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Unread 03-20-2006, 01:11 PM   #1
Hamhanded
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Unhappy Plywood overlay orientation

I have a sick pit in my stomach after I followed a subfloor installation guide to install layers of plywood perpendicular to each other, and now I learn that it is strongly recommended to install all plywood layers perpendicular to floor joists. I very much need and would appreciate anyone's advice on how to proceed.

I am planning to add 12" square ceramic or porcelain tile in an upstairs bathroom with 5/8" original plywood over top 2 x 12 joists on 16" centers. I estimated the joists span 17 - 19 feet. The original plywood was in good condition (not soft) but I could tell from some staining that it was exposed to water once or twice around the toilet. I was able to view that area of the plywood from below through an access panel in the adacent room and it showed no evidence of water damage. I also added some exterior screws in a couple areas along the joists to clean up some squeeks. Even so, I decided to add 1/2" ACX plywood because I didn't want to use the bare minimum subfloor recommended for DITRA. Also, the closet flange hole had been cut relatively far from the drain pipe in a square shape with a circular saw (there were partial depth cuts into the plywood at all 4 corners of the flange hole).

The ACX is installed parallel to the floor joists with 1 5/8" course thread exterior screws every 6 inches around their periphery and every 8 - 9" in the interior areas. Two sections are about 30" wide and span across two joists. 2 other sections are only 14" wide and span just one joist. I staggered the skinny and fat widths so their seams don't line up. I am pretty sure I didn't hit any joists with screws. I marked the joist lines fairly well and stayed at least an inch away.

Is it practical to unscrew the plywood and reinstall ? If so, what would be some good tools/methods to minimize damage on the original subfloor. I would prefer to avoid adding another 3/8" layer due to the added weight and the need to trim doors.
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Unread 03-20-2006, 05:25 PM   #2
jadnashua
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FIrst, you need to measure the length of those unsupported spans...at 19', it won't work, at 17' it just barely meets the deflection specs required for ceramic, and that assumes fir joists in good condition. If they are something else, then they won't work, either.

Plywood is MUCH stronger when layed across the joists. Since the goal is to make the floor stronger between the joists, laying it parallel to the joists means it isn't doing much. But, with the other layer in properly, I wouldn't tear it out, it just isn't adding that much, which it could have if installed the other way.

So, all is not lost, but the length of the span needs review and verification.

You'll need to be careful around the toilet flange to ensure it is fully supported - the flange should be screwed through the finished floor into the subfloor (the flange should sit on TOP of the finished floor and be anchored through it). That area might need some further thought and maybe some reinforcement.
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Unread 03-21-2006, 07:10 PM   #3
Hamhanded
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Thanks

Thanks much for your help and advice Jim.

I took some more careful measurements of the joist span and its about 17.5 - 18.5 feet joist span depending on if the span is measured from the inner or outer edges of the joist supports.

I could not make out any markings from my limited view of the joists, but a county building inspector said building codes for our area have required Douglir Fir or equivalent for many many years (my home is 31 years old).

With the improper plywood installation, close call on the deflection, and age of the home I am resigned to putting vinyl sheet back on the floor instead of tile.

I very much appreciate your help though.
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Unread 03-21-2006, 07:24 PM   #4
jadnashua
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There are ways to fix this if you wish, sister the joists (doesn't need to run the entire length - middle 2/3'rds is okay), or add a support beam, or a supporting wall. A closet built to support the joists works, too, and only needs to be about 2' deep (how about a recessed bookcase?).
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Unread 04-01-2006, 09:17 AM   #5
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Back in Business

Thanks again Jim. I opened up the ceiling underneath my bathroom to see how much is involved to sister my floor joists. When doing so I noticed that the joist support was 2 feet closer to the opposite wall than where I expected. The span is 16.5 feet and my joists are 2 x 12 (11+) Douglass Fir that appear in reasonable condition. No plumbing notches, etc.

I am now worried about that top layer of 1/2" ACX that I laid with the grain parrallel to the floor joists. The area to be tiled should have 4 joists beneath it, but 1 of the joists is positioned farther apart and under the wall seperating the bathroom and adjacent bedroom. As a result, this joist is actually 20" instead of 16" from the others under my bathroom. My toilet, several cabinets, and granite counter top will also be installed over this area and will only have 1 joist directly beneath them.

