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Partially rotten bathroom floor joist

bobioni's Avatar

Join Date: Sep 2009
Posts: 61

02-24-10, 08:46 AM   #1  
Partially rotten bathroom floor joist

We're remodeling our main bathroom, which includes a bathtub, vanity and all wall/floor tile removal/replacement. After having removed all old floor tiles as well as the plywood upon which the tiles were laid, we discovered a partially rotten floor joist which happens to run parallel with the outer portion of the left hand tub where the tub wall intersects the wall housing all the plumbing entering the tub.

Since this is a remodel, we want to do it correctly. I went into the crawl space to observe the subfloor condition. The house was built in 1960 with the subflooring installed on the diagonal and consisting of 1 x 12s. The subflooring appears in questionable condition and sometime in the past had been replaced in a piecemealed fashion. Due to the diagonal layout, the subfloor runs into a closet adjacent to the wall housing the tub plumbing and whose subfloor condition too is partially rotten. The flooring in the closet is hardwood.

Our intention is to replace the entire subfloor and to address the partially rotted floor joist. Please suggest the best subfloor material and tile substrate for both floor as well as walls. As far as subfloor diagonal layout goes, can we change that as well? Regarding the partially rotten joist (2 x 10); rotted section is about 6" long and 2" deep, what would you recommend to do the job correctly? About the hardwood flooring masking the partially rotted subfloor, any suggestions on that?

Last edited by bobioni; 02-24-10 at 11:12 AM.
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marksr's Avatar
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Join Date: Mar 2005
Posts: 42,787

02-24-10, 11:18 AM   #2  
Having the subfloor laid on a diagonal suggests that you have a better built house. It's ok to replace a square/rectangle section with plywood. More than likely you have 3/4" planks on the subfloor, so 3/4" plywood will work fine for a patch. Don't assume you have 3/4", it's possible they used lumber straight from the sawmill that is a full 1" in thickness.

Normally you would sister a new joist along the old one. How rotten is it?

Once the joist is repaired and new plywood subfloor install, cement board [laid in thinset] is all you should need to get it ready for tile.

retired painter/contractor avid DIYer

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