Skip the durock, add a layer of plywood and then membrane
The durock adds no strength and is kind of useless

 

Durock just adds weight to the floor assembly, adding to deflection. There are much better and lighter substrates available such as Ditra which is waterproof and acts as an uncoupling membrane. Plus it's dust free and one person can carry enough to cover over 300sf in one trip. Add kerdi band over the seams and you can have a fully waterproof install.

 

So a second layer of plywood with Ditra on top? Is that the best practice?

Do I need 3/4 plywood or would 1/2 typically be sufficient?

Thanks

 

I think Schluter says to use minimum 3/8. I don’t know why but I have used 1/4 BC many times. You say you have 3/4 underlayment so use whatever you need to get up to the height you need.

 

As nicko mentioned, add the thicknesses you need to get the finished floor on plane with the other floors.

There is both Ditra, Ditra XL, Ditra heat, 1/8, 5/16, 1/4 respectively. You can use the heat membrane without the heat cable.

Remember the mortar thickness properly collapsed is 1/2 the trowel notch height.

Tom

 

Only a fool would use Durock for anything.

 

If your subfloor is 3/4" as you indicated, then as long as your joist span and spacing meets minimum deflection requirements you are good to put Ditra on the existing subfloor. And as Tom said, use the approriate thickness Ditra for your new install to meet adjacent flooring. Between the Ditra and Ditra XL, you should be able to meet most adjacent flooring.

 

If your subfloor is 3/4" as you indicated, then as long as your joist span and spacing meets minimum deflection requirements you are good to put Ditra on the existing subfloor. And as Tom said, use the appropriate thickness Ditra for your new install to meet adjacent flooring. Between the Ditra and Ditra XL, you should be able to meet most adjacent flooring.

I believe the floor joists are 16" OC which appears to be what Schluter recommends. In terms of adjacent flooring, It's carpet on top of a pad on the same 3/4 plywood. I think the height of the thin set/ditra/thin set/tile will be fine where it meets the carpet and pad.

Back to the demolition ... the existing plywood had Durock adhered with thin set. My plan is to grind the thin set off. That will probably leave the plywood a little rough, bumpy and gouged. Will the thin set and Ditra be enough to bridge that rough surface? Or do I need to lay another layer of plywood to get a nice smooth surface?

Thanks

 

You’ll have to look at this as a whole.

Deflection is what matters, not 16” on center. The sub floor will deflect between the joists.

Make sure you use great duct extraction when grinding. I can’t recommend the Festool RENOFIX enough.

After you’re done removing the mortar you may need to do nothing to the sub floor, just use floor patch in a few areas, a layer of SLC, a layer of underlayment, then the membrane and tile.

A little rough is not a bad thing.

Tom

 

Is the demo started? Might find enough water damage to extract the original subfloor. (Likely not but anything’s possible) never mind just read the post. But still, might be easier to get down to the framing anyway. Little in from the walls with some solid blocking to lock in a 6” edge nailing.

 

You’ll have to look at this as a whole.

Deflection is what matters, not 16” on center. The sub floor will deflect between the joists.

Make sure you use great duct extraction when grinding. I can’t recommend the Festool RENOFIX enough.

After you’re done removing the mortar you may need to do nothing to the sub floor, just use floor patch in a few areas, a layer of SLC, a layer of underlayment, then the membrane and tile.

A little rough is not a bad thing.

Tom

Tom can you recommend specific products for the floor patch, self leveling compound and the underlayment?

Is the demo started? Might find enough water damage to extract the original subfloor. (Likely not but anything’s possible) never mind just read the post. But still, might be easier to get down to the framing anyway. Little in from the walls with some solid blocking to lock in a 6” edge nailing.

Yes the demo is started and the Durock is making us fight for every inch we get off. So far we have not found any water damage. However we will be moving the shower drain so I will have to patch the old drain opening. That old opening will be beneath the new shower floor. I am using a Kerdi shower tray. The framing is open from the basement below. I was going to add nailers and patch the old opening with plywood then do my new shower floor on top of that.

 

I use Schluter products.

Thefloor patch depends on the floor.

SLC, Custom from Home De[pot works just fine.

Tom

 

I have done a few jobs like that and I just removed the durock and the subfloor together because it was faster and left me a better surface. Grinding the thinset is very very messy.

 

I used a grinder on a kitchen floor a few times after removing cement board. I used a grinder dust shield hooked to a shop vac and had very little to almost no dust. If I was doing your job I probably get the worst of the thinset off with a grinder and cup wheel then use quarter or three eights plywood glued and screwed to the subfloor then just standard Ditra.