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Old 02-22-2018, 01:38 PM   #1
P38Light
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Exterior porch floor - sunroom

I am converting an exterior covered porch into a sun room. Existing floor is Tendura composite 1 x 4 TG, screwed and glued to 3/4 treated plywood subfloor that is screwed and glued to 2 x 10 joists, 14 foot span (10 year old construction). I am removing the Tendura (major PITA) as it has buckled in a couple of places, will remove glue strips (soft black stuff) with oscillating scraper tool. Subfloor looks OK to me, but I don't really know what to look for. Am now looking at 3 pre-tile floor prep options once windows are in:

1) Install Ditra with thinset directly on original subfloor.

2) Install 5/8 plywood or OSB underlayment over original subfloor, then Ditra with thinset.

3) Install CBU over original subfloor then tile.

Will probably install 12 x 12 tiles (or maybe rectangular tiles depending on my wife's selections). Would really appreciate your advice on one of these or another approach at this point.

Thanks for your help
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Old 02-22-2018, 06:14 PM   #2
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Welcome back, William.

I've not heard of that type decking material being installed over a plywood subfloor, but it's good that you're removing it.

The treated plywood is never a good idea under a ceramic tile installation, but if yours is ten years old it may have reached a steady-state moisture condition by now. It's still probably a CD grade, though, and still not the best for a tile installation.

You don't indicate the joist spacing, so it's difficult to judge the suitability of your structure, but if it meets L/360 I'd recommend you install at least nominal half-inch exterior glue plywood of grade CC or better over it and the tiling substrate of your choice. Guess that makes me a fan of your number 2 choice.

My opinion; worth price charged.
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Old 02-22-2018, 07:08 PM   #3
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Option two has my vote, preferably plywood for me
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Old 02-23-2018, 09:40 AM   #4
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Thanks CX and Robert. Spacing is 16, so I'm at L/380, Deflecto says OK for ceramic tile.

I have downloaded from the Liberry a article by Frank Woeste, P. E. and Peter A. Nielsen on Underlayment placement, and agree with all they said (especially the butt joint placement at 1/4 point, with substantial overlap of subfloor joints, but have a couple of questions:

1) They recommend nails or screws at 6" intervals throughout. I know these guys are engineers (as am I, so my apologies for all the detail worries - can't help myself) so I understand that their analyses may be a bit conservative. Does your experience indicate this many fasteners are really necessary, as that's a lot of knee work for an old guy like me.
2) They recommend that fasteners not penetrate joists, while I believe I read in another post re underlayment that joists should be engaged with long screws. Both arguments have their points, but what's your advice?
3) Regarding CBU over plywood underlayment, what's the general advice on placement and fasteners?

Thanks
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Old 02-23-2018, 10:00 AM   #5
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1. Yes, this will help eliminate voids between the two layers.

2. It depends on the situation. In yours it would be best to avoid the joists with your 2nd layer.

3. Most cbu has a fastener schedule marked on it, if not then refer to manufacturer specs for that particular brand. I believe hardi backer is 8" spacing while staying 2" back from corners and edges.
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Old 02-23-2018, 12:06 PM   #6
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IF the existing first layer is lousy, you may choose to essentially ignore that it is there, then, you'd want your second layer to engage the joists...otherwise, avoid them. Avoiding the joists on the second layer helps to decouple things a bit more, and is added for increasing stiffness between the joists and prevent the first layer's edges from telegraphing through. But, if it's compromised and you don't feel like removing it, ignore that it is there. You'd still want to offset the edges, but have it engage the joists.
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Old 02-23-2018, 02:34 PM   #7
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Hey, thanks for all the great input.

OK, so plan now is: Since the existing 3/4 plywood looks good (treated, but after 10 years screwed down beneath 1 x 4 T&G it is really dry and flat), I'll go over it with additional deck screws along joists to be sure it's fastened down well.

Then add 5/8 or 3/4 plywood underlayment (from contractor lumber yard to hopefully avoid cheap imports) short screwed to subfloor (1,100 or so screws for 14 x 17 floor - yikes!).

Now to choose between Ditra or CBU for decoupling ... looks like both involve a lot of knee work, but Ditra is about double the cost, not including thinset. I used Ditra in a bathroom as discussed in my prior thread, but this is sunroom much larger, no moisture issues. Would Ditra offer significant advantage over CBU for this? Which would be more tolerant of my amateur trowel work when setting tile??
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Old 02-23-2018, 03:22 PM   #8
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Cbu doesnt have any Uncoupling properties so they are already different there.

