How to tell if subflooring is bad? Or, how to add support....

[breathebeast]

Took the plunge, made the cut in the hardwood floor for the expansion of our level hearth - and it looks great! Used a 7 1/4" round saw blade with 60 teeth and it made a really clean cut with no need for scoring the floor beforehand. But I'm noticing that when my husband pries up the boards, the underlying plywood is flexing where the prybar is grounded. Is that normal for subfloor? Wondering if I should pop a nail in there and see how far it protrudes on the other side to see if we have decking + subfloor....oh, yeah, just went and did that, and we're lucky if we've got a full 3/4" decking w/o subfloor (or would it be the other way around).

So, the question now is, do we need to put another layer of plywood under the 1/2" micore and 1/2" cement board and 1/4" tile? We WILL be walking on this, though it's not a high traffic part of the room except for the 40 pound dogs. It flexes under the force of the prybar's job, MAYBE a little under my weight (more than the hardwood floor does, I'd say, but pretty subtle even with bouncing up and down).

It'll be tough to edge this out if we do. Already the micore put us 1/8" above the hardwood floor, 3/4" plywood would put the tiles an inch above the floor - too high for beveled wood trim? Can we add support underneath by cutting 2x8's and nailing between the joists? If so, how close would they need to be to each other?

 

[elmoleaf]

It is normal to have some deflection with just one layer of plywood....assuming your floor joists are 16" on center. If you're concerned it won't be stiff enough for tile, you can always add some blocking below between the existing floor joists. Blocking would simply be some 2x6 or 2x8 wood framing cut to fit the joist bay, hold tight to underside of plywood above, nail through floor joists into ends of blocking.

 

[Dakotas Dad]

It is normal to be able to flex the subfloor between joists with a crowbar. The reason you put down cement backerboard before tile is that the subfloor can flex, breaking both the grout and the tiles.

IF and I say IF you are really worried about adding another layer, do not add a layer of ply, but a layer of cement board. IF you are really worried about finished height.. take your saw and notch the joists down the amount you need to come out flush. Just set the blade depth as needed, then make many repeated cuts of the joist tops, until you nibble out enough to allow you to recess the added layers. I would then "skin" the joists with some plywood, at and about a foot both sides of the notch..

 

[breathebeast]

Awesome, this is still doable. So pleased.

I read one poster saying not to screw the ceramic or concrete board into the joists, just into the plywood - why is that? If I put in blocking, do I need to avoid putting the screws into that, too?

Have to say that I have learned that I suck at making a cross-grain cut with a chisel! I really had to use the saw almost the edge of my cut to get a clean cut.

 

[begreen]

Sounds like you are doing fine. This is the first time I have heard about not screwing into the joists, but that should be easy to avoid using 1 3/4" screws. Don't blame yourself for the the crossgrain chisel work. It takes a very sharp chisel and patience to do this well.

 

[breathebeast]

Ha! Did a little research on the cement board not-screwing-into-joists idea, and I wonder if it's from the history of using nails, where movement in the joists might encourage the nails to back out? Which suggests that screwing into joists would probably be fine, and even better if there's enough screw length to get a decent grip?