How to Re-Roof a Garage with EPDM Rubber
How to Re-Roof a Garage with EPDM Rubber
A single-garage flat roof is one of the most common EPDM jobs in the UK — and one of the most forgiving for a first-time installer. There are no complicated penetrations, the deck is usually easy to reach off a pair of trestles, and a standard 5m x 4m garage can be covered in one seamless sheet with no field joins at all. This guide takes you from stripping the old felt to trimming the final edge, with the exact quantities, tools and timings for a typical UK garage.
Before you start: is your garage a good candidate?
EPDM will go over almost any flat or gently pitched garage roof, but the deck underneath has to be sound. Rubber is a waterproofing layer, not a structural one — it follows the shape of whatever it is bonded to, including any sag, soft patch or protruding nail head. Walk the roof before you order anything. If it feels spongy underfoot, if you can see daylight from inside, or if the boards have delaminated at the edges, budget for a new deck as part of the job.
Typical UK garage roofs fall into three groups. Timber decks (usually 18mm OSB3 or WBP ply on joists) are the ideal substrate and take adhesive directly. Concrete decks are common on 1960s and 1970s garages and work well provided they are dry, dust-free and reasonably flat. Corrugated asbestos-cement or metal sheet roofs are not a bonding substrate at all — those need a new timber deck built over the top, or professional removal in the case of asbestos.
What you will need
The figures below are for a typical single garage measuring 5.0m x 3.0m externally. Always measure your own roof and add the upstand and drip-edge allowance described in the next section.
| Item | Quantity for a 5.0m x 3.0m garage | Notes |
|---|---|---|
| EPDM membrane, 1.2mm | One sheet, 5.6m x 3.6m | Cut to your exact size — no field seams needed |
| Water-based deck adhesive | 5 litres | Covers roughly 20m² over the field of the roof |
| Contact adhesive | 2.5 litres | For upstands, perimeter edges and any vertical face |
| Drip trim (edge trim) | Enough for the gutter edge, plus corners | Usually the front and one side on a garage |
| Wall trim / termination bar | Length of any abutting wall | Only if the garage adjoins a house or neighbour’s wall |
| Trim adhesive and lap sealant | 2 x 310ml cartridges | Bedding trims and sealing exposed edges |
| Fixings for trims | 1 box | Stainless or galvanised, at roughly 300mm centres |
Tools: a soft broom, a 9" paint roller and tray for the deck adhesive, a 4" brush for the contact adhesive, a sharp trimming knife with spare blades, a silicone seam roller, a tape measure, a chalk line, and a cordless drill. Our DIY installation kit bundles the roller, brush, knife and seam roller together, or you can jump to the professional kit if you are doing more than one roof.
Not sure of your quantities? Put your garage dimensions into the EPDM roof calculator and it will size the membrane, adhesives and trims for you.
Measuring a garage roof correctly
The single most common ordering mistake is measuring the deck and forgetting the edges. EPDM has to wrap down over every perimeter and up every abutting wall, and you want spare material to grip while you dress it into place.
The formula
Take the deck length and width. To each dimension, add 150mm for every edge that terminates in a drip trim, and 250mm for every edge that runs up against a wall. Then add 150mm of overall trimming allowance to each dimension.
Worked example
A deck measuring 5.0m x 3.0m, free on three sides with one wall abutment on a long edge:
- Length: 5.0m + 0.15m + 0.15m (two free ends) + 0.15m allowance = 5.45m → order 5.6m
- Width: 3.0m + 0.15m (free edge) + 0.25m (wall upstand) + 0.15m allowance = 3.55m → order 3.6m
Always round up to the next convenient size. Trimming off 100mm of surplus takes thirty seconds; being 100mm short means a seam you did not want or a whole new sheet.
Step-by-step installation
Step 1 — Strip the old covering
Remove the old felt, fibreglass or coating completely, along with every nail, tack and staple. Rubber follows the surface it sits on, so a single proud clout nail will eventually wear a hole through the membrane from underneath. Pull the old timber fascia and drip edges off too, since new trims need a clean face to fix to.
Step 2 — Repair and prepare the deck
Replace any soft or delaminated boards. Where you are re-decking, use 18mm OSB3 or exterior-grade ply laid with a 2–3mm expansion gap between sheets and staggered joints, fixed at 150mm centres around the edges and 300mm in the field. Punch down any proud fixings, fill gaps wider than 5mm, and sand off ridges at board joints. Finish by sweeping and then vacuuming the deck — adhesive will not bond through dust.
Check the falls while you are here. A flat roof should not be truly flat; aim for a minimum 1:80 fall towards the gutter. If water currently ponds on the garage roof, now is the time to add tapered firrings, because EPDM will happily hold that puddle forever once it is down.
Step 3 — Fit the perimeter trims
Fix the drip trim along the gutter edge and any other free edge, bedded on trim adhesive and mechanically fixed at roughly 300mm centres. At external corners, cut and lap the trim rather than butting it, and seal the joint. Getting trims on before the membrane means the rubber can be dressed straight into the trim leg for a clean, permanent edge.
Step 4 — Relax the membrane
Unroll the sheet over the deck and position it so the overhang is even on all sides. Leave it for 20–30 minutes in mild weather. EPDM comes off the roll with creases and a memory of being folded; it needs time to settle flat before you bond it, otherwise you will glue the creases in. On a cold day, allow longer.
Step 5 — Bond the first half
Fold half the sheet back on itself, cleanly, without dragging it. Roller water-based deck adhesive onto the exposed deck, stopping about 150mm short of every perimeter and wall — those areas get contact adhesive instead. Work it out to an even, wet film with no puddles and no dry patches.
