10.1 Single-Ply Membranes: EPDM, TPO & PVC
Key Takeaways
Thermoset (EPDM) and thermoplastic (TPO, PVC) single-ply roofs need a design slope of at least 1/4:12 (2%) for drainage.
EPDM is a thermoset rubber (ASTM D4637), joined with primer and seam tape or adhesive, and it cannot be heat-welded.
TPO (ASTM D6878) and PVC (ASTM D4434) are thermoplastics whose seams are fused by hot-air welding, and welded seams should be probed after cooling.
Single-ply membranes are ballasted, mechanically attached, or fully adhered, and the choice depends on the deck, wind exposure, weight limits, and the manufacturer's system approval.
Above-deck insulation under a membrane must meet FM 4450 or UL 1256 and be covered by an approved roof covering (R906).
Code Basics
| System | 2015 MRC section | Minimum design slope | Material standards |
|---|---|---|---|
| Thermoset single-ply (EPDM) | R905.12 | 1/4:12 (2%) | ASTM D4637, ASTM D5019, or CGSB 37-GP-52M |
| Thermoplastic single-ply (TPO, PVC, KEE) | R905.13 | 1/4:12 (2%) | ASTM D4434 (PVC), D6754 (ketone ethylene ester), D6878 (TPO), or CAN/CGSB 37.54 |
Both are installed "in accordance with this chapter and the manufacturer's instructions." Because the roof covering sections list no additional details, the membrane manufacturer's system instructions are the working specification. They cover deck and insulation requirements, attachment, seams, flashing heights, and edge details. The PSI roofing references include the NRCA Roofing Manual: Membrane Roof Systems.
Comparing the Membranes
| Property | EPDM | TPO | PVC |
|---|---|---|---|
| Polymer type | Thermoset (vulcanized rubber) | Thermoplastic polyolefin | Thermoplastic vinyl, with plasticizers |
| Seam method | Primer plus seam tape, or seam adhesive. Cannot be heat-welded | Hot-air welded | Hot-air welded |
| Typical colors | Black, also white | White, gray, tan | White and colors |
| Strengths | Flexible in cold, long record, easy repairs | Reflective, strong welded seams | Welded seams, good resistance to many chemicals and greases |
| Watch-outs | Petroleum products and some oils can swell it. Seams depend on clean, primed surfaces | Welding window depends on the product and weather. Needs clean, compatible surfaces | Plasticized PVC is incompatible with asphalt and some polystyrene insulations unless separated, per the manufacturer |
Attachment Methods
| Method | How it works | Notes |
|---|---|---|
| Ballasted | Loose-laid membrane held down by stone or pavers | Heavy. The structure must carry the load, and wind exposure limits its use. Rare on homes |
| Mechanically attached | Fasteners and plates at seams or through the membrane into the deck, covered by the overlapping sheet | Light. The membrane can billow in wind, so fastener rows follow the manufacturer's wind design |
| Fully adhered | Bonding adhesive over an approved substrate or cover board. Self-adhered membranes are also available | Best appearance and wind resistance. Adhesives have temperature and flash-off limits |
On a small residential porch roof, fully adhered EPDM or TPO over a clean, dry deck or insulation board is common. Follow the manufacturer's approved assembly for the deck type and wind zone.
Seams
EPDM
- Position sheets with the manufacturer's minimum lap.
- Clean and prime both surfaces with the specified primer, and let it flash off.
- Apply seam tape (or seam adhesive) and roll the seam firmly with a hand roller toward the edge.
- Seal the laps at T-joints and cut edges with lap sealant, and install T-joint covers where the manufacturer requires.
TPO and PVC
- Set the hot-air welder's temperature and speed for the day's conditions, and make test welds first. Peel them to confirm the membrane tears before the weld does.
- Weld the laps with an automatic welder or a hand welder and roller.
- After the seams cool, probe every seam with a blunt probe to find voids. Repair voids by rewelding or with a patch.
- Keep seams clean. Dirt, moisture, and old coatings cause weak welds. Use the manufacturer's cleaner on aged membrane before welding.
Flashings and Terminations
- Base flashings at walls and curbs extend up the wall and terminate under counterflashing or a termination bar sealed per the manufacturer. NRCA commonly recommends base flashing at least 8 inches above the finished roof surface. Follow the manufacturer's minimum.
- Penetrations: use prefabricated pipe boots where possible. For irregular shapes, use field-fabricated flashing or pitch pans filled with the manufacturer's sealant, and inspect them often.
- Edges: metal drip edge or gravel stop with the membrane stripped in. Where low-slope roofing meets a shingled slope, lap the membrane up under the shingle underlayment and ice barrier so water flows over the joint, not into it.
- Drains and scuppers: clamp or weld the membrane into drain bowls and scupper sleeves per the manufacturer (Section 10.3).
Insulation and Cover Boards
- R906.1: above-deck thermal insulation is allowed if it is covered by an approved roof covering and complies with FM 4450 or UL 1256.
- Board standards are in Table R906.2: polyiso (ASTM C1289), EPS and XPS (C578), perlite, wood fiberboard, and gypsum boards.
- Tapered insulation creates slope to drains (Section 10.3).
- Cover boards protect insulation from foot traffic and provide a smooth, compatible surface for adhered membranes.
- Never install over wet insulation or a wet deck. Replace wet insulation found during tear-off.
Weather and Temperature
Adhesives, primers, and tapes have minimum temperature and humidity limits, and cold slows flash-off and cure. Welding changes with ambient temperature and wind. Store adhesives warm, follow the instructions, and do not seam over frost or dew.
Maintenance and Repair Basics
- EPDM repairs: clean and prime around the damage, then apply a cured or uncured EPDM patch with seam tape. Round the corners and roll the patch firmly.
- TPO and PVC repairs: clean with the manufacturer's cleaner, then weld a patch of the same material. Aged membrane may need extra preparation.
- Keep walkway pads at service routes, such as around HVAC units, and clear debris from drains regularly.
A roofer plans to seam a new EPDM membrane with a hot-air welder. Why won't that work?
EPDM is a thermoset rubber that cannot be heat-welded, so its seams use primer with seam tape or seam adhesive
EPDM seams must be torched rather than hot-air welded
EPDM is thermoplastic and must be welded with solvent
EPDM seams must be sewn and then coated
What minimum design slope does the 2015 MRC require for a new TPO membrane roof on a residential addition?
1/4 unit vertical in 12 horizontal (2%)
1/8:12
1:12
2:12
After hot-air welding TPO seams, what should the installer do?
Coat each seam with asphalt roof cement
Apply seam tape over the welds
Nothing, because welded seams are always watertight
Let the seams cool and probe them with a blunt probe to find and repair voids
Sections you finish are checked off in the contents.