How to resolve a deflection problem [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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02-10-2010, 06:53 PM
Hello, this is my first post, and here is my situation in which I'm looking for some advice.

My wife wants me to install some commerical-grade 12x24 tile that is almost 1/2" thick. The deflecto calculator and my own jumping up and down on the floor tells me that I need to reinforce it before proceeding. However, there's no room to reinforce the 2x9 joists from below (too much ductwork and other things in the way). I've tried putting in some blocking between the joists, but that only seems to reduce the deflection marginally. The area I'm worried about is a 15' long span of joists.

My local tile shop told me to add 5/8" plywood on top of the existing 3/4" flooring to resolve the problem. However, if I do so, when I add the thickness of the tile, thinset, and possibly ditra, I won't be able to open my front door due to the buildup.

I was thinking maybe of cutting out the 3/4" plywood and installing 1" plywood instead. Not sure how much stiffness that would add, but it would allow me to fit everything on top without having to mess with door jams, etc.

Any advice is appreciated.

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02-10-2010, 07:08 PM
Welcome, Dan :)

You have options but not a lot of them are great if you have a lot of ductwork in the way.

You use that space down there? If not, you could build a support beam down clear of everything in the way, and put little cripple supports to hold each joist. Beam don't gotta be tight to the joists - can use individual blocks/wedges to support each joist.

Or you could try the 2x4-on-flat-under-each-joist method. Each joist gets a 2x4 glued and screwed underneath, on the flat, making an upside down T. Might not work tho, w/ the stuff you have down there.

Mebbe you post a picher of what you got and we can make some more ideas up for you? :)

cross-blocking, like you're doing, or adding more plywood, like your tile shop is recommending, only addresses across-joist deflection. Not the problem you have on your hands. More or less a waste of time in your case. Assuming that "commercial grade tile" is ceramic, not stone, you don't need another layer of 5/8. You could get away w/ 3/8". You could even possibly get away w/ no more plywood, assuming those 2x9 joists are 16" OC and the 3/4" ply is in really good shape.

Hope this helps!

02-11-2010, 01:22 PM
Thanks for the reply Ed. The space below is part of a furnace room with open access to the joists, so I could possibly do something with a support post. I was considering a telepost, but I thought that there needed to be some extra footings below the floor in order to do this, so I ruled it out. Maybe I'm mistaken about that?

I'm not quite sure what you mean about the beam and cripple supports? Could you explain that further?

In regards to the cross-blocking, I read that doing this can help to distribute the load over the joists, which is why I did it, hoping it would reduce the up/down deflection. There is a double-joist that I have not connected with yet with my blocking, so maybe if I can hook up into that one, it may help.

the plywood is in OK, shape, but when I was screwing the plywood floor into my blocking from above, a few screws went way to deep into the plywood, so there are some spots that may be weak for whatever reason.

I will grab my camera and post some pix soon. Thanks.

Dave Taylor
02-11-2010, 01:32 PM
when I was screwing the plywood floor into my blocking from above, a few screws went way to deep into the plywood,

When this happens, dan..... just move a tad away from the deep set screw and drive another properly. No big harm done at all.

02-11-2010, 07:15 PM
I'd be curious to see where you read cross blocking between joists would add structural strength and deflection resistance along the joists. Just nothin' there to add any support, Dan...

the beam concept is simple. If you can't add a mid-span beam across the joists due to ducting and pipes and wires, just build the beam wherever you can below the joists at the midspan area, and add blocking and shims up to each joist. It's kinda kludgy and requires a fair amount of extra bracing to make sure things don't ever get whacked out of position, but it works.

02-12-2010, 10:50 AM
I read about cross-blocking on an engineering site I found where it spoke about cross-blocking distributing the load across joists (

I wanted to make sure I understand your beam suggestion correctly. Is this one ore more a 2xNs (maybe doubled) attached flat-side across the bottom-side of the joists. So I end up with a grid of sorts? Where does the blocking and shims come into play? Would you just screw to the joists?

Also, does it need to be fir, or will spruce do? Sorry for all the novice questions, but I haven't tackled something like this before.



02-12-2010, 10:55 AM
I'll let the other guys provide fuidance on the support beam and such. I want to point out that you will need to add some kind of tile underlayment - 1/4" cement board, EasyMat, Ditra.

I like Ditra, only about an 1/8". You do need something though.

02-12-2010, 02:07 PM
Thanks Joe. I was planning on using Ditra, which my tile shop says adds 1/4" total buildup when you include the thinset layer. He also said I might be able to get away with adding just 3/8" plywood, as apparently Ditra provides a 3/8" plywood equivalent in floor stiffness.

If I can figure out how to do the under-joist support as well, plus keep my build-up height safely below my door-swing-path, I may just get this done (fingers crossed). The tile is obscenely expensive in my opion, so I don't want to blow this job.


02-13-2010, 05:27 AM
Sorry Dan, we're not communicatin'. No grid. Nothing like that.

You could do 2x4's on flat glued/screwed to the bottom of every joist individually, making each joist into an upside down "T". That's one method.

Another method is to add a single supporting beam ACROSS all the joists. Locate it approx midspan, and if there's "stuff" in the way (ductwork, wires, plumbing) you just have to add some blocking/shims to space the crossbeam below all the stuff it needs to clear, while still supporting each joist individually.

just search for supporting beam, there's pix out there.

02-13-2010, 01:38 PM
Thanks Ed, I think I have a better idea of what you are talking about now. I can't really do the 2x4s, as the ducting is square in the middle of the mid-point. There is a chance to do the beam across the joists, but I've only got maybe 7 inches of space in which to install it (there are two ducts running across the joists, with a 7 inch gap). Unfortunately, the joists in that area are heavily drilled through for wiring. Any suggestions on how to attach the beam, especially given the tight working space? Thanks.