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Mark in midwest
02-24-2008, 06:41 PM
Love reading you pros and hoping I can get some help! I'm a diy'er just about to start tiling a foyer but suddenly have doubts that I'm okay...

Joists: 11 & 1/16" high, 16 on center, 15'3.5" span gives me somewhere between 455 and 494 (only half foot increments in deflecto)

Floor: 3/4 tig, planning 1/2" ply and 1/4 CBB (although I'd love to go lower if possible to join wood floor)

Tile: Stone mosaic style on mesh, mostly 1x1 with some 4x4's and 2x1's mixed in. It's relatively thin (5/16")

I'm not sure why stone needs a higher rating on deflection - cause it's heavier? But am hoping this stone's "friendlier" in it's lower profile. I just read Bea's sistering of joists - wondering if I need to do the same. Any help on adequacy of my joists and sub would be greatly appreciated!


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Brian in San Diego
02-24-2008, 08:27 PM

The standard for stone installation over a joist system is a deflection rating of L/720. That satisfies 1/2 the equation the deflection of the joists. The second half of the equation is the deflection between the joists. To satisfy that half of the equation for a natural stone installation you need two layers of plywood totalling 1 1/4" in thickness. If you twist my arm I might say you could probably get by with 1 1/8". So the addition of the 1/2" plywood (CC or better Exposure 1) is a good thing. You then have your choice of installing a cbu or a membrane. In your case Ditra wouldn't work because of the tile size. Ditra limits tile size to a minimum of 2x2.

So your problem exists in your joist structure. You have a couple of conventional choices. One would be to sister the joists and the other would be to shorten the span. If you could shorten the span to 12.5' then you would be fine. If you sistered all the joists then you'd be in the L/900+ area. So it's a matter of picking your poison. There is a more "controversial" way of sistering that you may want to employ if you have access to the bottom of the joists (no finished room below).

Let us know what you have and perhaps there may be a solution.


Mark in midwest
02-24-2008, 10:18 PM
Thanks for the feedback, Brian!

I am redoing the ceiling in the basement below so the joists are exposed. There is ductwork through 2 of the three joist spaces which would make it tough to fit another joist in, but might be able to take out the ductwork and replace it after.

I'd be interested in hearing what the "controversial" way is. Also, though the ibeams are 15'3.5" apart there is a wall (in the basement) at about 11' with a door in it - might I be able to shore that up to count as a supporting wall? If so, what would that entail?

One other factor - the space I'm doing is a little over 4 feet wide. One of the two joists that outline that space rests on the foundation so it really is only three joists I'm talking about.

Is the reason I need the higher rating because of the weight of the stone?

Thanks - stone ready Tuesday and hoping I can do this Wed.

Brian in San Diego
02-25-2008, 08:15 AM

If you have a wall in the basement that is built perpendicular to the joists and the joists rest on top of this wall then the distance between the wall and the next support is your span. What the deflecto is looking for is the longest unsupported span, not the length of the joist. If the wall was built properly the door would have a header above it to span the width of the door. If you can determine that the wall is bearing the weight of the joists then you want to measure the span (inside edge of support to inside edge of support) and plug that into the deflecto.

Read through this thread (http://www.johnbridge.com/vbulletin/showthread.php?t=57494) that resides in the Pro's Hangout. You will read that the advocate of this system is an engineer and has used the method in his own house. He details that in another thread in the advice forum and he refers to it in the referenced thread.

Like I said in my last post, if your span is 12.5' or less then you don't need to do anything to shore up the joists. If you do have to do anything perhaps the methods discussed in the thread I referenced would be alternatives for you.


Mark in midwest
02-25-2008, 10:05 AM
Well, the light pierces the gloomy Monday outside Chicago! Nice to know I can focus my time elsewhere.

Yes, the wall is perpendicular to the joists and the door has a header, like most things in this house it was built to last. This makes the unsupported span 11' rather than 15+ and the rating jumps to 1020! Wow, what's heavier than stone that I could put down there? :)

We had been considering moving that wall in the basement but now I'm glad it's there. Interesting, my instinct told me there was little movement in the floor there but the results of the deflecto surprised me - now I know I used it incorrectly.

So I guess I only have two q's remaining. First - the subfloor. This will butt up against a wood floor that will be slightly lower and I'm trying to minimize the difference. I've got 3/4" tig planks - not plywood. You mention the need for 1-1/4" of plywood to properly support. It seems to me that the planks are better than plywood here - does it reduce what I need to add? Or do I still need the 1/2" plywood? (I will top it with 1/4" hardibacker.)

Second - someone suggested that I need unmodified thinset between the plywood and the cbu and modified between the cbu and the stone - they said the unmodified is firmer, creating a more solid base and that the modified gives a little, adheres better - that sound right?


Brian in San Diego
02-25-2008, 09:37 PM

The Ditra handbook says you could use 1/2" plywood over 3/4" plank subfloor. I'm not sure if the hardiebacker installation manual addresses that situation. I wouldn't put down anything less than 1/2" ply. The CBU adds no structural intergrity to the installation. It's function is to provide surface upon which to set tile.

The role of the thinset under the hardiebacker is to fill voids. It isn't to secure it...the screws do that. Here's what the hardiebacker installtion instructions state • Apply a supporting bed of mortar or modified thinset to subfloor using a 1/4" square-notched trowel. I have never heard the two different thinsets characterized like you have stated. I think the main difference is one (modified) needs air to dry and unmodified cures by chemical reaction. Never heard that one is firmer or one "gives" a little. If that were true I think many a floor would be in trouble.


