Down to joists [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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02-20-2010, 03:56 PM
Hey guys helping out a buddy on bathroom demo. He currently is tearing out subfloor which is essentially disingrated. When he is done he will be have nothing over the floor he will simply have joists exposed. On my recent project my subfloor was intact. My question is how many layers of plywood will be required?

Deflecto came up with the For joists that are SYP or Douglas Fir, in good condition, 7 inches tall, 1.5 inches wide, 16 inches on center, and 11 feet long between supports, the deflection calculated is 0.383 inches.

This translates to a deflection of L / 345.

Since the maximum deflection for tile is L / 360, and for natural stone is L / 720, your floor is rated for Sheet Vinyl or wood

He's on the cusp for tile. What are his options. How many layers of plywood etc will be required over the joists? Again he is down to bare joists so its a clean slate on how he can build back up. He was planing on using the ocatgon tile that comes in sheets

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Shawn Prentice
02-20-2010, 04:19 PM
Greetings Matt,

Plywood will do nothing to reduce deflection. You need to double up joists or build a support wall underneath to reduce the span. Then you can go back with 3/4" ply and 1/4" cbu as your tile substrate, unless the tiles are no smaller than 2x2, then you can use ditra.

02-20-2010, 04:35 PM
Matt, it's not a big project to sister up the joists, especially since the joists are exposed as they are now. A 2x6, some glue, and a few screws on each one is all that it would take.

I might want to hear from one of the resident engineers hereabouts, since the deflecto is known to be on the conservative side. The joists might be okay as they are without going to the extra trouble.

02-20-2010, 05:11 PM
Here's the thing..I'm not sure if they 2x8 or not? The measure like 7.125 with the tape measure does that quantify as 2 x8? When I change it to 2x8 in the deflecto he is ok for ceramic

The thing about sistering as was suggested. The current joists run from inside the rim joist to a wooden "I" beam. Now when I think of sisterng wouldn't one end need rest on the rim joist cavity? If so, the problem is the existing joists are cemented in and you cant get another one inside the rim joist cavity. So simply trying to place another one next to it is possible but it cant rest on the ledge of the cinder block foundation I would think that when your beefing up the joists one end needs to rest on the I beam and other end on the ledge of the foundation wall? Hopefully I'm wrong

If you are simply attaching the 2x6 directly to the existing joists irrespective if the ends rest on the existing I beam and foundation wall that would be very achieveable....

02-20-2010, 07:16 PM
Actually you would have to sister only the middle 2/3 of the joists to make it work, but....

Figuring those as 2x8's gives a good deflection rating. I'd put down a layer of 3/4" tongue and groove plywood, then CBU or Ditra, then tile, provided this is ceramic tile and not natural stone, of course.

02-21-2010, 09:29 AM
What I hear you saying in regards to sistering is the new wood can stop short of the beams that run perpendicular to the joists.

As or the plywood to be clear all that is required is 1 layer of .75 ply over the joists? The reason I ask is on my redo I had .75 sawn board existing over my joists and it was suggested I use 3/4 ply over my existing 3/4 sawn board. In essence I had 1.50 of wood over my joists.

On his project its only being suggested 1 layer of 3/4 and then CBU or Ditra? Seems light too me in lieu of me having existing 3/4 sawn board and yet it was highly suggested I had add another layer of 3/4ply?

John Bridge
02-21-2010, 09:59 AM
Hi Matt, :)

If you want your floor lower, remove the sawn lumber and install 3/4 in. plywood perpendicular to the joists. Then use 1/4 in. cement board or Schluter Ditra, available at Home Depot (about $80 for 54 square feet).

Going over the sawn boards with 3/4 in. plywood is fine if you don't mind the height. You'll still need to add cement board or the Ditra.

2x8s are never 8 inches tall. They are more like 7-1/4 in. :)

02-21-2010, 10:43 AM
Werid Dave Taylor just replied but now his post disappeared. What he said made sense...Essentially the rational for an additional layer of 3/4 over sawn floor boards may be more for more leveling than for adding rigidity, thus the recommendation on my project.

In terms of mu buddy's it does sound like simply adding 3/4 ply directly over the joists suffices. However if there are concerns regarding deflection (which he may have). In addition to the sistering suggestion perhaps another layer of ply say around .375 or .50 would be prudent over the 3/4 that is laid directly on the joists. For a total of bewteen 1.125 -1.25. Would this be correct. Of course then either Ditra or CBU for his underlayment.