I can carry 300 sqft of ditra on one shoulder while cbu is awkward, heavy, and a pain to cut. Cost is relative, if you include the added time for moving and cutting. Then add in cost for thinset below, mesh tape, and cbu screws its an easy decision for me every time.

Ditra isn't the only option these days. Laticrete, Mapei, and Ardex all have uncoupling mats as well.
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Old 04-01-2018, 01:56 PM   #9
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Slightly sloped flat floor

I am turning an exterior porch into a sunroom with tiled floor (20 in square ceramic tiles, beige) and part of one wall will be tiled for a wall mounted gas heater, using 6 in square tile (gloss black tiles with black grout). After installing new OSB subfloor (screwed and glued) and new drywall on short walls on both ends, the slight slope of the floor is more noticeable (but maybe I'm looking harder than I should). The room is 12 x 14 with a slope of 1 in per 6 ft in the 12 foot dimension (2 in overall, back to front).

Since the tiled wall section will have level lines, should I try to level the floor, or will this small slope only be noticeable by perfectionists?

If leveling is the recommendation, what methods do you recommend?

Thanks,
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Old 04-01-2018, 02:55 PM   #10
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Ok, there are multiple things to talk about. But let’s start with just a few basic questions to get this conversation underway.

I’m assuming this porch was previously much more exposed to the weather than what you’ve got planned? If so, the slope of this exterior porch was intentional to shed rain water away from the house. I’m assuming the joists are sloped away from the house?

Okay, next...
Exterior porches aren’t always built strong enough to handle tile. Can you tell us what you’ve got for a joist structure? How big are the joists, how far are the spaces from each other, and how far do they span?

And you said you installed a new layer of OSB. How thick? Is this installed directly atop the joists, or over another layer? If so, what’s the layer?

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Old 04-01-2018, 03:53 PM   #11
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More detail

The porch was always under roof, intended to be a sunroom initially, but we ran out of money. Joists are 16 OC, 2 x 6 I believe, 3/4 T&G OSB, recently installed by sunroom contractor after removing old floor (T&G plastic/wood composite flooring over 3/4 treated plywood - all removed because composite flooring had buckled somewhat and plywood was not flat). I did run it through Deflecto and came out OK.

You're right, the slope was intentional for drainage. Now have new exterior windows on three sides, one of which is floor to ceiling glass, sides have short stud wall with windows up from there to near ceiling, nice new flat OSB floor.

Hopefully photos are attached here. The 6 ft x 4 ft bumpout under windows will be black 6x6 tile, black grout with most of that area covered by stainless front gas stove.

So first question, will 2 inch drop over 12 feet be readily noticeable? If so what are my options now?

And thanks for taking time out of your Easter to help me.
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Old 04-01-2018, 04:14 PM   #12
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Along the lines of what Bubba is asking, what do the joists bear on or hang from?

I'm assuming it freezes where you are which would typically require footings below frost depth and stem walls of block or concrete. Is some cases helical piers can act the same, but it depends on how they're installed and manufacturer's recommendation.

If it bears on earth above frost depth, I'd look to something besides tile on floor.
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Old 04-01-2018, 05:09 PM   #13
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This is a second floor space, foundations go below frost line, first floor space is a screened porch, all below roof and supported as if enclosed (as was original plan), so no problem with support.
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Old 04-01-2018, 06:08 PM   #14
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Assuming your joist structure is adequate for tile, which you can gauge with the almost world-famous Deflectolator in blue band above, there's no real reason to level floor.

Chairs, tables and anything sitting on floor will lean a little, but it was like that before, no? Most obvious will be anything close to a plumb bit of the perimeter frame or windows. Many eyes would never see it, but you're asking a bunch of largely anally retentive house nerds so you be the judge.

To level now would require shimming in some fashion while maintaining structural integrity adequate for tile. I probably wouldn't do it if I was at the stage you are, as it's easiest and best to address at framing level rather than on top of subfloor.
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Old 04-01-2018, 06:29 PM   #15
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If the floor to ceiling glass is at the low end of the slope, can you even raise that side 2" without trapping the windows in an odd way?

If you want to remove the slope, the best option is to take up the 3/4" OSB and sister each 2x6 with an additional 2x6 that is level, then reinstall the 3/4" OSB.

Cheers, Wayne
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