how hot is too hot when welding TPO roofing?

I wouldn’t rely on those temp readouts alone, especially in direct sun. I always do a quick hand check on the seam - if it’s too hot to touch for more than a second, you’re probably running too hot. Also, I keep a spare thermometer handy just to double-check. Had a few “perfect” seams peel right up during a pull test when I trusted the machine too much... Learned that lesson the hard way.

I always do a quick hand check on the seam - if it’s too hot to touch for more than a second, you’re probably running too hot.

I get where you’re coming from, but I’ve seen some seams that felt just fine to the touch and still failed later. Maybe it’s just me, but I don’t totally trust the hand test, especially when my hands are already warm from being on the roof all day. I lean more on the pull test and visual cues - if the edge looks glossy or you see any bubbling, that’s usually a red flag for me. Direct sun really throws everything off, too... sometimes what feels “too hot” is actually just right for a solid weld.

That’s interesting - never thought about how your hands might already be too warm to notice. For someone just learning, is there a “safe” temp range you aim for, or does it really just come down to practice and watching the material? I’d hate to mess up my first try just guessing.

I totally get being nervous about messing up your first go - felt the same way when I tried patching a seam last summer. I watched a bunch of videos, but honestly, the temp settings on my heat gun were all over the place. Ended up just testing on a scrap piece first. The plastic kind of glosses up and gets flexible right before it’s ready, but if you see smoke or bubbling, that’s too hot. It took a little trial and error, but I didn’t ruin anything. Just go slow and keep an eye on how the TPO reacts.

The plastic kind of glosses up and gets flexible right before it’s ready, but if you see smoke or bubbling, that’s too hot.

This is super helpful - thanks for breaking it down like that. I was watching a couple of those YouTube tutorials too, and they all seemed to have different advice on what temp to use. Some said 500°F, others were way lower, and then I got nervous about melting a hole right through the stuff. I guess it really does come down to watching how the TPO reacts, like you mentioned.

I tried doing a tiny patch on my garage roof last weekend, and honestly, I was kind of paranoid about burning it. My heat gun doesn’t have a digital readout or anything, just a dial, so I kept fiddling with it. The first time, I definitely got it too hot - edges started curling and I smelled something weird. Backed off, let it cool, and then tried again with the gun further away. That seemed to work better, but I’m still not sure if I got it “right” or just “not wrong,” if that makes sense.

One thing I noticed - if it’s a little breezy out, the heat doesn’t seem to stay consistent at all. The patch I did on a calm day looked way smoother than the one I tried when the wind picked up. Not sure if anyone else had that issue? Also, I read somewhere that the color of the TPO can make a difference (like white vs. gray), but I haven’t really noticed much.

Anyway, I agree that testing on a scrap piece is the way to go. I wish I’d saved more leftovers from the install. I guess the main thing is not to rush it, and to be ready to back off if you see any bubbling or smell burning plastic. Still feels a bit like luck to me, but maybe that’s just because I’m new to this whole home repair thing.