EPDM Rubber Roofing Explained: Thickness, Attachment, Seams and Where It Fits
One sheet of synthetic rubber, three ways to hold it down, and a handful of details that decide whether it lasts.
- Advanced
- 1 to 2 days for a small flat roof
- Updated
The short answer
EPDM is a single-ply synthetic rubber roofing membrane, usually 45 or 60 mil thick, sold in wide rolls so a small roof can have no seams in the field. It is fully adhered with bonding adhesive, mechanically attached with bars and fasteners, or held down with ballast. Seams are joined with 3 inch splice tape. It commonly lasts 20 to 30 years when flashed well.
EPDM stands for ethylene propylene diene monomer, a synthetic rubber also used in car door seals and pond liners. As a roofing membrane it is a flexible black or white sheet that stays elastic across a wide temperature range, resists sunlight, ozone and weathering, and stretches with building movement. It is sold in rolls up to 20 feet wide or wider, so a small roof can often be covered by a single sheet with no field seams.
It is popular on garages, additions, carports, low slope house roofs and mobile homes because it is lighter than many other systems and installed without flame. The membrane itself rarely fails. The trouble comes at the details: seams, corners, pipes and edges, where the rubber has to be cut, folded and taped by hand. A good EPDM job is mostly about those details. It is not a walking surface: a roof deck people use needs a protected membrane under pavers or a different system altogether.
This guide explains the parts of the system so you can read a quote or judge a repair. The steps are in how to prep a roof deck for EPDM and how to install a fully adhered EPDM roof. If you are comparing systems, see how to choose a flat roof membrane and how torch-on roofing works.
Tools
- Utility knife with hook blade
- Paint roller or notched squeegee for adhesive
- Stiff push broom
- Hand seam roller
- Tape measure and chalk line
- Scissors for flashing cuts
Materials
- EPDM membrane, 45 or 60 mil, in the widest width that fits the roof
- Bonding adhesive (water-based or solvent-based)
- Seam tape and primer
- Uncured or cured EPDM flashing
- Termination bar and sealant
- Pre-molded pipe boots
- Cover board or insulation as required
Safety first
- Solvent-based bonding adhesives and primers are flammable and give off fumes. Keep them away from flames, sparks and pilot lights, work with ventilation, and wear gloves and eye protection.
- EPDM is slippery when wet, dusty or frosty. Wear clean, soft-soled footwear and keep the membrane free of debris.
- Large sheets catch wind. Do not unroll a full width sheet in a strong breeze, and keep clear of roof edges, since WorkSafeBC fall protection applies at 3 metres (about 10 feet) and wherever a fall could cause serious injury.
Step by Step
Step 1: Choose the thickness
EPDM is sold by thickness in mils, where one mil is a thousandth of an inch. 45 mil is common on residential flat roofs and is the usual minimum. 60 mil is the most common commercial and heavier residential choice, and gives better puncture resistance. 90 mil is used where there is regular foot traffic, equipment or debris.
Thicker costs more per square foot and is stiffer to handle in the cold. Choose based on traffic, exposure and budget, and confirm the thickness the manufacturer approves for the system and the warranty you want.
Step 2: Choose the attachment method
A fully adhered system glues the sheet to the deck or insulation with bonding adhesive, which resists wind uplift and suits small residential roofs. A mechanically attached system fastens the sheet through bars or plates, which is used on larger roofs where gluing is slow. A ballasted system lays the sheet loose and holds it down with washed stone or pavers, which needs a structure that can carry the weight, commonly about 10 pounds per square foot of stone.
Most house roofs in the Fraser Valley are fully adhered. See how to install a fully adhered EPDM roof.
Step 3: Prepare the deck
The deck must be smooth, dry, sound and free of oil, dust and sharp edges. Rough boards and protruding nails show through the rubber and puncture it. Insulation and cover board add thermal performance and a smooth surface.
See how to prep a roof deck for EPDM for the detailed preparation.
Step 4: Lay out and install the sheet
The sheet is unrolled and left to relax flat, because rubber rolled tight for storage has a memory. It is then folded back in halves and adhered in sections, or fastened, or ballasted. Wrinkles and tension are worked out as it goes.
Seams between sheets are joined with splice tape and primer, typically over a 3 inch overlap. See how to seam EPDM with seam tape.
Step 5: Flash corners, pipes and edges
These are the details that decide whether an EPDM roof lasts. Corners use pre-formed pieces or folded uncured flashing, pipes use pre-molded boots or field-fabricated wraps, and edges use metal drip edges, termination bars and counter flashing. See corners, pipes and edges and walls.
Step 6: Inspect and plan maintenance
Check every seam edge and flashing termination for lifting, and look for ponding. Keep the roof clean, and avoid contact with oils, solvents, fats and asphalt products, which damage EPDM. A periodic look after storms and in autumn catches small problems while they are easy to fix. See how to repair an EPDM roof.
EPDM Compared With Torch-On and TPO
- EPDM: no flame, flexible in the cold, few seams on a small roof, weak points at seams and flashings, tape and adhesive are temperature limited.
