Designing Kitchens and Bathrooms - Found mold under vanity, need help on how far to go...

I am in the middle of doing a minor remodel of my master bath, but it is heading for major. I pulled out the old vanity last night and found there was a small amount of mold underneath (about a 1' x 2' area). The people before us had placed 1/4" plywood over the old sub floor, which doesn't look to be in the best shape, then placed adhesive backed 12" vinyl tile directly to the new plywood. They didn't place the tiles under the vanity, and it seems the mold is only on the new sub floor and not the old. The caulk on the back splash of the sink was cracked, and if I had to guess I would say water ran down the back and got the sub floor wet, which instigated the mold. There doesn't seem to be a leak in the plumbing or trap that I can see.

So here is my question: how should I proceed? Should I get a mold test to make sure their isn't mold under the old floor? Or is a mold test a waste of money? Should I rip out all of the old sub floor (and new) and replace everything (this would require taking out the tub and everything)? Or should I just take out new sub floor, replace it with backer board and then lay tile? I really don't want to have to take out the tub if I don't have to, but I wanted to get some peoples opinions first.

Thanks for any and all help with this. I'm new to home remodeling.

Brian

Welcome to the forums Brian!

If it is only a small amount of mold on the surface of the raw plywood - I wouldn't get overly concerned. You should be able to remove it with a bleach/water solution. Painting or otherwise covering the raw plywood may help in detering mold growth although taking away the moisture may be enough.

If you feel you need to replace the subfloor, you may be able to replace all of it except what is under the tub [a lot depends on joist placement] it isn't uncommon to do this in a remodel if the tub is left inplace.

As always if you or anyone in your house has asthma or other breathing problems - extra precautions need to be taken.

Thank you for the help, I appreciate it.

Over the weekend I decided to explore this subfloor a little further just to make sure, and you were right. There was just surface mold because it was not covered properly. Anyway, I'm glad I investigated because what I found was even more interesting.

The people who lived in the house before at one point in time had a leak in the toilet because they caulked around the bottom of the base and the water soaked into the plywood. Everything is dry now of course, but instead of taking out the damaged sub floor and replacing it they covered it with new 3/4 ply. And then another layer of 1/4 ply over that. Needless to say I had a pretty excessive amount of ply under my flooring :)

So I took out all of the old ply including the damaged original and then I'm putting down the hardibacker this week, to get ready for tiling.

You "Are" putting down new plywood prior to the hardiboard, right? That stuff is not structural....

yes, I have put in a new sub floor first. all of the old stuff was separating because of the old moisture damage.

The Hardiebacker MUST be installed in a bed of fresh thinset. Your new tile installation will be doomed otherwise.:)

Thanks Bud. Yes, I was planning on doing the thinset under the hardibacker and using the special coated square drive screws for it. Is 1/4" notch ok for the thinset?