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04-10-2017, 06:43 PM
Hello. I have been frequenting/stalking the forum for a while, but it is my first time posting. I started a bathroom vanity replacement project that turned into a "let's install tile" project since it would be "easier" without the vanity present. Famous last words...

My home was built in 2000. The bathroom is on the second floor of the house. After removing the current flooring (laminate wood, not original), I found the 3/4 OSB subfloor was damaged and flaking around the toilet from previous water damage. The floor was dry, but I was concerned about the damage and wanted to fix it. When I cut out the damaged portion (plus a few inches around), I found that the floor system was a 2x4 open web truss system. I *think* it's a fan style truss, but I'm not sure. They are 24in o.c. and 18in deep(tall?). The longest span looks like around 64in. There appears to be framed box like bracing at each end of the truss (I've attached a picture for clarification since I'm not too familiar with the proper terminology.) From looking all over the web, I can see that my span length seems unusually short which either means I'm wrong in my observations or that there are special circumstances why this area has shorter spans...? The wall in which the toilet supply line is located is the wall that separates the garage from the rest of the house (on the first floor), and the opposite side of the bathroom is adjacent to a staircase which may have needed the added reinforcement. I tried to find the manufacturer, but I can't find markings except for an "F12" handwritten on a 2x4. The truss system looks in great shape with no water damage or notches. I called a local truss company to ask for advice on how to identify the company, and was informed that the manufacturer of my truss went out of business in 2008 after the housing collapse.

When I dry fitted the osb patch, I found that it deflected a lot more than expected. So I added a couple 2x4 supports below the unsupported ends of the osb which helped.

My questions:
1) I had planned to use 1/4in modified thinset, 1/4in Hardie backer, 1/2in modified thinset, and 12inx24in (.31in thick) porcelain tile. Given the truss information, do you think it can accommodate it? (I tried using the calculator, but it seemed more useful for traditional joists and not the trusses)
2a) Is there another (better) way to support the patched OSB? I thought about making an OSB cleat to try to connect the patch to the existing OSB.
2b) Would cutting the patch to make use of the tongue and groove slot left where I cut the hole be beneficial? I happened to cut right on the line and found I could clean out the groove. I thought I wouldn't be able to fit the tongue in the existing slot, so I didn't try it. But now that the board deflected so much more than the existing stuff, I'm wondering if that would perform better?
3) Is it crazy to consider ditra instead of the backer board to save on floor height?

Sorry for all the details, I just wanted to provide as much info as possible, and I didn't wanted to filter anything you'd deem relevant. Thank you for any insight... I really need it.

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04-10-2017, 06:51 PM
I cant really speak to your joist deflection I don't deal a lot with that style.

With your joists being on 24" centers I would reccomend another layer of plywood. Minimum would be 3/8" but I would go ahead and use 1/2". Then you could go with CBU or ditra whichever you choose. A single layer of 3/4" osb on 24" centers has too much between joist deflection in my opinion.

Houston Remodeler
04-10-2017, 07:49 PM
1/2" CC or better over the top would be a good idea, installed this way (http://www.johnbridge.com/images/mike2/For%20Liberry%20Stuff/Underlayment-Nielsen-Woeste-0604.pdf..pdf).

04-10-2017, 09:21 PM
I'm not surprised that you had deflection of the patch! Ideally, it's best to bridge two bays when making a patch, or, do as you appear to have done, add bracing across the span so that the patch isn't hanging by the fasteners on two ends without a T&G to support the edges, it really is necessary. Tiling on 24" centers almost always requires two layers of ply to get the between span strength strong enough.

04-11-2017, 02:53 PM
Thank you Ryan, Paul, and Jim. I appreciate your responses, and I also really enjoyed reading the technical write up about positioning the underlayment that you linked, Paul. I missed that in the Liberry when I was looking for info, so thanks for showing me the way. :)

I was afraid you all would suggest the added layer of ply... With the tile installation, I'm already looking at just under .7" OVER the height of the adjacent hallway laminate flooring. (That's why I was asking about the ditra to try to reduce that as much as possible) Adding the extra .5" to that would make it unreasonably high at almost 1.2"... The whole house is done in the laminate flooring, so it's not just a small portion that would need to be redone to raise it up. I thought since my home is newer that builders would plan the support system for tiling since it is such a common flooring choice, but maybe that isn't the case at all.

I'll have to think about this more... Have any of you guys ever encountered this on a job and told the homeowner that tiling isn't an option?

Houston Remodeler
04-11-2017, 03:32 PM
Many times, especially when they want natural stone where two layers of plywood are required to prevent cracking. But some folks don't listen.

04-11-2017, 04:25 PM
If your spacing was 16 or 19.2", you could use your single layer of ply, but from what I remember, 24" OC requires that second layer to achieve the deflection requirements between the trusses. Hopefully, they're strong enough along them for tile...generally, they're designed for in the order of L/480...good for ceramic, but not natural stone.

04-11-2017, 05:24 PM
That's EXACTLY what I'm afraid of happening, Paul! :/

Thanks, Jim. I got a bit of promising news... I went down the street to a neighbor's house this afternoon (who has the same style of home as me) and asked them if they had tile flooring in their upstairs bathrooms. They actually have tile, and it IS original to the home. They've had no problems with it. My neighborhood was built by KB homes, and it seems hard to believe that KB would tailor each home's truss system to the buyer's flooring picks since it would probably be more time consuming (and thus costly) to make them differently. Maybe their tiler reinforced the flooring(?), but I'm more likely to believe they would have everything cookie cutter and easy to speed up finishing the homes... And I was planning on using ceramic (porcelain) too, not a natural stone...

04-11-2017, 05:42 PM
Your other option would be to pull the subfloor and add joists to cut down on your between joist deflection. Then you could go back with a single layer of 3/4" tounge and groove, followed by whatever underlayment you choose.

You could look into Blanke Permat. I believe that they approve installation over a single layer with 24" oc joists. Many are skeptical of its ability, and I have yet to use it but I've heard its good stuff.

04-11-2017, 07:59 PM
I just looked at the Ditra Installation handbook, and it does allow installing ceramic tile over one layer of a minimum of nominal 3/4" ply with spans of 24". A second layer would be more insurance, but you could use what you have and Ditra.

04-11-2017, 08:04 PM
You could look into Blanke Permat. I believe that they approve installation over a single layer with 24" oc joists. Many are skeptical of its ability, and I have yet to use it but I've heard its good stuff.
I've used it on a handful jobs over the past year and I've been impressed. It may be a viable option if your subfloor is in good condition.

04-12-2017, 03:24 PM
Thanks for the idea about adding extra joists and letting me know about Blanke Permat. I hadn't even heard of that product. I'll double check what they're recommendations are for the 24" oc joists... Especially since I find the thought of adding more joists pretty intimidating...

Thanks for noting that about Ditra, Jim. I'm definitely going to look into that more since it looks like that might be the only way I could move forward as-is (or if the Blanke Permat checks out).

I have to delay the project for a week since I will be out of town. But it will give me some time to decide how to move forward.

Thank you all for your helpful advice, and I'll let you know how it goes...