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Unread 07-09-2006, 06:26 PM   #1
Mtn Mama
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Location: Elkins, WV
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Nails versus Screws -- Plywood underlayment

New here, so will give a bit of background on one of our projects (I've got a bunch with a complete reno, but will start with this one )

In the kitchen, we're installing hydronic radiant tubing, sandwiched within the floor. Over the floor we plan to install large slate tiles (either 16", 18", or 24" depending on what we can get in the style we want).

We have a 3/4" plank subfloor that is original. On that we have installed 8" wide, 3/4" exterior grade plywood strips with an inch space for the tubing. On top of that we will put 1/2" exterior grade plywood sheets, with Ditra (over thinset) on top of that. Then goes the slate over thinset.

We're only halfway through our layering, and from what I've been reading here there are two things that we've already done contrary to best practices. I'm here to ask, is it that important, and if it is, what can we do to remedy.

First, the installers have used a nailgun (versus screws) to secure the 8" plywood strips to the joists and the underlying planks. The nails are ring shank coated, so you'd think they wouldn't be moving. But do we really need to go through and screw it all down every 6" also?

The other thing is most of the 8" wide plywood planks have their grain running parallel to the joists. The 1/2" layer to go on top of the tubing I'll definitely install perpendicular to the joists. All in all we'll have almost 2" of wood below the Ditra, but with expected expansion and contraction due to the radiant, I just wasn't sure whether the parallel grain was going to be a problem.

I've also read on this site that you screw the lower layer of plywood (in our case the strips between the radiant tubes) to the joists, but the upper layer you do not. Yet on another site I read that you want to secure both layers to the joists with long screws. What's the real deal and why?

Here are a couple pictures to show you what we've got going.... thanks in advance for your advice! I'll be coming back to the well for many more parts of our project! (3 bathrooms, foyer and mudroom in addition to the kitchen , and yes, one custom shower that you've already convinced me to go Kerdi )

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Unread 07-10-2006, 05:58 AM   #2
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Welcome, Larisa!

Ring shanked cement coated nails are fine for securing the plywood, including the top 1/2" layer. Just make sure they are no more than 6 to 8 inches apart.

The middle layer (with the tubing) is just a spacer. There's no appreciable strength there, since it's all cut up. It doesn't matter which way the grain runs. I don't think there's going to be very much expansion one way or the other.

The top layer should be oriented with the face grain perpendicular with the joists as you said. The nails or screws should penetetrate the subfloor planks. You don't need to worry about hitting the joists. Avoiding the joists with the second layer fasteners comes from the direct-to-plywood method of tile setting. It's simply easier to keep this rule than to remember the exceptions.
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Unread 07-10-2006, 07:28 AM   #3
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You're OK so far

From a structural concern, you don't need to worry about the nails used for the spacer ply, Larrisa, but it would be a good idea to make sure the spacing is consistent and no more than 8 inches apart. More constructive would be to insure that there are no errant nails or loose edges which could cause slight movement sqeaks under the tile, eventually leading to a popped tile, or cracks in the grout lines of your slate. I would use 2 1/2" decking screws for your top 1/2" layer, attaching through the subfloor planks, without any special regard to the joists. I would, however pre-drill, countersink and prime with wood glue or caulking each screw when securing the top 1/2 ply. I would also use a little P&L or similar glue in between the 1/2" & 3/4" strips over high traffic areas.

At the least,Tape & mud (thinset) the joints formed accross your 1/2". You could go one further and use a crack isolation treatment as well. You have the right idea with the grain.

Looks like a fun project...
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Unread 07-10-2006, 07:49 AM   #4
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Welcome, Larissa.

Mostly what Injineer Bob said. Of course.

I'd like to see you run this by Schluter for an opinion (800-472-4588). You've technically met the requirements for a stone installation if you didn't have the extra "spacer" layer in there with the hydronics. I'm curious to know if they think that makes any difference in their specs.

Tell'em just what you told us. They might wanna see your most excellent photos, too. info@schluter.com

Then please tell us what they tole you.

My opinion; worth price charged.

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Unread 07-10-2006, 11:09 AM   #5
Mtn Mama
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Hey, thanks for all the great advice. We'll definitely do the thinset on the end loops, that's a great idea. They groaned and fussed when I mentioned pre-drilling each hole and priming with glue (it's a 17' x 17' area), so I don't think I won that one. They also plan to use 2" versus 2-1/2" screws, and especially with countersinking, I think that should get it through the 2" of lumber (3/4 + 3/4 + 1/2 ... and of course those are spec measures, not true).

I sent an e-mail to Schluter as suggested, and attached this latest photo of where we're at now.

And oh yes, this is a really fun project. I'm in hog heaven
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