This probably falls solidly into "stupid newbie questions," but it's a stumper to me. I've been helping my mom spruce up her kitchen. The house was built in 1963, and it still has the original built-in hardwood cabinets. We recently stripped & restained the cabinet frames and put on new doors. I was going to install roll-out trays in the lower cabinets when I discovered some pretty serious water damage to the floor from an old plumbing leak, so I decided to replace it first. Ripping it out without damaging the fronts was no picnic, but I did it. (Plunge saws are your friends!) I took a piece of the old floor--far removed from the water damage--to the hardware store to get replacement plywood in the exact same size. The old floor was exactly flush with the lower front cabinet frame, but now both the new piece AND the old one sit at least an eighth of an inch higher than the frame. Is it possible that the entire front frame of the cabinet is sagging without the floor nailed into it, or is there some obscure carpentry maxim that I'm missing here? I am entirely sure that the chunk of old floor is not swollen or misshapen, because it was one of the pieces that fit perfectly flush before I yanked it out (as opposed to the water-damaged parts, which were warped).

Should I 1) get thinner plywood for the floor; 2) use a router to trim 1/8 inch away from the bottom wherever it sits on the supporting frame so it will lie flush with the front; or 3) use the thicker stuff, nail it flush to the front, then force the back down flat and hope it doesn't rip the nails out?

Thanks for suggestions. This is just seriously weird!