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Unread 11-21-2014, 10:42 PM   #1
Lou_MA
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Subfloor orientation parallel to joists - problem?

I'm starting a project where the subfloor has already been installed. It's 3/4" tongue-and-groove Advantech, glued and nailed down, but with the long dimension parallel to the joists. Total room size is about 10' x 12'.

I found this load table from the manufacturer http://www.huberwood.com/assets/user...%282014%29.pdf.

The joists are 16" o.c. and meet L/360. From the table, I can see that the load table value for strength grain installed parallel is only about 40% of what it would be had it been installed perpendicular, but I don't know what the situation calls for.

Is there a certain load value I need to meet for a tiled installation?

If I'm not meeting that value, would a layer of properly installed 3/8" ACX ply over the Advantech solve the problem?

As an aside, the project will be Ditra Heat, and then 6" x 36" plank tile.
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Unread 11-21-2014, 11:28 PM   #2
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I'm sure the pros will have something to say about it, and If I had to guess I don't think it's going to be favorable.

I suspect an additional layer will be able to make up for the deficiency, but I think you will need at least 1/2" if not 5/8". Just guessing - I'm sure CX will have an opinion.
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Unread 11-22-2014, 12:13 AM   #3
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I wonder if installing a sub-floor the correct direction over the incorrectly directing subfloor might make up a bit but I don't think I'd sleep well at night knowing my subfloor was weaker than it should be....
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Unread 11-22-2014, 12:52 AM   #4
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Lou, for the point load requirement for ceramic tile, that subflooring would be the equivalent of something less than nominal half-inch plywood. I'm presuming it's a five-ply panel, which I think is correct, but I don't have any Advantec material in the shop.

That being the case, I'd accept nothing less than nominal half-inch material, properly oriented, over what you've got and I'd want it glued and screwed to the existing.

Keep in mind also that all your current between-joist seams will not be T&G and should have had blocking installed under them.

I sure gotta wonder why someone would install that material that way.

My opinion; worth price charged.
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Unread 11-22-2014, 07:38 AM   #5
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CX - I couldn't find anything in their online literature, and their Help line is closed today, but my understanding was that Advantech, as an OSB-type product, didn't have ply layers.

Quote:
Originally Posted by CX
for the point load requirement for ceramic tile, that subflooring would be the equivalent of something less than nominal half-inch plywood
  1. How much of a point load is a ceramic tile install required to meet? Is there a minimum value?
  2. How did you calculate point load of Advantech? From their uniform load table?

    Quote:
    Originally Posted by CX
    I'd accept nothing less than nominal half-inch material, properly oriented, over what you've got and I'd want it glued and screwed to the existing.
  3. Using a full-spread of Tite Bond or similar? For fastening, following the 4" perimeter / 6" field spacing and no-fastener-penetration-into-joists?

    Quote:
    Originally Posted by CX
    Keep in mind also that all your current between-joist seams will not be T&G and should have had blocking installed under them.
  4. My understanding (and I forget the exact source) was that the long dimension of the panel should either be (1.) tongue and groove, OR (2.) supported by blocking from underneath, OR (3) have a second layer of ply, 3/8" minimum, installed over the subfloor, properly fastened and with seams properly offset between layers.

    If I'm doing #3, would I still need to do #1?
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Unread 11-22-2014, 09:14 AM   #6
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I've seen both OSB and plywood marketed as Advantec, Lou. The difference between the rigidity between the two orientations might be a little less simple to determine in the OSB, but it's the same principle.

1. 'Bout 300 pounds.

2. You can't calculate it for any material, far as I know, you get the data from lab testing, generally using a Robinson machine - ASTM C627.

3. That woulda been my recommendation for plywood, but I know of no really successful way to bond to the OSB. Might work, might not. I wouldn't even bother trying to glue, were it mine.

4. Not an either/or kinda thing. You must have the between-joist seams of the first layer of subflooring panels blocked, either by panel edge design or as an added feature. That's supposed to apply to sawn board subflooring, too, but we overlook that pretty regularly. I would not consider nominal 3/8ths" plywood in any way acceptable in this application. But that's up to you.

My opinion; worth price charged.
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Unread 11-24-2014, 06:08 AM   #7
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Thanks CX. Aside from the point load requirement, is there a uniform load value that needs to be met?

I'll be contacting the manufacturer today to get their input. But what info should I be asking for? Point load values when the strength axis is installed parallel to joists? Anything else?
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Unread 11-24-2014, 11:55 AM   #8
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The joist structure needs to meet the common uniform live load requirement of the residential building code, which is 40 psf for most traffic areas of the house, plus the standard 10 psf dead load plus any additional dead load added by the tile installation.

Building code generally specifies only a minimum thickness for subflooring and there is no loading or deflection specification that I've ever seen.

Keep in mind that the building code requirements for floor deflection are based upon protecting the ceiling below rather than any floor surface materials.

My opinion; worth price charged.
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Unread 11-25-2014, 07:25 AM   #9
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CX - Current status on the project is that the subfloor will be staying down as-is.

