T&G 2x6 subfloor rot repair, subfloor then tile?

[wjw1961]

Greetings!

My wife and I gleaned LOTS of good information from this forum when we redid our master bathroom last year (gutted to studs and slab, removed moldy insulation, installed new, added a vapor barrier in walls (duh!), 1/2 inch CBU, built custom shower pan, tiled shower surround & wainscoting, etc., etc.). This years project, the hall bath, SEEMED like it ought to be easier, but I still have a few questions.

Our house is a 1965 ranch, build with piers and beams. (The master bath was in a mid-70s addition, thus the slab.) We have tongue and groove 2x6s running north-south across the east-west beams. I have not actually been down in the crawlspace to measure the beams. To make things more fun down in the crawlspace, we have R25 (?) insulation under the subfloor.

We pulled up the sheet vinyl flooring in the bathroom, found some water damage in particleboard atop the 2x6s had been repaired with plywood. It looks like the damage was just due to water spillage from the alcove tub, but after 40 years, enough had gotten through to the 2x6s to cause some rot. We pulled up all of the particleboard and the plywood patch, so are now down to the bare 2x6s. A couple have some rot, in one down almost through the 2x6 (that deep in a finger-wide place along a joint, in a spot of rot that's overall about as big as my hand) and in another spot, just down to the tongue & groove. I thought "well, that's no good!" and attached the rot with a scraper (easy) and chisel (easier). Perhaps instead I should have just tried to stabilize the rotted wood with some sort of epoxy resin? Regardless, the bad spots are gone now, but I have a few questions:

1) Do I need to treat the rot with something? The local lumberyard has only "exterior use ONLY" stuff for decks and fenceposts. (Copper ____?) In another thread, someone mentioned a boric acid treatment.

2) What do I use to fill the holes? On the earlier repair job, they seem to have used plaster to fill the (then smaller) holes. We used "Megalite crack prevention mortar" in the master bath to hold the 13x13 tiles to the backerboard, and I thought that might work. As the holes are near the walls (tub overflow) and the 2x6 T&G runs under the walls, it'd be no fun at all to try to remove and replace any of the 2x6s. It'd also be tough to run a router around to square off the holes and fit in 1-bys. Thoughts?

3) It sounds like I'll actually need to go down into the crawlspace to measure the beams and their spacing, and use Deflecto to see if I need to try to stiffen up the floor. The measurement won't be fun, but it'd be a delight compared to adding stiffening, particularly with the presence of insulation.

4) It also sounds like I may need to put down plywood on top of the 2x6s before adding backerboard. Correct? If so, do you recommend a thickness? 3/4, or can I get away with less? I do NOT want to have to redo this, so am willing to have a little extra thickness if need be. I'd rather keep the hight down, though, because the hallway has 1 inch T&G hardwood flooring, and a big transition would be less than ideal. Guessing here, but it seems like I'll need the plywood to make a flat enough floor for the backerboard.

5) Water barrier? In the master bath, we used 4 mil (or 6?) plastic sheeting behind the backerboard in the shower walls, and thick (50 mil?) PVC when building up the shower pan. For the floor of this bathroom, I worry that plastic sheeting would either be slippery (kind of eliminates the idea of getting any bonding from thinset under the backerboard) or insufficient (if pierced with backerboard screws). Perhaps a painted on water barrier would be better in this case?

6) Reading some other threads, it sounds like the backerboard doesn't actually add strength, so could I back down to using the 1/4 inch stuff on the floor? This has appeal if I need to add plywood.

7) Can I, should I, or should I not run the backerboard under the tub? The advantage is that we could put down all the floor materials before we have the tub in hand, so would not need to worry about cutting to match the edge of the tub apron. It's only a standard five foot width tub, so the extra cost would only be a sheet of backerboard, plus (likely) some additional plywood.

8) We have a 5 foot, 32 inch whirlpool tub on order. The rough-in and installation materials found on the manufacturer web site make all seem fairly clear. We're thinking of putting plastic sheeting on the walls behind the backerboard, and are tempted to have it hang over the lip of the tub (where it will get covered by tile). The installation notes down show this water barrier at all (!), let alone this detail. Any thoughts? Any other warnings about a whirlpool tub?

9) Do I need to build up the toilet drain pipe to handle the additional floor height, or will a tall wax ring suffice? I do NOT want to wrestle with the cast iron drain pipe if I can possibly avoid it.

Thanks in advance. This forum is a GREAT resource!

William

P. S. I'm a DIY homeowner, not a pro. (As if you couldn't tell!) This is my first post to the forum.

P. P. S. FYI/FYA, the old tub surround had ugly formica poorly glued to sheetrock, on top of another layer of sheetrock. We found remnants of the pink flowered wallpaper behind the vanity when we pulled it.

[John Bridge]

Welcome, William.

There is another thread going on concerning the West Coast post and beam framing method. Try the search.

Don't worry about patching anything on the subfloor. The little spots you mention won't cause any problems. Also, don't worry about treating dry rot. Once the source of moisture is eliminated, the rot stops. Forget it.

Put down a minimum of 3/8 ply over the subfloor. A good membrane is then recommended, either Schluter "Ditra" or Noble Company "Nobleseal TS." Finally, the tile bonded to the membrane with thinset mortar.