Erich
Erich UberDork
7/30/18 11:44 a.m.

I have my small bathroom down to the studs and joists, and am preparing to install subfloor and tile. Looking for advice. 

I have a sag towards the center of the bathroom that might more than  1/4" (have to measure when I get home) - Plan was to do: 5/8 plywood, 1/2" ply, thinset (even out the dip with some extra) and then cement backing board screwed to the 1/2" plywood. Thinset and tile on top of that. 

Sound ok from the experts or should I use mortar at some point to level the floor?

The tile is 4x5 hexagon so not terribly worried about cracking. 

SVreX
SVreX MegaDork
7/30/18 12:12 p.m.

Either your sag is much more than 1/4”, or you may have the most sensitive feet on the planet!

Your plan is fine except the plywood. The 1/2” ply isn’t really doing anything. I’d use a single layer of 3/4” Advatech, with 1/2” cement backing board on top. 

Most likely, the Advantech will take care of the dip. If it’s bad, I’d shim the joist before laying the subfloor. 

I am not a fan of thinset between layers of plywood.  It can get crunchy. If you need to do it, do it on top of the cement backing board. 

SVreX
SVreX MegaDork
7/30/18 12:14 p.m.

Don’t use mortar. Thinset is fine up to 1/2” thick. 

914Driver
914Driver MegaDork
7/30/18 1:52 p.m.

The floor is down to the joices?  Just marry up new 2X6s next to the sagging ones, just make them level.  I just finished something like that.

APEowner
APEowner HalfDork
7/30/18 3:07 p.m.

First you need to fix the root cause of the dip then sister new level joists onto the old ones.  If the problem was undersized joists then maybe all you need to do is install that new ones.

SVreX
SVreX MegaDork
7/30/18 3:17 p.m.

1/4” is not really a sag. That is within the tolerance or the normal crown/ bow of virtually every 2x available. 

A floorjoist installed upside down (with the crown down) would exceed that. 

Erich
Erich UberDork
7/30/18 4:57 p.m.

yeah it's 1/4", I can only tell because I laid a 4' level across the floor. I'm not screwing with sister joists as I'm past the point where that would be done. 

Probably do a little extra thinset in the low area, above the backerboard, and call it a day. 

Every place I see online calls for a layer of thinset between the underlayment and the cement board. Not how I've done it in the past and seems like it would make it a bear to renovate in the future. Opinions?

 

Nice job 914driver!

SVreX
SVreX MegaDork
7/30/18 5:23 p.m.

In reply to Erich :

I have never done it that way. Mechanical fasteners (nails or screws) have always sufficed. 

However, I do see the manufacturer recommends it:

Hardie Board installation

However, this could vary based on the subfloor. Thinset will not bond to some subfloors (like Advantech). 

I have seen the thinset bed loose it’s bond and crumble between the layers. 

The right answer is probably to do it, but confirm the thinset will bond to you subfloor material. 

coexist
coexist Reader
7/31/18 9:15 p.m.

I always use thinset (versabond, fortified) under the 1/4 HB, cuz I don't trust the 1/4 integrity on it's own. 1/2" durock sometimes bond, sometimes not, depends on application. 

"thinset" is a big category, and some is better at bulk filling than others. In your situation, I would also consider imbedding a layer of mesh in the top of the leveling filler.

coexist
coexist Reader
7/31/18 9:16 p.m.

Should also say that I am always working on CDX, rather than Advantech.

You'll need to log in to post.

Our Preferred Partners
O6Go1E9PITEMBXkEEnBscNTm132vQL8wdfMsJp1TgUgjNOGUa1bdA4hmm7uMPhoK