I'm installing sheet vinyl flooring in the bathroom. Home Depot told me to get this hard plywood, just under 1/4" thick for underlayment. I'm concerned though because my sub floor is 10" or 12" wide boards with a good amount of gaps between them. I was thinking of throwing another sheet of 3/8" OSB on top of the subfloor, then the 1/4" underlayment. Is that a good idea, or bad idea? Really needed? I'm wondering if there will be a problem with the nails going through multiple layers of plywood?