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2long4u 01-22-2009 02:21 AM

Bathroom subfloor question
I am going to be redoing my bathroom. We are going to be putting tile on the floor. My mother in law's boyfriend is going to be doing it and is a tile setter by profession.

My question is this: He said to remove the plywood subfloor and put down 1/4" hardy backer. I was confused and said that that wasn't ridged enough. I asked him if he was going to float concrete on top of it and he said yes.

Is that rigid enough for the floor?

2long4u 01-22-2009 02:27 AM

I hear a lot about people using concrete board for the shower. He always puts a moisture barrier on the drywall then puts up chicken wire then floats the concrete. He says it will be more level, plumb, and will be more water proof. Is that correct?

buletbob 01-22-2009 05:28 AM

I personally wood not rely on the 1/4" hardi board just as a subfloor with a mud job!. any mud jobs that I have come across was installed on 3/4" 1x8 T&G over the tops of the joists or the subfloor recessed 2" down between the joists with the tops chopped at a slight angle and then floated. or just 3/4" plywood.
I would not do it that way. leave the subfloor and float over it. if you remove the subfloor You will compromise the integrity of the entire floor system. with proper framing there should be a double joist under each wall partition but I have come across plenty of remolding jobs where there was not, if this is true in your case what happens to the wall that's on top of the subfloor when you cut the subfloor out with no double joist?? :huh: BOB

angus242 01-22-2009 06:50 AM


The subfloor needs to be at least 3/4" T&G plywood. The "mud job" would get laid directly over the ply with a layer of lath nailed or stapled down. The mortar bed needs to be at least 3/4" thick. Now you can worry about tiling.
Are you sure he's planning on an actual mortar bed? If not, 1/4" cement back board over the 3/4" T&G and then thinset for tile is an acceptable method too. This is, of course, if the joists meet minimum deflection ratios.

JazMan 01-22-2009 09:03 PM


I'm going to bet you didn't mean so say you were going to remove the subfloor. I think you were told to remove the underlayment. :thumbsup: I don't understand about the floating part though. If he's planning a mud job, you don't need the 1/4" backer first.

If the 'shower' is a stall as apposed to a tub/shower, the details can get lengthy. If there is a membrane behind the wall board, (plastic or faced insulation), he should not put a membrane over it too. It would be better to do the mud job, then apply a membrane over that.

Please reply if it is a stall shower as there is more to that then a tub surround.



2long4u 01-22-2009 11:25 PM

In our house he is just doing the tub surround which he will float the concrete on the wall. For my mother in law's shower he put a special drain, put come concrete down with a slope towards the drain then put down a piece of rubber then more concrete.

The toilet leaked in my bathroom. I don't know what you would call the floor I say subfloor but it is the plywood over the joists. It didn't really feel soft but I don't want to take chances. I was just going to set the skill saw to the depth of the plywood and cut around the room and put down new plywood.

Thanks Jon.

buletbob 01-23-2009 05:50 AM

I would just cut out a section of the area around the toilet. just a section. don't need to remove the entire floor and compromise the intire floor system. just install some blocking under the seams of the piece you installed.
I have seen parallel walls belly down the subfloor with no double floor joist. IF THIS IS TRUE IN YOUR CASE ONLY. when you cut out the entire subfloor and install new your seams are on one joist instead of being 4' staggered. and the weight of the mud and tile on your floor will settle even more over time. resulting with cracks on the walls and even the floor along the wall and floor joint. BOB

2long4u 02-02-2009 08:47 PM

Update. I'm just going to remove the underlayment and install new 1/4" backer board.

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