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Old 09-07-2017, 05:37 PM   #1
BillGriggs
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Advice Needed: Porcelain Tile over 48"oc

I'm about to tile two areas and wanted to check with some pros about whether I'll be ok. (Geographic area: Northwest)

One area is a 7'x3' entry pad and one a 6'x2' fireplace hearth. I've discovered the joists in our house are 48" oc. The subfloor is 2"x6" TNG planks with a 1/2 particleboard underlayment over them.

We have 12"x12"x3/8" porcelain tiles.

The spot in front of the fireplace was previously 8x8 tiles stuck to fiberboard with adhesive. It did just fine (no sagging in the beams).

My plan is to cut out the particleboard and replace with 1/2" ply, then put down unmodified thinset, ditra, flexbond, and the tiles.
  • Is this ok on my floor? It's been super solid so far, and the two areas aren't all that big. I've seen some posters be quite skittish about 48" oc.
  • What spacing should I shoot for between the tiles?
  • With the floor setup like this, should I shy away from tiling a larger bathroom and dining room/kitchen area?

Thank you
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Last edited by BillGriggs; 09-07-2017 at 05:37 PM. Reason: Clarification and formatting fix
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Old 09-07-2017, 07:09 PM   #2
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Unfortunately, our deflecto calculators only cover standard floor assemblies. Hard to say how much deflection there is in a T&G 2 by deck. It's your house and your call, worst case you will end up with a small failed floor. Otherwise, your approach is sound. Grout lines should be minimum 3 times the size difference between tiles ( determined by holding a straight edge across a stack of tiles). Generally 1/8" is a pleasing size.
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Old 09-07-2017, 09:50 PM   #3
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Welcome, Bill.

If you'll put a geographic location into your User Profile it'll remain permanently in view to aid in answering some types of questions. We know you're on the Left coast just by the description of your floor, but get us a little closer, eh?

You'll need to look at your joist structure to determine for sure whether you qualify for a ceramic tile installation, but usually those floors are suitable with the addition of half-inch exterior glue plywood. The particle board you have will need to be removed, of course.

No need to avoid tiling anywhere if your framing structure is adequate.

You've got your thinset mortars exactly backwards on the Ditra installation. Best to go to the Schluter website and download the Installation Handbook. You must always, always use a modified thinset mortar meeting ANSI A118.11 to bond anything to a wood subfloor. Schluter wants you should use an un-modified to set your tiles.

My opinion; worth price charged.
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Old 09-08-2017, 11:10 AM   #4
BillGriggs
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Thanks Cx!

I did wonder what type of plywood I should be looking for, so that is helpful as well.

Sorry about mixing up the thinset. Yes that is my understanding: modified to bond ditra to plywood and then unmodified to bond ditra to tile (so that it can actually dry because of ditra being waterproof).
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Old 09-08-2017, 11:45 AM   #5
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Again, be sure the mortar for bonding to plywood says it meets A118.11. Very important, that. There are no many 118.4 (modified) mortars that do not also meet A118.11, but there are some out there. Read the bag.

Up to you what mortar you use to set the tiles. Schluter wants unmodified, the tile manufacturer wants modified.

My opinion; worth price charged.
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Old 09-08-2017, 01:19 PM   #6
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FWIW, if you insist, Schluter now has a modified that you can use over their membranes. The issue is, most people do not realize that there are at least four classes of modifiers used in mortar, and not all of them will work, so since testing by them has shown an unmodified works, they didn't want the confusion and chance for failure when people didn't use one they were certain that worked. You can't tell by the ANSI spec, either (but you can in say the Eurozone, since they rate and test them to a different spec). So, they can't just say use one that meets ANSI 118.11, since it may not have the 'right' modifiers in it. For people that want a modified, they've now got one specified and branded under their name. Since they control the configuration, they don't have to worry that some other manufacturer's product might work today, but tomorrow, they change the configuration. Since they are under no obligation to tell anyone about it, that could mean it worked, then the new stock won't...not a good solution...thus, they decided to have one made to their specs that should always work. IMHO, in most cases, it just results in more expense, but there may be situations where it would be better. Your money, your choice.
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Old 09-15-2017, 12:54 PM   #7
BillGriggs
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Thanks jadnashua! I'd always wondered if the manufacturer's own brand was just an attempt at up-selling, but you've offered a great perspective.
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