I want to put a tile floor over an existing wood sub floor, problem is the 3/4 tg plywood sub floor is on joist 24″ o.c. both cement board and ditra recommend max 16 o.c. joists under 3/4″ plywood. dont want to overlay the plywood if possible due to problems matching height with non tile portions of the floor. I do have access to the underside of the floor but a lot of pipes, hvac ducts and really dont want to add more joists.
existing joist are 2×6 and span is about 7 1/2 foot. what about adding a 2×4 between each pair of joists.
any other suggestions?
Replies
One way would be to rip up the existing ply and use 1 1/8" t and g. Another option would be to put in a lot of soldi blocking (2x6 or 2x4, since it's only spanning 22.5"). Adding 2x4s parallel to the existing joists and not attached to them wouldn't add much strength.
zak
If I support the 2x4 at each end for a 7.5 ft span that would cut the span on the plywood down to 12" which I think might solve the problem. thanks
I think that the 2x4s spanning 7.5' would do very little to make the floor stiffer, because it's too much of a span for them. At the very least, try to transfer some of the load from the 2x4 to neighboring 2x6s mid span. Also- concrete board doesn't strengthen the floor, so use 1/4 or 1/2, IMHO. 12" tiles will be more likely to crack in the middle of a tile than a smaller tile would, since there aren't as many grout lines to give.
zak
Here's my take. Tear out the existing flooring. Sister 2X4's to the existing 2X6 joists 3/4 inch below the top of the existing joists. 2X6's would be better. Screw the 2X4's to the 2X6's. Lay in 3/4 inch flooring grade plywood between the joists. Screw that down. Now you have a flat surface. Add an additional layer of 3/4 inch flooring grade plywood. Screw that down. Now you have a pretty strong floor sytem. Add cement board or ditra.........screw that down but try to miss the joists and install your tile. I would try to stay away from 12x12's for the reasons mentioned. Use a smaller tile.....................much safer. There is a great flooring site by John Bridges. Stop in, you will like it. Mike L.
2 x 6 on 24" centers?
Ugh.
Yer not putting a tub/jacuzzi in there I hope?
Floor height be damned....at the very least I'd be adding a layer of 1 1/8" ply.
If you do have a tub going in, I'd be lookin' to completely gut the room and start by reframing the floor.
J. D. Reynolds
Home Improvements
2x6 @ 24"oc are absolutely maxed out (for deflection) at 7'-6" for a 50#/sq ft (total) uniform load......assuming decent lumber.
Like posted.....is this a bathroom??? are you planning on a tub??? If so, I would take more drastic measures than those below.
If no tub.....put either solid blkg or diag (criss-cross) bracing all along the center of the 7'-6" span.
I would also strongly reccomend 1/2" (NOT 1/4") "concrete backer board" set in thinset, and screwed at 8" oc.
Even w/ all this....I would use smaller 4" to 6" tiles, since larger ones might be more prone to cracking from deflection (bounce) in the floor.
Good luck
No tub but my wife want 12" tiles. I had discounted the diagonal bracing since this doesnt reduce the span on the plywood.
will the 1/2" cb help? they tell me it has no structural value. have you used the Ditra product that was just featured in FHB?
will the 1/2" cb help? they tell me it has no structural value.
Lots of debates on that. I have given this topic some thought, and IMHO.......
Indeterminate structure is the technical term for the "total" being greater than the sum of the parts (by a factor of an indeterminate value). Here's my PERSONAL THEORY.
A layer of 1/2 CB, though by itself is not "structural", does add "rigidity" to a floor. I have noticed this in practice, and my theory is as follows:
Food for thought....how much "structural" capacity does 5/8" osb have when laid flat over a 4 ft span. Hardly any, but glue that osb like a skin to the bottom and top of a 4x8 sheet of 6" expanded polystyrene foam (another NON STRUCTURAL material) and all of a sudden we have a "stress skin panel" which can miraculously carry hundreds of pounds over a 4' span.
You can do the same w/ paper....it's called corrugated cardboard.
Is 3/4 ply more "structural" than 5/8 ply? Why is that? The extra 1/8" layer (half of which has the grain running in the wrong direction) is not structural.
When CB is laid on thinset and screwed to the floor, it becomes a "top layer skin". When loads are applied to the floor, the floor flexes downward, and the "top layer" is in lateral compression.
Adding 1/2 of concrete to that layer, (whose magical qualities are...you guessed it....outstanding performance in COMPRESSION) will undoubtedly stiffen it. The reality is that the compression forces that are handled by the CB are in a horizontal direction through the sheet itself (think standing the sheet on edge and then applying force to the upper edge), not in a vertical axis.
It basically performs the same function as the top layer of OSB on a stress skin panel, or the fiberglass skin on a honey comb composite material, or the outer layer of paper on a corrugated cardboard box, or the top flange of a steel I-beam......etc etc etc.
have you used the Ditra product that was just featured in FHB?
Sorry I have not, though many here, who do way more tiling than I do, have recommended it.
Thank you for the analysis professor pickings. obviously then when we put on the tile and thinset this adds another layer to the stress skin panel. the tile also good in lateral compression does ok, but as the adjacent span flexes up that tile comes under tension and crack. interesting the differance between the two products cb then trys to resist the movement while the ditra, which would add little strength to the assemble, trys to absorbe the movement.
very helpful
I would remove the plywood and attach 2X4's on both sides of the 2X6 joists. The 2X4's would be low enough to allow 2X to be laid flat so the top surface would be flush with the top of the floor joists. Tar paper would go on top and then the tile man would be able to install an inch or more of mortar. The 2X4's would stiffen up the existing joist and the lowered floor just might keep bathroom water from flowing into adjacent rooms. There is also a chance the install an emergency floor drain somewhere.
Do a dry pack. Of course use mesh. If the grout cracks when you're done, go beneath and beef up the joists. The dry pack will span over the 24" centers but the joists seem to be already at their max.
Sistering 2x4's will help stiffen each joist too.
blue