why on earth would you want to use roof tar - just because its cheap? Bad idea....

if your going to put an actual coating on on the floor pans use a spray on bedliner & be done with it. You've got it that far torn down so mask off everything in the car you dont want it on & spray it - the spray guns are pretty cheap, the materials not terribly expensive & it cures to a hardened state - roof tar will NEVER do that. Do you want gooey, warmed tar under your feet & carpet - not trying to be critical here but personally I think its a horrible idea but hey - its your car.

Lizard Skin, SPI bedliner, google search it..... a lot of it I think you can roll apply as well so no spraying - dont use tar!