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Unread 05-03-2013, 01:01 AM   #1
dappelha
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Tiling over 24" o.c. engineered joists with minimal underlayment

Hi all,

I'm trying replace the trashed linoleum flooring in my kitchen with 12" porcelain tile.

House was built in 1979, I bought it last year. The total square footage that I'm trying to tile is about 50 sqf, so pretty small. I have an engineered trust system of 2x4's (12" high) that is 24" o.c., and a subfloor of old 3/4 plywood that looks like it has had some water damage.

Here is my dilemma: I feel limited on the amount of underlayment I can use because of the dishwasher and the cabinets. The cabinets are sitting on the subfloor with the bottom of the counter top 34" from the subfloor. The dishwasher looks original and barely slid in and out with the 3/4" of underlayment, old wood floor, and linoleum that I just ripped down to the subfloor. Yes I screwed the wheels up. So I don't want to make the floor higher than 3/4" because I know I'm going to have to replace the dishwasher someday. And I don't want to raise the cabinets because I'd have to redo the backsplash and the plumming and risk destroying them in the process. I'll probably replace the cabinets in 5-10 years.

The plywood also flexes quite a bit between the trusses because they are spaced so far apart. Since I'm expecting not to be able to build up a layer of underlayment we are planning to beef up the area between the trusses.

What is my best option for laying tile that will be alright? It doesn't have to be the most timeless job, but I want it to have a reasonable chance of lasting 10-20 years.

My current plan is to rip out as many sections of subfloor as we can replace on the trusses without going under the cabinets, bridge and block the truss every 1' while we can access them, and replace with new plywood. Then I was hoping it would be solid enough to thinset/ditra/thinset/tile right on the subfloor. Is this a terrible idea? How thick is the thinset/ditra/thinset layer, and can I afford to put a 1/4" OSB underlayment and still be at 3/4" total?

Am I better off skipping the ditra and putting a thicker plywood underlayment and tiling directly on that? Or are there tricks to building the floor up but setting the tile in such a way not to lock in the dishwasher? Will thinset on the subfloor mean a hell of a time later if I want to re-tile?

Thanks for your help,

David
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Unread 05-03-2013, 09:33 AM   #2
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Welcome, David.

You can't have ceramic tile floors if you don't have a way to build an adequate subfloor any more than you can expect to live happily near the ocean if you don't wanna hear the roar of the waves.

With the engineered joist system, it's usually not even feasible to reinforce the subflooring from below or to add the blocking I think you're envisioning from above. I can't see your joists from here, though.

I think your most reasonable approach would be to select one of the newer vinyl floor coverings in a tile look. That would keep you at peace with your dishwasher, which is a very common problem when changing floor coverings in kitchen areas.

You could try the Ditra XL approach over your existing, old, damaged subflooring, or your new replacement subflooring, but I wouldn't. Schluter says it'll work, though.

My opinion; worth price charged.
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Unread 05-03-2013, 04:26 PM   #3
dappelha
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Unhappy

You know you are making me sad, right? I don't want to rush into disaster, but I also think DIYers throw tile down on all kinds of surfaces and it seems to turn out ok.

The truss is very solid, it is just when you bounce up and down between them that the floor flexes because the plywood isn't very rigid. We were going to beef them up by adding 2x6s perpendicular to the truss space about 2 feet apart. We were going to attach them with over joist hangers Simpson-Strong-Tie-Z-Max-2x6-Post-Frame-Hanger-.

Won't this seriously beef up the floor and make it reasonable for tile? I think we can also add a layer of 1/4" OSB and still fit the dishwasher. If we did all that would you still say it is too bad of a subfloor? What if locking in the dishwasher wasn't an issue, what would it take to have a good surface you would be happy installing tile over?
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Unread 05-03-2013, 06:41 PM   #4
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Wasn't my intent to spoil your day, David, was more in line with not wanting your to spoil lots of your own days later. Really it was.

I'm not familiar with joist hangers, Simpson or otherwise, that are made for attaching to engineered wood joists. Doesn't mean there aren't any, just means I don't know about'em. If you'll give us a Simpson model number for the hangers you have in mind I'll be happy to look'em up and get a lesson, maybe.

And if they are suitable to your application, adding blocking between your joists at two-foot centers would just mean you now have two-foot blocking spacing under the weak axis of your subflooring as well as under the strong axis. The amount of improvement, presuming there would be some, would be precious little.

Adding quarter-inch anything to the top of your existing subflooring would yield similar improvement. Actually, it would probably be less unless you could effectively glue the quarter-inch OSB to the existing subflooring, which you can't because it's OSB. Even were it plywood, the very thinnest structural plywood is nominal three-eighths of an inch.

Your kitchen base cabinets should have started life being 34 1/2" tall. If you measure only 34" it's likely youre measuring a lip on the front of the counter top. Perhaps you could remove said lip and gain another 1/2" of height.

You might also be able to raise the front of your countertop temporarily for installation and removal of the dishwasher if you don't tile under that area.

Or...........something else.

I would not consider tiling over your existing subfloor without an additional layer of at least nominal half-inch plywood. But it's your house and you can.

My opinion; worth price charged.
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Unread 05-03-2013, 08:38 PM   #5
dappelha
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Just got back from HD. I've decided to follow your advice and go with the vinyl 1' square flooring. We are going to forget about beefing up the truss and just put down nominal 3/4" underlayment and high quality grout-able vinyl tiles.

Now I'm trying to decide if I should grout the vinyl tiles or not... I think the grout looks nicer, but don't know if it is less durable. Any tips?

Also, should I use OSB or plywood for the underlayer? I was going to screw it into the subfloor (but not joist) with dry wall screws, does that sound right?
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