How do I approach replacing subfloor in a small space (23 sq. ft.)?

Question

I’ve undertaken redoing my laundry room in a house I bought as-is. Yesterday, the plumber came to remove the washer and dryer, which were sitting on a loose piece of plywood on top of the floor- suspicious. Sure enough, underneath where the washer sat, the subfloor had completely rotted out under the peel and stick flooring. Per plumber, there are no current leakage issues with my washer, so this was obviously an old issue and shouldn’t recur.

I can’t afford to hire someone, so I need to DIY this. The joists are in good condition, thankfully, no signs of rotting or moisture damage or mold. I’ve never done subflooring before, but I’ve researched for several hours over the last two days. My issue is, the sites I’ve read have all been discussing whole-house installation or replacement, and my laundry room is only about 23 sq. ft. (really it’s more of a closet). A joist sits almost directly center in the room. The joists are sistered (two planks thick) and solid wood (not I-framed). The house has a crawl space and a wood foundation. This is a midwestern house built in 1959 if that matters.

Because the dimensions of the room are 52 3/8” x 63 1/2”, a single piece of 4x8 plywood obviously won’t work. What is my best approach structurally? Should I do two pieces that both attach to the center joist and then the outer joists near the walls? Is there a better approach?

I’m doing porcelain tile over a DITRA uncoupling membrane, so my plan was to do a lower-grade 3/4” plywood for the subfloor and attach the membrane directly to that. My dad suggested doing 1/2” plywood, a moisture barrier, then another layer of plywood before the membrane, but the floor would then be higher than the hallway floor, and I’m pretty sure the DITRA membrane takes care of the waterproofing. Am I wrong?

It’s below freezing here, so I’m aiming for a quick turnaround on cutting out the old and installing the new. Any advice is greatly appreciated.

Answer 1

I'd give preference toward choosing a subfloor material that matches (in thickness) the subfloor that's used in the adjacent space. I couldn't guess what kind yours used, but for reference, today OSB "wafer board" is common for subfloor in USA. It is nominally called 3/4" but is actually 23/32" thick. I'd suggest getting two sheets of the tongue and groove type and orienting them so that the tongue and groove run perpendicularly to the floor joists. The tongue and groove is nice because it minimizes/prevents flex at the joints between sheets.

Support the edges of the new subfloor material -- a question asked just yesterday may give you ideas how to implement that.