Do you think it would be best to remove that ACX and install new plywood with the proper grain orientation or add another 1/4 or 3/8 ACX with the correct orientation or just proceed onto Ditra installation with 5/8" original plywood and incorrectly installed 1/2" subfloor. I installed the 1/2 ACX with course thread exterior decking screws with coverage being about 3 - 4 per square foot. I can unscrew these with a screw driver.

Thanks again for any help an advice.

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Unread 04-01-2006, 10:14 AM   #6
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Personally I think you will be OK, bare minimums for sure but OK. The Deflector tool is built using conservative 50 lb load factors. And while that top layer of ply is definitely oriented wrong resulting in a weaker sub-floor that would otherwise be the case, Ditra over 5/8" ply alone meets mins. Assuming Doug. Fir joists, my recommendation; move forward with ceramic tile.

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Unread 04-01-2006, 09:14 PM   #7
Hamhanded
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Lightbulb A new thought

Thanks Mike.

Like yourself, I'm not too fond of building things to the minimum standard. I also hate to waste this chance to take care of my floor and structure with the whole bathroom gutted and torn down.

I had two other ideas that I would appreciate your thoughts on:

I have room to add 1/4" or a little more to the subfloor without a serious mismatch to the adjoining hallway floor (adding 1/4" will put the bathroom about 1/16 higher than the hallway). My idea is to put in baltic birch aircraft grade 1/4" plywood - oriented proper. I believe its strength and bending resistance is greater than 3/8 AC grade plywood. Because the birch does not have an external grade glue, I would give both sides and all exposed edges a good soaking with JASCO anti-mold sealer, and then paint some polyeurathane on top of this in the area and edges around the closet flange opening and along the periphery that will run next to the bath tub. Is this just busy work for piece of mind or do you think it would provide significant help to the subfloor floor structure. I am not keen to use hardibacker or cement board - I very much prefer to avoid the cutting hassles, related airboirne fibers, and taping.

Also, because I am at or near the bare minimums, I am thinking to use a modified thinset under the ditra that is specifically designed for flexibility (the manufacturer even claims it is good down to L/240 deflection).

From what I understand of Ditra and thinsets, unmodified is recommend above the Ditra, but if I wait a couple weeks before grouting I can use the flexible modified thinset there as well.

Thanks for any help on these points
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Unread 04-02-2006, 11:05 AM   #8
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Given the choice between removing and reinstalling the 1/2" and installing 1/4" ply that doesn't meet the standards, I'd reinstall the 1/2". And given that you have another 1/4" to play with, I'd consider installing 5/8" or 3/4" instead.

Or, I'd simply go with what you've already got, since it meets the minimums.

Better thinset is good. However, if you want to use a modified thinset over the Ditra, use Versabond. It's plenty good for your application, and it won't take forever to cure.
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Unread 04-09-2006, 07:42 PM   #9
Hamhanded
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Smile Much thanks Bob

Thanks for the advice on plywood types and versabond thin set.

I removed the improperly laid plywood - the decking screws weren't so bad to remove with a power drill and screw bit. The difficult part was laying out a screw pattern on the new 5/8 ACX so I avoided my previous screw points (and the joists). All is much better now.

Before I start putting in the Ditra/Thinset layers, I am inclined to paint an oil based sealer on the plywood, and in addition paint some polyeurethane overtop of this in the area of the closet flange hole - including the hole's inner edge. The only thing stopping me is whether this might prevent proper curing of the modified thinset under the Ditra. Would it hurt to apply such coatings, or perhaps just around the closet flange/sinks/tub.

Thanks again for your help previously. I have a much better piece of mind with the plywood installed correctly.
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Unread 04-09-2006, 08:17 PM   #10
jadnashua
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Probably not a good idea. If you want the floor to be waterproof, you could use kerdi band on the Ditra seams.
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Unread 04-11-2006, 03:26 PM   #11
Hamhanded
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Rounding Second

Thanks again Jim.

I was going a little overboard with the coatings and definitely overthinking the project. I appreciate your sanity check.

