# Underlayment for marble tile



## SMLWinds (Dec 19, 2012)

I am putting down marble floor tile for the first time. I have the deflection of the floor figured out and need to put down 3/4" underlayment over 3/4" heart pine tongue and groove prior to ditra and tile.

I have gotten very mixed reviews on how to get down the underlayment. The first thing people have told me is that I want to MISS the joists. Several people have said this but it makes no sense to me. Why do you miss the joists? I assume it is something with the deflection of the floor.

Secondly, I haven't gotten a great answer about what screws to use. Everyone says "deck or underlayment" screws but not wood screws.

I have a Simpson Quik Drive that I was going to use. I was looking at these screws, which are under "deck/dock" but also say they are wood screws:
http://www.fastoolnow.com/wsntl134s.html

If those will not work or are not what you would recommend here is the complete line up of Quik Drive "deck/dock" screws:
http://www.fastoolnow.com/quikdrive-fasteners-deck-dock-screws.html

Thanks in advance for your assistance!


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## PrecisionFloors (Jan 24, 2006)

Is your plan to lay the marble directly to the plywood?


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## Inner10 (Mar 12, 2009)

SMLWinds said:


> I am putting down marble floor tile for the first time. I have the deflection of the floor figured out and need to put down 3/4" underlayment over 3/4" heart pine tongue and groove prior to ditra and tile.
> 
> I have gotten very mixed reviews on how to get down the underlayment. The first thing people have told me is that I want to MISS the joists. Several people have said this but it makes no sense to me. Why do you miss the joists? I assume it is something with the deflection of the floor.
> 
> ...


Using a screw with a short shoulder on the joist can pull the subflooring off the joist and break the glue bond.


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## SMLWinds (Dec 19, 2012)

No, the tile is not going directly over the plywood. I am putting thinset, then ditra, then thinset, then the marble tile.

I think I understand what you are saying about pulling the subfloor off the joists. So you guys do confirm that I should intentionally miss the joists, right?

Now...what screws do I use to do that?


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## cleveman (Dec 28, 2007)

This screw business is pretty serious.

Lets take a screw, for example, with threads all along the shank, from the head to the point.

Let's assume that the plywood is above the floor sheathing by 1/4" and you put the screw in that way, with the gap there.

You go to suck down the plywood and it can't happen. The threads are holding the plywood up.

The plywood can't slip down because the threads on the screw are holding it up.

Now let's say you have a screw with a 1" smooth shank on the top and 1" of thread towards the end. You screw it down and the threads bite the floor sheathing or joist and the smooth shank is going through the plywood above. The head will force the plywood down. There are no threads on the screw in the plywood holding it up.


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## astor (Dec 19, 2008)

There are special screws for deckings I have used. The screw has two direction of treads. I will look in my truck to see the name/brand tomorrow.
Ok it is called Reverse Thread Trim Head Screws. I am not sure if they are available with regular head though.


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## Inner10 (Mar 12, 2009)

SMLWinds said:


> No, the tile is not going directly over the plywood. I am putting thinset, then ditra, then thinset, then the marble tile.
> 
> I think I understand what you are saying about pulling the subfloor off the joists. So you guys do confirm that I should intentionally miss the joists, right?
> 
> Now...what screws do I use to do that?


Or I suppose you could use really long screws where the joists are...


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## GO Remodeling (Apr 5, 2005)

Your 3/4"t+g pine is strip flooring on a subfloor?

What you are referring to with missing the joists is in the Schulter handbook. Basically, there is more stress over the joists. In addition, there is more deflection between the joists. To better relieve the stresses, the plywood is laid 1/4 of the joists bay past the joists. For a 16" o.c. spacing, the plywood (4' side) is moved 4" past the joists. You do not screw into the joists. Your screws are only long enough to connect the two plywoods.

On the long edge of the plywood, the seam overlap is 24". The plywood is the strongest when the long edge is laid perpendicular to the joists.


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## SMLWinds (Dec 19, 2012)

That was an extremely helpful post! Thanks so much for providing that information!

The T&G heart pine is the floor from when the house was built in the 1800's. It is in great shape but is laid directly only to the joists. I am putting the 3/4" exposure 1 underlayment grade plywood over that. When calculated with the joist size, unsupported span, etc. I easily meet the deflection standards for marble.

So, back to the question of screws. Do those I picked out for my Quik Drive look like a good choice? If not, are there any Quik Drive screws (see the link I posted) that would be better? Thanks!


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## bowtie (Feb 4, 2012)

Ring shank nails are the best option ...


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## GO Remodeling (Apr 5, 2005)

I'd use screws.


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