By July, commercial roofs across Missouri and Illinois have already spent weeks absorbing direct sun, with surface temperatures regularly climbing past 150°F. Most conversations about summer roof stress focus on the membrane as a whole, but the area that tends to fail first on flat and low-slope roofs is more specific than that: the seams. Seams are where two pieces of roofing material meet, and they are also where a roofing system is most vulnerable to heat-driven movement and separation.
Why Seams Are the First Point of Failure
A roofing membrane is manufactured as a continuous sheet engineered to expand and contract with temperature swings. Seams, on the other hand, are joints created in the field using adhesive, seam tape, or heat welding. Every time the roof surface heats up and cools back down, the membrane pulls at these joints. Over enough cycles, that repeated movement finds the weakest connection point on the roof — and that is almost always a seam rather than the field membrane itself.
How Different Seam Types React to Extreme Heat
- Heat-welded seams (TPO and PVC): These seams are fused during installation, but extended UV and thermal exposure over the years can still cause edges to lift or the weld to weaken, especially at seam ends and detail work around penetrations.
- Adhered and taped seams (EPDM): EPDM systems often rely on seam tape or liquid adhesive. High heat can soften adhesive over time, and if the seam was not fully cured or prepped correctly during installation, summer heat is often what exposes that weakness.
- Modified bitumen laps: These seams depend on heat or adhesive-applied laps. Sustained surface heat can soften the asphalt within the lap, allowing slow slippage or separation at the edge.
Warning Signs Property Managers Can Look For
- Lifting, curling, or waving at seam edges
- Visible gaps or daylight along a seam line
- Bubbling or blistering concentrated near seams
- Interior water stains that line up with a seam pattern
- Soft, tacky, or shifted laps on modified bitumen roofs
Why Small Seam Issues Escalate Quickly in Summer
A seam that is only slightly compromised in early summer rarely stays that way. Continued heat cycling widens small gaps, and wind-driven rain or rooftop ponding can then work its way into the opening. Once water reaches the insulation layer beneath the membrane, it spreads laterally, which means the visible sign of trouble is often far smaller than the actual area affected underneath.
What You Can Do Now
A mid-summer inspection that specifically checks seam condition — not just the general membrane surface — is one of the most effective ways to catch this early. Minor seam separation is a straightforward repair. Left through several more weeks of extreme heat, it can turn into a much larger, more expensive problem.
At Jewett Roofing, our crews inspect seam integrity as a standard part of commercial roof maintenance across Missouri and Illinois. If it has been a while since your last inspection, mid-summer is a good time to have your seams checked before the next heat wave puts more stress on them.