I also want to thank everyone for the help with the plywood orientation. It may be just in my mind, but the floor feels so much better with the face grain laid across the joists. I also did a little more plywood research and quantified what everyone warned me about. Depending mostly on the wood type, ply thicknesses, and ply alternating pattern, the sheets only have 10 - 25% bending strength across the face grain direction compared to bending the face grain. From what I read, many plywoods (including mine) have thick face plys and the cross grain bending strength provided by the thinner interior ply(s) put these sheets at the low end of this range.

I'd very much appreciate anyone's help with two other questions. First, what is considered acceptable levelness ? I have a downward slope running from about 2" in front of the toilet drain to the opposite wall. I laid a 4 foot level along the floor and the bubble is within the markings but definitely nudging one of the markings. To perfectly center the bubble I have to raise the level end about 3/8. I also found a low point along the slope that is about 1/8 lower than the slope line. I think this is unacceptable and wonder if it is correctable with SLC.

I also wonder if I should tape/cover/silcone caulk the gaps I spaced between the plywood sheets I installed. Even if I avoid SLC, I will be putting thinset under the Ditra and will likely fill in much of the gaps.

With my limited woodworking skills, I was able to space the plywood sheets about 1/16 to 1/8 between each other, about 1/8 to 3/8 from the walls, and about 1/16 around the toilet drain pipe.

JohnC
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Unread 04-11-2006, 03:34 PM   #12
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Hi John,

I think you're over-thinking this small project. Are you an engineer per chance?

No silicone, no sealer, and I'm not worried about your joist length in a small room like that. The walls will hold the joists in line.
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Unread 04-11-2006, 09:13 PM   #13
Hamhanded
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Not overthinking Anymore

Hi Jim,

I'm not an engineer, its a far more serious problem than that - Pittsburgh steel mill apprentice ("stilmil prentice") turned thermodynamicist and patent lawyer when the mills started closing down. Lots of issues here.

I enjoy learning the skills I never had the chance to develop. Wood and stone working are totally foreign to me. If I'm lucky maybe I can pass something on to my very young son when he's older. For now, he enjoys water coloring the bare bathroom walls at the other end of the room and asking more questions than I do. I have some high hiding primer when I'm ready to paint, but I'm going to save his work that will be under the sink cabinets.

I'm surprised about not covering the gaps. For a novice, it was quite a job to make the sheets fit with proper gaps and walls out of plumb. Then I might be filling some of them in, especially with the slc.



Thanks again
JohnC
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Unread 04-11-2006, 09:31 PM   #14
jadnashua
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SLC would fix the floor for level if you decide to go that way. You could do it with thinset (well, mediumset will handle the thickness you need, otherwise you'd need a couple of layers of thinset), but slc would be faster and cost more. If you use only the cbu, the reinforcing tape and thinset joins the sheet together to make it one monolithic sheet. The Ditra is a little different, but the combination of the embedding fleece and the flexible towers allow the tile to be decoupled from the subflooring.

If you go with slc, you need to prep the floor properly - this includes sealing holes around pipes (toilet?, other water, registers, etc.) and the room doorways to keep the stuff in the room of interest. You should use sill-seal (1/4" foam) around the edges, and the manufacturer's primer within the requisite timeframe prior to pouring. Then, when you pour, have people and assets arranged so you can make it an efficient assembly line to get it all done quickly. Literally, once you start, you'll be done and cleaning up the mixing bucket(s) probably in less than 1/2 hour, or you did something wrong. Now, getting to that point will take you awhile. SLC, done right (I seemed to get it right the first time, you proably can, too) makes a very nice, flat surface to tile on, very quickly.
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Unread 04-11-2006, 10:19 PM   #15
Hamhanded
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Smile Thanks again for the heads up

John,

Thanks for getting back to me, I was confused by the thread and thought it was jim that wrote first.

Jim,

Very much appreciate the heads up and details on the SLC. I also forgot to say thanks for the previous joist sistering and closet/book case ideas. Before I properly identified the joist span, my wife was warming up to a book case downstairs. It would have been a huge challenge for my skill level, but it sure sold well to a skeptical boss!

JohnC
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