Illinois Roofing License IDFPR #19HC00108700 · GAF Certified Contractor
Commercial buildings – and many residential additions, garage roofs, and low-slope porch roofs – use single-ply membrane systems instead of asphalt shingles. The two dominant products are TPO (thermoplastic polyolefin) and EPDM (ethylene propylene diene monomer, a synthetic rubber). This guide compares them, explains when each makes sense, and covers what’s involved in installation and warranty.
Why single-ply membrane on low-slope roofs
Asphalt shingles require minimum 2/12 pitch to shed water effectively. Below that, water can pool and find its way under the shingles. For roofs in the 1/4:12 to 2:12 range (most commercial buildings, many residential additions and porches), shingles aren’t an option.
Single-ply membrane systems use a continuous sheet of synthetic material covering the entire roof. The seams are heat-welded (TPO) or seam-taped (EPDM) into watertight joints. Because there are no shingles to lift in wind or accumulate water in low spots, single-ply works on essentially flat roofs.
The two products dominate this market: TPO and EPDM.
TPO (thermoplastic polyolefin)
TPO is a white reflective single-ply membrane introduced commercially in the early 1990s. It’s now the dominant choice for new commercial flat roofs in the US.
Specs:
- Color: typically white (reflective); gray and tan also available
- Thickness: 45, 60, or 80 mil – thicker grades have longer life and warranty
- Lifespan: 20-30 years
- Cost: $5-$10 per sq ft installed
- Installation: heat-welded seams (hot air gun creates a continuous bond)
- Reflectance: high (white surface reduces summer cooling load – meaningful for commercial cooling costs)
Pros:
- Reflective surface reduces cooling costs
- White appearance can qualify for ENERGY STAR roofing
- Heat-welded seams are very strong and watertight
- Good chemical resistance
Cons:
- White surface shows dirt easily
- Some debate about long-term UV degradation (newer formulations are improved)
- Specialized installation equipment (heat gun) and training required
EPDM (synthetic rubber)
EPDM is a black synthetic rubber membrane that’s been on commercial roofs since the 1970s. It’s older technology than TPO but has a long track record.
Specs:
- Color: typically black (occasionally white-coated)
- Thickness: 45, 60, or 90 mil
- Lifespan: 20-40 years (proven in field installs from the 1980s still performing today)
- Cost: $4-$8 per sq ft installed
- Installation: seam-taped or seam-adhesive (no heat welding)
- Reflectance: low (black surface absorbs heat)
Pros:
- Long field-proven track record
- Easy to repair (patch with EPDM and seam tape)
- Slightly lower cost than TPO
- Flexible across temperature ranges (handles IL freeze-thaw well)
Cons:
- Black surface increases summer cooling load
- Seam adhesion can fail over time if installation wasn’t perfect (especially older installs)
- Doesn’t qualify for ENERGY STAR reflective roof credit
TPO vs EPDM – which to choose
For new commercial flat roof installations in Illinois, TPO is usually the right choice:
- Heat-welded seams are more reliable long-term than EPDM seam tape
- White reflective surface reduces cooling costs (meaningful on commercial buildings)
- Energy code credit and potential rebates for reflective roofing
- Cleaner appearance for buildings where the roof is visible
EPDM still makes sense for:
- Replacing an existing EPDM roof (compatibility, easier transition)
- Buildings with very specific chemical exposure concerns (some chemicals attack TPO)
- Budget-constrained projects where small cost savings matter
- Lower-profile installs where aesthetics aren’t a concern
Residential applications
Most Trill Roofing work is residential asphalt shingle, but TPO and EPDM come up in specific residential scenarios:
- Low-slope porch roofs – when a porch addition has slope below 2/12, single-ply is the correct material
- Garage roofs with very low slope
- Flat-roofed additions – common on contemporary architecture
- Bay window or dormer flat tops – where the structure has a small flat section
For these applications we use EPDM more often than TPO – the small scale makes the heat-welding equipment investment less worthwhile, and EPDM’s seam tape works fine on small areas.
Installation considerations
Both TPO and EPDM require:
- Clean, sound roof deck (replace any soft or rotted sheathing first)
- Underlayment or insulation board (varies by spec)
- Proper edge metal (drip edge equivalent for low-slope roofs)
- Penetration flashing (pipes, vents, drains) – typically pre-fabricated boots
- Roof drains or scuppers for genuinely flat roofs (water has to go somewhere)
Most failures we see on existing low-slope roofs come from inadequate drainage (water pools and finds seam weaknesses), failed penetration flashing, or installation in cold weather (TPO seams can fail to weld properly below 40°F).
Schedule an evaluation
For commercial buildings or low-slope residential sections, schedule a Trill Roofing free inspection at /free-inspection/. We assess the existing roof condition, identify drainage issues, and recommend the appropriate single-ply system for the application.
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