Subfloor Questions

Thanks again oz. I am really starting to think about putting down 1/4" plywood and the ditra giving me 3/4" doing this would mean I do not have to re-hang the doors which is a bonus.

The kitchen is 12' x 12'6" that leads to a hall way that will be 9' 9" long x 3'3"

 

1. assess your floor stiffness: L/360 midpoint deflection or better is good for ceramic/porcelein, .. L/720 or better for stone..

2. adding a second plywood layer (1/2" or thicker) can substantially reduce any deflection occurring in the subfloor "between" the joists...

3. www.johnbridge.com is your friend!

 

First you might want to explain the concept of L / 360 , L / 720

 

Definitely go use the deflecto calculator over on the johnbridge forum.

I ended up needing to add another layer of 1/2" ply to the 3/4" in the bathroom I'm doing bc of the 24" oc joist spacing even though I passed the deflecto.

The 1/4" ply isn't going to do much.

Make sure you lay your ply with the face grain perpendicular to your joists. Stagger your seams from the existing seams.

 

Get underneath and reinforce the framing , there is no way in hell you are going to acheive L/720 with 2x8's on 16" centres spanning 12'

 

I like the concept of the Ditra product where it allows the tile assembly to "float" independently of the subfloor.
Looking at some reviews it appears to be important to use the correct products with the Ditra. Apparently above and below the Ditra different products are required, with a wood subfloor.

The 1/4" ply is not going to give you much added value for your effort.
A stated previously add a 1/2" layer to your existing 5/8" if that thickness doesn't complicate your transitions. Do not "stack" joints with the existing 5/8". Use construction adhesive and screws to laminate the two layers.

 

depending on substrate , yes

Read the install instructions OP

 

midpoint deflection (joists), as a function of unsupported span, "L",... basically it's derived from the "beam theory" (i.e. mechanics of materials, beam deflection)... this is significant from a structural standpoint because more deflection is indicative of more "curvature" of the beam/joist axis, under load, ...it could cause tile to crack... www.johnbridge.com has a "calculator" for this, ..i can't remember if it assumes a point load at midspan or a uniformly distributed load, ..or something to that effect...

...the notion is that problems "could" arise if you don't follow this spec... i.e. tile or grout could crack over time, loading, ...if the actual deflections are higher than "allowable" in the specs..

 

I know what it is , I meant the OP probably does not

 

This is correct I did not know.

I used the deflection calculator on john bridge website. The unsupported length of the joists are 11' 1" and the calculator gives me a deflection of L / 377 when I use the closest option of 11"

If I use 11'6" which is the other closest option the deflection is L / 337 which isnt enough.

So it seems like the floor is boarderline from a deflection perspective. I feel like I should add some blocking as well.

 

Do you have bracing between the floor joists (X-bracing). That can stiffen a floor up quite a bit because it makes all the joists move and deflect as a single unit rather than individually.

Also, keep in mind those tolerances are what you'd see in a commercial building spec. Your kitchen shouldn't see nearly the amount of traffic or load as a commercial building.

The Ditra product does basically the same thing. It doesn't allow the subfloor to deflect because in order to do so it would need to tear the product. It's not a miracle worker but sounds like a good idea if your floor is 'borderline'.

 

So it turns out that a that subfloor is 19/32 which I found by getting up on a ladder and reading the stamp on the wood. Im learing just a little slow.

As for bracing I have metal x between the joists just not alot of bracing.

 

That metal bracing does make the floor more rigid. It makes it so in order for a single floor joist to deflect (bend under load), the adjacent floor joists must deflect with it. I'm not sure the calculator takes that into account (I didn't go through and find the calculator on the John Bridge website).

 

So the question is should I add additional blocking to the floor? If so should I use 2x4, 2x6, 2x8 and should I screw and glue from the top and toe nail the bottom? Also if I do add the blocking how far apart should it be.

 

If it were me, I'd only add blocking to support the edge of the new sub floor you're installing and it'd only be a 2x4 sitting flat. It looks like you've got plenty of support there for your floor and you can decide if you want to mess with the ditra product. I haven't looked into the price of it myself but it seems like a decent insurance policy for laying tile (there may be other brands that offer alternatives if the ditra product is too expensive).

 

Your existing metal bracing should be sufficient. If any of it has been comprimised just add some wood blocking. You could do 2x4's X-braced or just use 2x8 blocks. If you use 2x8 blocks just stagger them so you can nail from the side rather than toe-nail them. Add some construction adhesive before you put the block in to glue to the subfloor. Then try to screw from above if possible. There's no such thing as overkill haha.

 

Very true...

 

I think I am going to leave the subfloor at 19/32" as it is solid and doesnt creak and bounce and add a row of 2x6 blocking down the seams if there is any and use ditra and call it good as ditra is supposed to work if the deflection is over L360 with a 5/8" minimum subfloor. I really if possible do not want to add additional subfloor as it will make for some nasty transitions and I will have to rehang 3 doors

 

Why would you have to rehang them? Just undercut your jambs with an undercut saw and possibly trim a little off the bottom of the door if needed.

 

Schluter Ditra is the way to go. No cracking. I would contact them directly and give them an idea of what you have as far as the floor is concerned. They have guidelines for just about every situation. They would be able to tell you if your subfloor, joists, and joist spacing is adequate. They are professionals and they know this stuff in and out.

Side note: make sure you use the proper thinset under the tiles if you use Ditra. It is different than if you went directly to a cement board or on the plywood itself. Again, Schluter will be able to better guide you on that.

That's my 2 cents worth.