overlaps the manufacturer specifies, usually 8 to 10 centimetres along the length and more at the head laps
- Mineral finished cap sheet on top, with the seams staggered relative to the underlayer (so never seam on seam)
- Roller down every seam with a seam roller
- Upstands and details in strips, with reinforcement at the corners
Where bitumen goes wrong: laying seam on seam, too little overlap, working in the cold so the self-adhesive layer does not bond, and running the upstands too low.
The details: this is where your roof leaks
Nine out of ten leaks are not in the roof field but at the abutments. Work through this list.
Upstand height. UK practice, following BS 6229 and the roofing federation guidance, is a minimum upstand of 150 millimetres above the finished roof surface. The Dutch trade guidance puts its floor at 120 millimetres, likewise measured from the top of the newly laid roof covering system. So measured above the finished roof, not above the deck. If you are insulating with 12 centimetres of PIR, your roof edge has to go up 12 centimetres with it. This is the mistake most often made when people insulate their own roof: they lay insulation on the deck but forget to raise the roof edge and the upstands. Then in a proper downpour you have water standing against an upstand of 3 centimetres.
Abutment against a wall. Where your flat roof meets the wall of your house, the covering has to run up the wall and above it there has to be something that stops water getting in behind: a chased-in cover flashing, a lead flashing or an aluminium wall trim. Simply bonding the covering to the brickwork and putting a blob of sealant over the top will hold for a few years and then it will not.
Penetrations. Every pipe, every cable, every outlet: prefabricated collar. Sealant is a finish, not a seal.
Edge trim. An aluminium trim makes the edge crisp, holds the membrane in place and makes water drip clear instead of running down the wall. Fit it with the right fixings and allow for expansion: aluminium moves, so do not butt the sections up hard against each other.
Ballast. On loose laid systems there is gravel or paving on top as ballast against wind uplift. On a fully bonded EPDM roof that is not needed. If you want gravel anyway, first check your structure carries the weight and lay a protection layer under the gravel.
Solar panels on a flat roof. Use a ballasted mounting system that does not penetrate the roof, put protection mats or extra membrane under the feet, and then do go for 1.52 millimetre EPDM. And think ahead: once panels are on, you can no longer reach a leak underneath them.
What does it cost?
Material prices move around, so these are ranges to plan with, not to hold anyone to.
| Item | DIY (materials) | Done for you (complete) |
|---|---|---|
| EPDM roof, bare roof field | £22 to £40 per m² including adhesive and edge trim | £50 to £95 per m² |
| Bitumen, 2 layers | £18 to £30 per m² | £22 to £50 per m² |
| PIR insulation (Rd 3.5, around 12 cm) | £25 to £45 per m² | included in the roof price |
| New roof deck, 18 mm | £18 to £30 per m² | included |
| Tower scaffold hire | £45 to £100 per weekend | included |
| Disposing of the old covering | £90 to £260 (skip) | often included, ask |
Doing it yourself, reckon on roughly 40 to 60 per cent of what a roofer would charge. And with a roofer, always count the guarantee in: an established firm warrants both the material and the workmanship, and on a flat roof over your living room that is worth real money.
Seven mistakes that have your roof leaking within five years
- Working on a deck that is not completely dry. Moisture under the covering never gets out again and starts blistering as soon as the sun hits it.
- Not raising the roof edge when you insulate. Upstand too low, water over the top, done.
- Seam on seam with bitumen. Between the underlayer and the cap sheet, the seams have to be staggered.
- Starting in the cold or with rain coming. Adhesive will not bond, sealant will not cure, and one shower on fresh adhesive is a start-again job.
- Pulling the membrane tight. EPDM has to lie relaxed. Rubber under tension contracts in the cold and pulls your edges loose.
- No emergency overflow. One blocked outlet and you have thousands of kilos of water on your roof.
- Using sealant as the seal. Sealant finishes the seal off. Prefabricated corners and collars do the sealing.
Maintenance: twice a year, ten minutes
A flat roof asks for little, but not for nothing.
