how hot is too hot when welding TPO roofing?

Surface temp can be deceiving, especially with different TPO colors or cloudy days.

That right there hits home. I remember a job last summer—white TPO, middle of July, and the clouds kept rolling in and out. One minute the surface felt cool enough to touch, next minute it was almost burning my palm. We had a new guy on the crew who swore by the “hand test,” but he ended up chasing his tail half the day because the temp kept shifting. The seams looked fine at first glance, but when we did our pull tests later, a couple spots just didn’t hold up.

I’m with you that gadgets aren’t perfect either. My temp gun’s been dropped more times than I care to admit, and sometimes it gives me numbers that make no sense at all. Still, it’s better than guessing, especially when you’re dealing with darker TPO or a roof that’s been baking for hours. I’ve seen folks get burned (literally and figuratively) thinking they could eyeball it every time.

But I do get the appeal of keeping things simple. There’s something to be said for experience and just knowing when things feel “off.” I guess for me, it’s about using both—trusting your gut but backing it up with a quick check from a tool if you’ve got one handy. Especially when you’re trying to keep those welds green (pun intended) and avoid call-backs down the road.

Funny thing is, even after all these years, I still get tripped up by those little microclimates on a big roof. One corner’s in full sun, another’s shaded by an HVAC unit... It’s never as straightforward as you’d hope.

Anyway, just wanted to say I hear both sides here. Sometimes simple works, sometimes you need a little tech backup. Either way, nothing beats taking your time and double-checking before you move on to the next seam.

Funny thing is, even after all these years, I still get tripped up by those little microclimates on a big roof.

Ain’t that the truth. I’ve had welds go from perfect to “what the heck happened here?” just by moving ten feet over. I’ll admit, I used to think my hand was a pretty good thermometer until I roasted myself on a black TPO job. Now I keep a temp gun handy, even if it’s seen better days. For me, if the surface is pushing 150°F or more, I start dialing back the heat and slow down—otherwise you’re just cooking the membrane and asking for trouble later. But yeah, nothing beats a good pull test before you call it done.

I hear you on the temp gun—mine’s got duct tape holding the battery door shut, but it still does the trick. I’ve definitely fried a few seams thinking “eh, it’s probably fine,” only to find out later it wasn’t. Honestly, I get nervous when the roof feels like it could cook an egg. I know some folks say 150°F is the magic number, but I’ve had issues even a bit lower if there’s no breeze and the sun’s just beating down. Pull tests are a lifesaver, though... nothing like that little moment of truth to keep you humble.

- Totally get the “eh, it’s probably fine” feeling—been there, regretted that.
-

“I know some folks say 150°F is the magic number, but I’ve had issues even a bit lower if there’s no breeze and the sun’s just beating down.”

I’ve definitely run into the “sun’s out, seams toast” problem—especially on my garage roof, which is dark gray TPO. I used to think as long as I wasn’t hitting 150°F, I was good, but after a couple of ugly seams last summer (one peeled up after just a week), I started checking with an IR thermometer more often. On those still, sunny days, it’s wild how fast the surface temp jumps. Even when the air’s only in the 80s, that membrane can get way hotter than you’d expect.

I’m with you on the scrap weld test—saved me from redoing a whole section more than once. Pull tests are great too, but sometimes I wonder if there’s a better way to judge when it’s just too hot to bother trying. Like, is there a point where you just call it and wait for some cloud cover or a breeze? Or do most folks just adjust their welder settings and hope for the best?

Also curious if anyone’s tried putting up some kind of shade while working. I thought about rigging up a tarp to block direct sun on smaller jobs, but not sure if that would actually help or just make things awkward.

For what it’s worth, I’ve found lighter TPO doesn’t get quite as crazy-hot as the darker stuff, but still not foolproof. Anyone else notice certain brands or colors holding up better in the heat? Just trying to avoid wasting material (and money) on seams that’ll fail before winter even hits...