- Torch-on: thicker layered membrane, tough underfoot, easy to patch, but needs trained crews and fire precautions.
- TPO and PVC: heat-welded seams that fuse the sheet, white reflective surface, but needs welding equipment and trained installers.
Slope and Drainage
EPDM is made for low-slope roofs, but it is not a pond liner. Water that sits on a roof for days loads the structure, keeps seams wet and holds dirt that breaks down the surface. Plan for positive drainage, commonly at least 1/4 inch per foot, so that water is gone within about 48 hours after rain, which is the usual test for ponding. Every inch of standing water weighs about 5.2 pounds per square foot.
Where an old flat roof sags or drains the wrong way, fix the slope before the rubber goes on, with tapered insulation, crickets between drains or reframing. See how to plan flat roof drainage and how to install tapered insulation. A new membrane on a roof that ponds carries the old roof's problem into the new one.
Reading an EPDM Quote
Two EPDM quotes for the same roof can describe very different jobs. These are the lines worth comparing.
- Thickness: 45, 60 or 90 mil, and whether the sheet is reinforced. Reinforced sheet is used for mechanically attached systems.
- Attachment: fully adhered, mechanically attached or ballasted, and what the sheet is attached to.
- What is under it: whether wet or rotten decking and insulation are being replaced, and whether there is a cover board over the insulation.
- Seams: splice tape with primer is the current standard for field seams, and the lap width should be stated.
- Flashings: pre-molded corners and pipe boots, or field-formed uncured flashing, and how walls and curbs are terminated.
- Edges and drains: new drip edge or edge metal, termination bars at walls, and new drain clamping rings or scupper flashing.
- Colour: black absorbs heat and is the common choice, while white EPDM reflects sunlight and runs cooler in summer.
What Damages EPDM
Petroleum products, cooking grease, some solvents and contact with asphalt-based materials attack EPDM. Sharp objects puncture it, and standing water shortens the life of seams. Do not set hot pipes, asphalt or roof cement against it unless the manufacturer says it is compatible.
Fraser Valley Notes
Adhesives and primers need dry surfaces and mild temperatures, and the wet season gives fewer good days. Moss and needle debris collect on flat roofs under trees and hold water against seams, so keep drains and surfaces clear. EPDM stays flexible through freeze-thaw, which is one reason it does well on the coast.
Common Mistakes
- Choosing 45 mil for a roof that gets foot traffic. It punctures, and patches are a constant chore.
- Skipping the deck prep. Rough boards, nails and dirt show through and puncture the sheet.
- Gluing in cold or wet weather. Adhesive does not cure properly and the sheet lifts.
- Leaving slack and wrinkles at corners. They become weak points or get pulled tight when the sheet shrinks.
- Contact with asphalt or oils. Petroleum attacks the rubber and softens it.
- Skimping on flashing details. Most EPDM leaks are at corners, pipes and edges, not in the field.
- Putting new rubber on a roof that ponds. The water that sat on the old roof sits on the new one, and seams under standing water fail first.
When to Call a Professional
EPDM is within the reach of a capable builder on a small, simple roof, but the flashing work and the adhesive limits are where most installations fail. If the roof has many penetrations, a complex shape, wet insulation or a structure that needs ponding and slope correction, call a roofer.
If you are replacing an old roof, ask what is under the membrane, whether there is a cover board, and how the corners and edges will be flashed.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Flat Roofing and Torch On service or the towns we cover.
EPDM Rubber Roofing Explained: Questions
What is EPDM roofing made of?
EPDM roofing is made from ethylene propylene diene monomer, a synthetic rubber. It is made into flexible sheets, usually 45 or 60 mil thick and black or white, and sold in rolls up to 20 feet wide or more. It resists sunlight, ozone and weathering and stays flexible in cold weather, which is why it is widely used on flat and low slope roofs.
How thick should an EPDM roof be?
The two most common thicknesses are 45 mil and 60 mil. 45 mil is a common minimum for residential roofs, 60 mil gives better puncture resistance and is the usual commercial choice, and 90 mil is used where there is foot traffic or debris. Check the manufacturer's requirements for the system you are installing.
What is the difference between fully adhered and ballasted EPDM?
Fully adhered EPDM is glued to the deck or insulation with bonding adhesive, so it is light and resists wind uplift. Ballasted EPDM is laid loose and held down by washed stone or pavers, which adds a lot of weight, commonly about 10 pounds per square foot. Mechanically attached EPDM is fastened through bars or plates.
How long does an EPDM roof last?
A well installed EPDM roof commonly lasts 20 to 30 years. Most early failures are at seams, corners, pipes and edges rather than in the sheet itself. Ponding water, foot traffic, oils and poor drainage shorten the life, and regular inspection and small repairs extend it.
Can EPDM be installed in winter?
It can, within limits. Adhesives, primers and tapes have minimum temperatures, commonly about 5 degrees Celsius for bonding adhesive, and some products work lower with special primers. The surface must be completely dry and free of frost. In the Fraser Valley wet season, dry days above the limit are scarce, so planning around the weather matters.
Related Guides
All EPDM Rubber Roofing guides|The full How-To Library
This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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