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Dyoung 03-19-2010 06:15 PM

........remaking the "BOMB SHELTER!"
The house was built in 1959.

Our bathroom studs had over 1.25 inches of cement poured for the walls and floor. A 3/4 subfloor existed over the joists. Atop that lay the 1.25 inches of cement....atop of that lay the mortar and tile. When both the wooden and cement subfloor are added up, we get about 2 inches in height. With the mortar and tile the entire floor was about 2.3 inches overall. We affectionately called the bathroom with all that cement the Bomb Shelter.. (and deep down there might be something to it given that we live near Washing DC and an air force base....hmm)

We had to replace the wooden subfloor which was incredibly water damaged, and thus the cement subfloor went also.


The repair (to at least bring the bathroom floor height up to a similar height) called for 2 x 12 pressure treated planks that span five joists spanning the bathroom. The rationale for the pressure treated was the potential of water damage and frankly because the quality of the lumber was better than the standard stuff. I talked to various contractors who thought the idea of using PT was just fine.

Thus the PT planks sit off the joists just shy of about 1.75 inch.

When the .25 inch hardiebacker is layed atop it.......the combined PT and hardiebacker come to just about 2 inches.

Now, here is my question.

Given that I have a 1.75 thick wooden subfloor and will be using the hardiebacker, do you all think its necessary to put mortar down UNDER the hardiebacker as is called for in various "official" instructions (which I suspect are based on 3/4 subfloors....but we have another 3/4 more thickness and thus stability). There is a rationale in that it helps to provide a continuously smooth underlying base for the hardiebacker, but somehow I think it may be overkill in this case .....rather that the mortar is best placed under the tile, to give it a smooth base.

Note that we are using these kinds of floor tiles

Ideas and thoughts welcome gents!

Im hoping to get this done this weekend...


tpolk 03-19-2010 07:51 PM

i dont think thin haribacker on press treated planks without at least a layer of 5/8 ply under hardibacker is such a grand idea. I'm thinking lots of movement is on the way. no ply on top of jousts? are these planks press treated t&g decking?

oh'mike 03-19-2010 08:30 PM

Oh,Boy.--Sadly pressure treated treated wood shrinks--Did you notice whether the boards were heavier than a non treated board of the same size?

If it was heavier --it was wet--

A better way to raise a "sunken" floor would have been to sister on new joists--That method also lets you level and flatten the floor.

On to Hardie backer or Durrock--That should not be attached directly to planks of any kind--to much movement in natural wood. You will need a layer of ply-wood on top of the planks.

You also asked about firm-setting down the Hardy backer---That is a must-do.
Hardie is so stiff that it will "bridge" over any low spots--You put tile over that spot and a heavy person steps there--CRACK--

Durrock is much more forgiving --it is soft enough to sag into the low spots and not "bridge"

I always firm-set any of the backer boards--Very cheap insurance -----Mike----

Dyoung 03-19-2010 09:24 PM

Few things I forgot to add which you guys brought up:

Yes PT does shrink (well all wood does). But I should have noted that the PT planks have been down on the joists for over a month now....and there is a heating duct that was removed, so its blown under the planks steadily (thank you God). My gut tells me that most of the structural shrinkage has already occurred; and its winter. I can see nice 1/8 to 1/4 gaps between the planks much like the original floor.

In anticipation of any cupping or gaps, I also sanded down some cups in a few planks and its pretty dry down as far as 1/8 inch. No wetness to the touch.

I would tend to think that PT planks around 3/4 inch (as with original subfloor) would give thus requiring the Durrock. In this case the planks are literally twice that thickness and the joists average about 14 inches apart or less (and the outside joists are doubled up). This summer I plan on putting in another joist anyway. 3.5 inch screws are driven into the joists. My sense is that the planks are pretty well planted. I am 6'7 and weigh about 350. When I put weight on them, no movement or dipping in the wood that is apparent. I suspect with the mortar and hardiebacker it should be stiff as a rock. And my hunch is that the smaller tiles will give and flex more than a larger solid piece. ...seems logical anyway.

oh'mike 03-20-2010 08:13 AM

Hi there DYOUNG--I stand by what is said about backer board over natural wood planks------

Only time will tell---I certainly don't know every thing. I only speak from my own experiences.

Tile work is like a tattoo--it's going to be there for a long time---best to get it done right---Mike--

Dyoung 03-20-2010 09:09 AM


Originally Posted by oh'mike (Post 417205)
Hi there DYOUNG--I stand by what is said about backer board over natural wood planks------

Only time will tell---I certainly don't know every thing. I only speak from my own experiences.

Tile work is like a tattoo--it's going to be there for a long time---best to get it done right---Mike--

Indeed I generally agree Mike. From what I can tell however the planks are pretty solidly planted

I also took the belt sander to the cupped planks (2 or 3) and sanded them flush with the others. Right now there is, at most, about a 1/8 gap between the height of each butted up plank....which is probably about what most subfloors are anyway. With the mortar it should even off quite well. And I doubled checked the spacing in between the planks. They have dried to a healthy 3/16 to 1/4 gapping, most likely due to winter and the heating duct blowing directly underneath the planks for the better part of 4-5 weeks straight.

Wish me luck! Im gonna lay the mortar and hardiebacker down today.

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