Going to install vinyl floor tile in an old house. There was sheet material in place, and the edges were stuck down with something asphaltic that reminds me of the stuff they used under floor tile years ago. It won't come up without tearing the plywood. Is there a product that will soften this material that can be used indoors (non-flammable)? I'm thinking I'll have to just add another layer of luan to cover it up - I don't think the tile will stick, and it's too rough to leave anyhow. There's only the one stripe across the entire floor - the rest of the floor is clean. Thanks for ANY advice.