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Motordoctor 02-09-2015 06:02 PM

What size screws for underlayment
2 Attachment(s)
Hello all, I thought I would ask this question here instead of dogging the poor guy that's been helping me on another forum. He pointed me this way so here I am. I'm redoing a kitchen floor, had to take it down to the subfloor. It had some hideous dark brown wood tiles in a house that has Oak hardwood floors in the rest of the house. Under the wood tiles was black felt, followed by glue, followed by vinyl tiles, followed by some gray gasket looking paper, followed by more felt that I couldn't get off. I figured it would be faster and do a better job just tearing up the old plywood and putting down some new stuff. So I sawed it into manageable chunks and pried it off. Probably a good idea since the subfloor had a couple of water damaged planks that I replaced. These are 7" wide and overlap, not a true tongue and groove. I put down some Poplar 1X8, actual size was 3/4 X 7 1/4. Hopefully that is the right stuff to use, didn't have a lot of choices other than pine, poplar or red oak.

I'm going to put down on top of the 3/4" subfloor, sheets of 1/2" "Plytanium. It's rated CD and exterior. On top of that will be 1/4" Hardibacker and then 18X18 tiles.

So on to the questions: What length screws should I be using on the plywood? I had gotten 1 1/4" deck screws but those will only give 1/2" engagement into the subfloor. Better to get 1.5" deck screws? I would have like to put down some thicker plywood but it will create a bigger transitional problem from the living room into the kitchen. With what I described, the tile will be above the existing oak floor about 7/16". The only other option I saw was to put down 3/8 plywood but I felt that might be a little too flexy. Thanks.

More info
House built in 1950
Deflecto rated L /842
2x8 joists on 16"
140 sq ft kitchen
subfloor 3/4" overlapping planks laid on diagonal

Steve in Denver 02-09-2015 06:58 PM

I'm not a pro, but I'll take a stab at what they are going to say.

- Make sure the existing boards are securely fastened to the joists.
- The CDX plywood isn't suitable - it can have voids that leave some areas unsupported, so you need something with all layers with a C grade or better (and with exposure 1 / exterior glue rating)
- You want to screw your top layer into the subfloor only, not into the joists.
- Your floor will need to be very flat to get good results with the tiles you are planning on using.

I've gotten this detail wrong in the past (and CX or someone will correct me if I'm wrong) but I think the spec is a maximum 1/4" deviation in any 10 feet and 1/8" in any 2 feet.

Have you considered removing the existing subfloor, taking that opportunity to correct any flatness issues with the joists, and installing a 3/4" T&G plywood subfloor?

cx 02-09-2015 07:09 PM

Welcome, Dan. :)

That deflection rating is a bit suspect. You do understand that you're not looking at the dimension of the room or tile installation, but the unsupported span of the joists regardless what's on top of that span?

Steve's got you covered on your plywood selection.

But for the flatness requirement for 18" tiles the tile industry requires no deviation from intended plane of more than 1/8th-inch in ten feet nor 1/16th-inch in two feet measured from the high points in the floor. That is a very, very flat floor and you'll wish it were even flatter when you begin setting those big tiles.

My opinion; worth price charged.

Motordoctor 02-09-2015 08:07 PM

2 Attachment(s)
The deflection is based on 14ft joists, 16" O.C. and the joists are supported @ midpoint by a basement bathroom wall. The wall is framed with 2X4s and is sheet rock with tile, may have backer board, not sure. Sorry I was wrong on the plywood rating, had to search for it. Included a couple of more pictures. I was going to put a dial indicator on the bottom of the center joist and have about 400lbs worth of people stand right above it to measure the deflection. That sound like a reasonable way to measure it?

Motordoctor 02-09-2015 08:33 PM


Make sure the existing boards are securely fastened to the joists.
I'm planning on putting a deck screw through each plank into the joists.
- You want to screw your top layer into the subfloor only, not into the joists.
- Your floor will need to be very flat to get good results with the tiles you are planning on using.
To check the floor, what would be a reasonable method? A ten foot level or ?

Have you considered removing the existing subfloor, taking that opportunity to correct any flatness issues with the joists, and installing a 3/4" T&G plywood subfloor?

I hope it doesn't come to that but if that's the only way to make this work I would do it. I don't know where to get that plywood at. Here in NYC we are technologically backward, I'm stuck shopping at Home Depots and Lowes.
If I check the floor and it's not to spec, flatness wise, is the self leveling compound a viable option?

Steve in Denver 02-09-2015 08:52 PM


Originally Posted by CX
1/8th-inch in ten feet nor 1/16th-inch in two feet measured from the high points in the floor.

I'm not sure I'll ever get that right. I think it has something to do with my apparent inability to actually achieve those results...flat, indeed.


Originally Posted by CX
and you'll wish it were even flatter when you begin setting those big tiles

And correct use of the subjunctive to boot. Who is this guy? ;)

cx 02-09-2015 09:40 PM

Steve, the requirement changes when your tile has one side measuring 15 inches. Smaller than that it's 1/4" in ten feet and 1/16th" in one foot. When you get to 15 inches on a side, the tolerance is exactly half that or 1/8th" in ten feet and 1/16th" in two feet.

