Originally Posted by JohnZ3MC
I think you're doomed Bunky.
Those tarps are usually woven polyethylene and I don't think there's any adhesive that's going to work like you want it to with poly.
Poly does melt though, so you might be able to weld them together.
A plastic welder (expensive),
A soldering iron/gun with a rheostat,
A hot glue gun,
could be used to weld them together with your lap joint.
Too little heat and you wouldn't get the weld. Too much heat and you'd burn holes into the poly. Hence the rheostat to control the heat.
Then there's the practice time to get the temp and technique right.
Economy and time wise, I'd go and buy a larger tarp, and I have a plastic welder.
I think you are right. I priced out some larger tarps and actually quite cheap -less than $50. I figure I woudl spend $10 to $15 in adhesive.