Mark in midwest
02-25-2008, 10:16 PM
Thanks, Brian, for again answering my q's. :bow:

Then the use of modified vs. unmod is strickly personal preference? Do I need to let the thinset dry under the cbu before starting the tile? Am I right in estimating two layers of thinset (on each side of the cbu) will give me an additional 1/4" in height - I'm estimating in order to trim six jambs.

I do understand from my research that cbu offers no structural support. I'll do the right thing and add 1/2" plywood and manage the height difference as best I can with a transition piece.

Thanks again for your help, Brian. You've put my mind at ease - a great thing for this diy'er!


02-25-2008, 10:55 PM
Welcome, Mark. :)

Looks like Brian's takin' good care of you here.

Only comment I'll add is in response to your questions about the weight of your stone tiles. That has nothing to do with the decreased deflection requirement. That's to compensate for the natural stone tiles being less consistent and more fragile than ceramic tiles.

I'd recommend you not make a final cut on those jambs and casings until you have your CBU installed. Then you can lay one of your tiles on a scrap of cardboard or some such to account for thinset up against that trim and mark your cuts, or just let your hand saw ride on the (upside down) tile. Get a nice fit that way.

Most CBU manufacturers say you can tile immediately upon setting their boards. Do read the instructions for the actual board you'll be using, though. Good habit to get into, that instruction reading. :)

My opinion; worth price charged.

Mark in midwest
02-25-2008, 11:19 PM
Thanks CX - yes, Brian has taken good care of me.

Makes sense about the reason for the different rating for stone. I never thought stone might be more fragile than ceramic.

I'm still 2% short of sure on the choice of thinset... I'm planning on plywood - unmodified thinset - hardieboard - modified thinset - stone... that sound right?

Thanks for the tip on cutting the jambs... was hoping to avoid two cuts on each (have to cut to slide under plywood and cbu) but a better fit is what I'm after - it is the front foyer and my diy abilities will be on display every morning on the way to the kitchen. To me and my wife! :)

Thanks much.


Brian in San Diego
02-26-2008, 12:05 AM

I would definitely set the stone with a modified thinset. You may want to consider using a medium bed mortar (which is modified). It helps prevent stone from "settling" once it's been set. You may be o.k. without it since your stone sounds like it's on 12x12 sheets. (You're going to do an entire floor with those itty bitty tiles?) I would use a white thinset either way and it would definitely be modified.


Mark in midwest
02-26-2008, 06:24 AM
Yep. It's only the foyer - 55 sq ft including closet space.

Why the white thinset? Will likely use a gray grout. (is Flexbond Fortified Thin-set Mortar from HD gonna work okay?)

Reading the thread on the "controversial" method of reinforcing the joists (we've since realized I don't need to do that), it was interesting to me that solid wood was stronger than plywood of the same thickness. I wondered if that applied to strengthening the subfloor, too. Does having the 3/4" planks give me any additional strength over 3/4 plywood that would allow me to need to add only 3/8" plywood on top rather than the full 1/2"? Sorry if this seems redundant - this question just seems to have never had a fork stuck in it.

Pickin' up the tile tonight.

Brian in San Diego
02-26-2008, 07:18 AM

I take back what I said about white thinset. I for some reason envisioned a travertine type material (lighter) stone. I used grey thinset with my beige porcelain tile and where I missed cleaning the grout in a couple of places it shows. If it were white it wouldn't be noticeable. If the tiles are darker and you are using a grey grout, then the grey thinset should be fine. Be diligent in getting grout off the tile prior to letting it set up. I would recommend you get a v-notch trowel either 1/4x3/16 or 5/16x5/16 to set those tiles. You want complete coverage but you don't want excess thinset oozing up into the grout joints.

I am not going to tell you that your floor will fail if you use 3/8" plywood. I am telling you what the Schluter Ditra Installation Handbook states for minimum plywood thickness over plank subflooring. If the 1/8" less in height is going to help you and you want to assume the risk that it might not be enough then like we sometimes say..."your house, your money". I will add that the Schluter guideline states that the maximum joist spacing for their scenario is 24" OC. so you may want to make the assumption that your floor will be o.k. with 3/8" plywood because you are on 16" centers. Make sure the planks are securely fastened to the joists and check out the thread in the (Liberry) about Plywood, CDX, etc. There's a link to a great article in post #3 of that thread.


Mark in midwest
02-26-2008, 11:08 AM
Okay - I'm going to live with the different floor heights and go with the 1/2". Too many who know have concerns - I've learned the hard way you can't change the way it is with good intentions and hoping. The edge between the wood floor and this new tile will be a curve with a height difference of nearly 5/16 - have to do some research on that transition piece now.

The article was good - I'll keep in mind the 4" rule on seaming the plywood. Interesting pix on post 2, too - 3/8" plywood has 3 glued layers - 2 going the same way and one the other way. That's why it's got to be the right direction. Is it the same with 1/2" I wonder? It's 4 glued layers so it seems to be 2&2 so I wonder if it matters. But I'll play it safe and put it the "right" way.

The "experts" at HD gave me a square 1/4x1/4 trowel but you give me a good warning - last thing I want to do is try to clean the thinset that's oozing through the gaps. I'll run to the local Ace and get one you suggest. They didn't steer me wrong with the Flexbond Fortified Thin-set Mortar, did they?

Brian in San Diego
02-26-2008, 01:43 PM
Flexbond is good pookie but almost twice the price of Versabond. I like medium bed mortars for stone but you don't have to use them. At HD that would be Custom's Marble & Granite.