Achieving the higher standard is, indeed, quite difficult. It's generally accepted in the industry that only with a mud bed or properly installed SLC can it actually be done effectively. I've heard it can be done with poured gypsum, but that's not something usually encountered or considered in residential work.

As for the grammar, if y'all was from Texas y'all would know we most all gots real good grammar hereabouts. :)

Dan, your plywood grade and exposure rating are fine. The nominal 1/2" is the minimum industry requirement for installation over board subflooring, but that presumes your boards to be T&G and to be oriented perpendicular to the joist structure. I generally recommend a thicker material when the boards on on a diagonal because their span is a good bit greater. I have no authority for that recommendation.

The support wall doesn't really qualify. To act as a load bearing wall it must have at least a doubled top plate or have the studs stacked directly below the joists. You could add some studs, with appropriate bracing, and also ensure you have firm contact at the bottom of each joist. You may need some shimming for the later.

Trying to measure joist deflection in situ is not a realistic endeavor in most cases. The design deflection, which is what building code and the tile industry rely upon, is calculated using a fixed load, most commonly 40 pounds per square foot) and known values for joist size, wood species, and grade. There are other factors, too, such as the dead load and material condition that also factor in. You can certainly do as you suggest, but the result won't actually tell you what you need to know.

If you improve your support wall you should have very good joist support for your tile installation.

My opinion; worth price charged.

Motordoctor 02-09-2015 09:41 PM

How many screws should I be putting down in each sheet (4x8)? According to this site, it's about 220 per sheet :scratch:


cx 02-09-2015 09:48 PM

I recommend you not pay any attention to that site, Dan. Different kinda product for different kinda applications.

Here is a good article from our Liberry that I think shows the very best way to install a second layer of subflooring for a ceramic tile installation. I recommend you adapt as much of their method as you can make fit your application.

My opinion; worth price charged.

Motordoctor 02-10-2015 09:04 AM

Well, I'm not married to the 18" tiles, I think I could take them back since I only opened one box. The fun part is going to be carrying 700lbs (70 lbs/case) worth of tile out of the garage down a snowpacked ice covered driveway to load them into the back of the Jeep.

I bought them as a typical rookie would: "they were on sale". Knew only enough about tile installation to be dangerous. Thought putting down bigger tiles would be less work than 12X12s. Walking through the local HD with the wife, saw the tiles, said "do you think these would look nice in the kitchen?" She liked them and the color so I came back and bought them. HD had them stacked in a display so I was thinking that they were closeouts and I should buy them before they were gone. Rookie mistake #2. Oh well, live and learn. Better to know now then spent hours putting tiles down only to have them start cracking. What started out as thinking that putting down some nice tile shouldn't be too much of a pain and doing the work myself would save lots of money has become "the floortile monster project". Regardless of what direction I take, the old floor had to go, it was hideous and starting to come off so something had to be done.

So on to the questions:

• I can get to the bottom of the floor joists and put a 2X8 support column under each one at the midpoint next to the existing bathroom wall. Should I do it while the floor is at its lightest, with only the subfloor on the joists or do it after I put the plywood down and Hardibacker?
• Should I predrill the screwholes in the plywood? My thought is having the pilot holes will reduce the subfloor deflecting away from the plywood when you drive the screws.
Thanks for your advice.

cx 02-10-2015 09:28 AM

1. I suppose you could, but looks like it'd be a lot easier to add a few studs to your existing wall. And single stand-alone 2x8s won't really provide the support you need. Too much flex if not at least stabilized mid-span. You'd actually do better with just doubled 2x4s in that application and that would be somewhat questionable.

2. That's referred to as screw-jacking and yes, pre-drilling the new layer will help prevent it.

My opinion; worth price charged.

Motordoctor 02-10-2015 10:09 AM

The reason I suggested that method was of course because it would be the easiest. The bathroom wall isn't accessible the full width of the kitchen floor to put anything else. I would have to take out the furnace, take out the return ducting to get to it and probably knock down a wall in the process. But if it won't work, wishing it would doesn't change anything. I guess maybe I should consider some other floor options. Too bad, it would have looked good with the tile in there.

cx 02-10-2015 10:19 AM

Dan, I'm judging only from your posted photo and description. I can't see the rest of the problems. All I can tell you is that you must have a mid-span support under those joists to achieve the necessary rigidity. How you do that is negotiable.

If you have access to a line under the span that is clear enough to install a support under each joist, make that a wall with a double top plate or studs stacked directly under the joists and sheath one side or add adequate blocking. Is that feasible? You need to reduce the unsupported span to ten feet or so.

Or tile over what you've got. Your house, your tile, your decision. :)

cpad007 02-10-2015 10:52 AM

Do things change at all for you if you removed the solid wood plank floor? Does having access from above help you? You might be able to sister new joists to the old ones or do a combo of improving the support and sistering. You'll need at least 2/3 span for sistering to help. I dunno if you have a bunch of plumbing and/or electrical in your way for sistering.

Removing the solid wood planks might get you to the flatness those large tiles need because you can then fix things at the joist level.

Motordoctor 02-10-2015 06:08 PM

5 Attachment(s)
Doing the sister thing would be hard since it's over the bathroom with all the electrical wiring and can light poking up under the subfloor. Here's some more pictures, it's the backside of the bathroom wall, in the laundry room, from left to right.

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