Will roofing tar used as undercoating start on fire?

Hello,

Im replacing the floorboards of the 93 XJ.

I used roofing tar to seal the joint of the various overlaps.
Then painted the inside with rustoleum paint.

I am thinking of water proofing the underside with roofing tar then 3m undercoat.

I had read that undercoat is a soundproofer not rust preventer

My question is, will the roofing tar start on fire if its over the muffler?

I had it happen years ago when I was 16. I patched a hole in my trunk on my first car. Couple of week latter I was driving and thought is smelled cherry pie baking, next thing I knew the car was full of smoke, my roof tar undercoat was on fire.

Thanks,

I thought that made sense.

What alternatives would you use?

POR-15