- Spring and autumn: clear leaves, moss and twigs, and check the outlet and the emergency overflow
- After a storm: have a quick look that nothing has pulled loose at the edges
- Every year: walk the upstands and abutments and feel whether anything is lifting
- With gravel: rake it aside at the outlet once, because that is where the dirt collects
If you see blistering, there is air or moisture underneath. Small blisters in EPDM are usually cosmetic, big bulges in bitumen are a sign of moisture in the build-up. That is the point to get someone to look at it, before it comes through your ceiling.
Frequently asked questions
Can I lay EPDM myself?
Yes. On a shed, garage, dormer or small extension up to roughly 25 square metres, EPDM is very doable yourself. You order the membrane cut to size, so you have no seams in the roof field, and you work without open flame. The hard part is the edges, corners and penetrations. Use prefabricated parts and contact adhesive there, and give yourself time.
Can I lay a new covering over the old one?
Sometimes, but only if the old layer is dry, well bonded and flat, and if the structure carries the extra weight. If there is moisture under the old layer you seal that moisture in and the build-up will start blistering. If in doubt, take it off. You then see straight away what condition your deck is in, and you want to know that anyway.
How much fall does a flat roof need?
BS 6229 works on a design fall of 1:40 so that the finished fall is never less than 1:80, which is 12.5 millimetres per metre. The Dutch trade guidance for fully supported roof coverings sets its minimum at 1.6 per cent, so 16 millimetres per metre. Over 5 metres of depth, 1.6 per cent is 8 centimetres of height difference. Less fall means ponding, and ponding is the biggest single accelerator of ageing in every type of roof covering.
How long does a flat roof last?
Laid well, EPDM lasts 40 to 50 years. With bitumen the service life is lower: roughly 20 to 25 years for APP systems and 25 to 30 years for SBS systems, provided the design, the workmanship and the maintenance are right. Ponding, a lot of foot traffic and damage bring those numbers down sharply.
Should I insulate at the same time as replacing the covering?
Practically speaking: yes. You are opening the roof up anyway, and opening it a second time is double the work. On top of that, re-covering a flat roof and renewing its insulation is treated as renovating a thermal element, with a target U-value of 0.18 W/(m²·K) under Approved Document L in England; in the Netherlands the legal floor is 2.1 m²·K/W, and for ISDE support the new material needs at least Rd 3.5 m²·K/W over a minimum of 20 square metres. Either way, remember that your roof edges and upstands have to rise with it.
Can I use a gas torch on my own roof?
You can, but not on a timber deck. The first layer on timber gets mechanically fixed or laid self-adhesive, precisely because timber can smoulder under the covering without you seeing it. If you still want bitumen, choose a self-adhesive system. Check your buildings insurance too: hot works can come with conditions attached.
What is an emergency overflow and do I need one?
An emergency overflow is a short pipe through the roof edge, set just above the normal outlet, that lets water escape when the regular outlet is blocked. It is not compulsory in every situation, but it is the cheapest protection against water building up that there is. A roof filling with water carries hundreds of kilos for every centimetre of depth. On any new roof covering I would put one in.
In short: choose EPDM if you are doing it yourself, check and repair your deck first, make sure you have at least 16 millimetres of fall per metre and preferably the full design fall, insulate while you are up there anyway, raise your roof edge with it so the upstand stays right, use prefabricated corners and collars instead of sealant, and fit an emergency overflow.
Do that, and you have a roof you will not have to think about for decades. Skip the details and in three years you will be hunting for where that mark on your ceiling came from.
Are you building a shed, garage, extension or shelter that this roof is going on? At Fred’s DIY Plans you will find fully worked building plans with the structure, material list, cutting list and a step-by-step plan, including the roof build-up and the fall, so the dimensions are right before you cut the first board. Have a look here.
Regards from Fred Schouten, handyman and DIYer through and through.
PS: before you start, tip a bucket of water onto your old roof and watch where it goes. If a puddle stays anywhere, you know exactly where your fall falls short and where your roof will age first. Two minutes of work, and it saves you guessing about your structure. Small tip. Big difference.