Roll the membrane back into the wet adhesive slowly, working from the fold outwards, and sweep it down with a soft broom from the centre to the edges as you go. That sweeping motion is what pushes trapped air out; if you drop the sheet flat all at once you will chase bubbles for the rest of the afternoon. Repeat for the second half.
Step 6 — Bond the perimeters and any upstand
Now go back to the 150mm you left dry. Lift each edge, brush contact adhesive onto both the deck and the underside of the membrane, and let it flash off until it is tacky rather than wet — usually 5–15 minutes depending on temperature. Then bring the two surfaces together. Contact adhesive grabs instantly and does not give you a second chance, so position carefully before you touch down.
Where the garage abuts a house or neighbour’s wall, carry the membrane a minimum of 150mm up the vertical face and finish it into a wall trim or under a termination bar with sealant along the top edge. At internal corners, fold the material into the corner and dress a corner patch over it rather than cutting the sheet.
Step 7 — Dress the edges and trim off
Work around the perimeter dressing the membrane down into the drip trim leg, then trim the surplus off with a sharp knife held against the trim. Run a bead of lap sealant along any exposed cut edge at a wall or trim junction. Finally, walk the whole roof and look across it at a low angle — any remaining air will show as a shadow. Bubbles found while the adhesive is still green can usually be swept out towards the nearest edge.
Garage-specific details worth getting right
The up-and-over door head
The front edge of a garage roof sits directly above the door opening, and it is the edge most likely to be seen and most likely to be knocked. Fit the drip trim there properly with the leg overhanging into the gutter or clear of the door head, and do not rely on sealant alone to hold the membrane down at that edge.
Shared and party walls
Semi-detached and terraced garages often share a wall with a neighbour. Take the upstand to 150mm minimum, terminate it mechanically, and never dress the membrane over onto the neighbour’s side. If there is a shared parapet, both sides need weathering — a cap flashing or coping over the top is the usual answer.
Existing rainwater outlets
If the garage drains through an outlet in the deck rather than to an eaves gutter, use a proper bonded outlet rather than dressing the membrane into a pipe. Our horizontal drain kit gives you a factory-made flange that bonds to the field sheet.
Five mistakes that cause garage roof leaks
1. Bonding onto a damp or dusty deck
Water-based adhesive needs a dry, clean surface. Laying onto a deck that got rained on the night before is the fastest way to a sheet that lifts in the first gale. Test by taping a square of polythene down for an hour — condensation underneath means the deck is still wet.
2. Using deck adhesive on a vertical face
Water-based adhesive relies on the weight of the membrane holding it in contact while it cures. On an upstand there is no weight, so it slides. Every vertical surface gets contact adhesive.
3. Skipping the relaxation period
Bonding a creased sheet locks the creases in permanently, and creases channel water and collect debris. Twenty minutes of patience is cheaper than a new sheet.
4. Cutting the membrane at internal corners
Every cut is a potential leak path. Fold the material into the corner and cover it with a purpose-made internal corner patch. If you must cut, the cut edge has to be sealed and covered.
5. Leaving ponding water unaddressed
EPDM tolerates standing water better than felt, but ponding still traps grit, promotes moss and hides the point at which a detail is failing. Fix the fall before you fix the covering.
How long it takes and what it costs
A competent DIY installer should allow a full weekend for a single garage: one day to strip, repair and prepare the deck, and one day to fit trims and bond the membrane. Two people make the membrane-handling stage substantially easier — a 5.6m x 3.6m sheet is heavy and awkward for one pair of hands. A professional crew will typically complete the same roof in a single day.
For a materials-only budget on a typical single garage, see the EPDM roofing cost guide, which breaks costs down per square metre including adhesives and trims.
Frequently asked questions
Can I lay EPDM straight over old felt?
No. Old felt is not a stable bonding surface, it may be damp underneath, and any trapped moisture will blister. Strip it back to the deck.
Do I need to prime a concrete garage roof?
Concrete decks need to be dry, dust-free and sound. Depending on the surface condition a primer may be recommended before adhesive — see the adhesives and primers guide for when priming is and is not required.
What thickness should I use on a garage?
1.2mm is the standard choice for domestic garages and gives a 50-year design life. Step up to 1.5mm if the roof will see foot traffic, if it is a commercial unit, or if you simply want the heavier sheet. The thickness guide covers the differences in detail.
Can I fit EPDM in winter?
You can install down to around 5°C provided the deck is dry and it is not raining, but adhesives take longer to flash off and the membrane is stiffer and slower to relax. Above 10°C and dry is much easier.
Will the roof take my weight afterwards?
EPDM is walkable for maintenance access once fully bonded. It is not designed as a wear surface — if the garage roof is going to be used as a terrace or balcony it needs a protective layer over the membrane.
How do I deal with the gutter?
Fit a drip trim along the gutter edge so the membrane discharges cleanly into the gutter rather than running behind it. If the existing gutter is failing, replace it before the roof goes on — it is far easier with the deck exposed.
Can one sheet really cover a whole garage?
Yes. EPDM is available in large one-piece sheets, so a typical single or double garage is covered without any field seams. Seams are the most common failure point on any flat roof, so eliminating them entirely is the biggest single advantage of rubber over felt.
Shop related products
- Complete EPDM flat roof kit — membrane, adhesives and trims in one order
- 1.2mm EPDM membrane — cut to your garage dimensions
- 1.5mm heavy-duty EPDM membrane — for trafficked or commercial garages
- Water-based deck adhesive
- Contact adhesive
- Flashing and trims
- DIY installation tool kit