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Unread 07-09-2006, 06:26 PM   #1
Mtn Mama
Registered User
Join Date: Jul 2006
Location: Elkins, WV
Posts: 5
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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