I contacted Huber - the Advantech manufacturer - and they acknowledged the stiffness is significantly compromised, but couldn't offer any specific solutions (aside from removing and re-installing correctly).

I contacted the NTCA, and they said this is a question for a structural engineer (which isn't an option on this project).

Given that - what would give the best chance of success? Finished surface is 6" x 36" porcelain over Ditra Heat.

My plan was to install second layer of underlayment grade ACX ply, 1/2" nominal thickness, properly oriented, offset, gapped, and fastened per the Woeste and Nielsen article.

Should I laminate the two layers together, or not? If so, with what? Huber recommended a solvent or urethane adhesive, and the only full-spread one I'm aware of is an FRP adhesive. I could either use that, use Tite Bond, or skip it altogether.

Any other steps I should take to maximize chance of success here?
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Unread 11-25-2014, 07:38 AM   #10
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Hi Lou,

Been following your thread, what a pain they installed the plywood of the wrong axis.

I'd go with a full lamination of the second sheet to the Advantech using Titebond wood glue. Make sure you get a full spread of the glue on both panels for best results. 6" drywall blade works great to quickly spread out the glue.

Predrill the holes on a 4" separations along the edge and 6" grid in the field. You can use 1 5/8" drywall screws (versus deck screws normally used) since the screws are there to ensure a solid clamping of the plywood sections until the glue sets up.
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Unread 11-25-2014, 08:03 AM   #11
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Lou, there are two things I would do in your situation.

First, I'd use something thicker than nominal 1/2" plywood. 1/2" might be enough, but I'd really want to be sure. I don't have any test results or special training to back up that opinion, but I'd be willing to give up the additional 1/8" of height, or more, to gain the extra strength. That assumes you can afford the additional height. You can use one of the tile membranes to help reduce the overall thickness.

Second, I would use deck screws to attach the top layer of plywood to the subfloor. One of the problems with drywall screws is that they just don't have the strength when being driven through two layers of plywood. What you'll find is that some of them will snap as you try to drive them in flush. And if they happen to snap above the surface, you'll have another problem to deal with.
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Unread 11-25-2014, 10:46 AM   #12
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Quote:
Originally Posted by Kevin
First, I'd use something thicker than nominal 1/2" plywood. 1/2" might be enough, but I'd really want to be sure. I don't have any test results or special training to back up that opinion, but I'd be willing to give up the additional 1/8" of height, or more, to gain the extra strength. That assumes you can afford the additional height. You can use one of the tile membranes to help reduce the overall thickness.
Agree with Kevin, 1/2 may be good but 5/8" is much better. You could also increase the screw grid size to 8" in the field with the thicker plywood.

Quote:
Originally Posted by Kevin
Second, I would use deck screws to attach the top layer of plywood to the subfloor. One of the problems with drywall screws is that they just don't have the strength when being driven through two layers of plywood. What you'll find is that some of them will snap as you try to drive them in flush. And if they happen to snap above the surface, you'll have another problem to deal with.
Kevin has another good point. Certainly doesn't hurt to use deck screws but I don't know they buy you anything when doing a full spread glue AND you pre-drill the top layer. If you don't pre-drill , the drywall screws can snap off or worse, result in the second layer being lifted or "screw jacked" off of the lower layer. Bad, Bad, Bad.

This is typically not an issue with deck screws since the 1 5/8" screws have a long un-threaded shoulders allowing them to pull the layers together without pre-drilling. Personally, I'd still pre-drill even then because the short deck screws are a pain in @$$ to start into the ply otherwise.

edit: Hey, just reached my 2K post! Do I get unlimited use of the BOLD and underline controls now?

Last edited by PC7060; 11-25-2014 at 11:10 AM.
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Unread 11-25-2014, 11:00 AM   #13
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Starting with a first layer of subflooring that is OSB I would not try to glue anything, Lou.

Congratulations, PC. I'll hafta watch my six to be sure you don't catch up to me in the next few months.

You just feel free to bold or underline anything you want so long as you're helping our visitors with their questions.
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Unread 11-25-2014, 11:05 AM   #14
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Quote:
Originally Posted by CX
Starting with a first layer of subflooring that is OSB I would not try to glue anything, Lou.
Huh, missed that the original floor is OSB ("beaver barf"); guess it would be almost impossible to get glue to stick to that stuff.

So..., I'd go with your original plan. Using the 5/8" thick material would be better if you can toloerate the extra 1/8".

Quote:
Originally Posted by Lou
My plan was to install second layer of underlayment grade ACX ply, 1/2" nominal thickness, properly oriented, offset, gapped, and fastened per the Woeste and Nielsen article
.

Quote:
Originally Posted by CX
Congratulations, PC. I'll hafta watch my six to be sure you don't catch up to me in the next few months.
Ha, I'd hafta get a whole lot faster on my little phone.

Last edited by PC7060; 11-25-2014 at 11:11 AM.
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Unread 11-25-2014, 12:59 PM   #15
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Little late to the party here, but

1- what they all said, mostly...
2- how long are the floor joists?
3- what is the room used for?

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