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Old 05-25-2014, 11:51 AM   #1
centerisl
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Cementitious floor w/ electric heat over wood

I'm remodeling our master bath and could use some advice on the floor.

It's on the 2nd floor, maximum 16' x 10' but tapering at one end (see attached Sketchup image). Circa 1979 construction is (stapled) 3/4" ply over 2x12s 16"oc, and there was a sunken Jacuzzi tub on the right that we removed (itself sitting on ply over 2x10s). I'm acting as the general contractor, but I have an experienced LEEDS contractor working with me on most parts (and making sure I keep to code and generally accepted practices).

I would like a colored / etched cementitious floor - we've currently identified ARDEX SD-T as a candidate but are open to anything else. I have the TCNA handbook and understand that I should segment the area into 8' sections with control joints - currently thinking about going from the side wall of the shower area straight towards the sinks.

Complicating this is that I'd like to use radiant heat throughout, including the shower (probably SunTouch though I'm considering SlabHeat because of the 3x thicker jacket and possibility of a more than 3/4" buildup to surface). We obviously need a decoupling membrane (and a waterproofing membrane in the wet area) - and the current thinking is a Schluter member over the heating (they won't warranty anything that's not theirs on top of the membrane, but Schluter says they have no experience with cement without stone/tile and - despite numerous customer requests - will likely never support a construction without stone/tile). So...we're going to have to go "off-label" here at some point anyway.

My contractor is a big fan of HardiBacker, and a friend of mine in the flooring industry also recommended cementitious over HardiBacker (he's used it in numerous commercial installations, but never over plywood).

I'm currently thinking of pulling up the stapled 3/4" plywood and re-installing with liquid nails and screws. We used a dial caliper last week and are within the 1/360 deflection between joists using our combined weights (an approximation for 2 people in the shower, probably the highest dynamic loading), but I would probably take that opportunity to hang some blocks between the joists in a few locations.

So what should go above that? First choice is the usual 2nd layer of plywood, then tarpaper / waterproof membrane, then screwed lathe to tie all the wood together.

Then I think we need a physical decoupling membrane. Both DitraHeat and RPM mats can act this way, or we could use a Schluter membrane and...HardiBacker?...and hot glue the wire (just to position the wire - electric heat doesn't get above 130 degrees). We would then use thinset (if using a mat), and another membrane? (particularly a waterproofing membrane for the wet area) and...HardiBacker on top of that, and then the cementitious coating on top?

My problems are this - if I mechanically fasten the HardiBacker through to the plywood, then I think I've lost the decoupling between the cementboard/cement and the wood that I really wanted, especially with the thermal expansion impact of radiant heat.

I think you tilers (is that a term? ) are concerned about decoupling your hard tile area from the cement or wood layers - I don't have a tile area, so I'm only concerned about not cracking the cementitious finish (particularly in the wet area!) by decoupling from the plywood.

I've seen some advice about putting the cementitious coating over mortar or self-leveling. I think I'm somewhere between the constructions of RH130/135/140 - but I can't find a single specification that matches my requirements.

So...how would YOU do it? How would you build this floor to last for 20+ years without cracking?

(Note: the interior dividers are planned to be 1/2" tempered glass, the counter-looking areas are actually cabinets with a 2cm slab).
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Old 05-25-2014, 04:17 PM   #2
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Hi Keith.

You may want to look into a Mapei system, as they have waterproofing, ant-fracture, and wear-layer cement product lines. No going "off label" as one manufacturer makes it all. I'll see if i can get Mapei's Tech guy Dan to look in here.
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Old 05-25-2014, 05:05 PM   #3
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Hi Dana!

Thank you for the response! But looking briefly at the Mapei site they don't have radiant flooring (in fact, there's not a single search result on their site that contains "radiant"), they have no solutions for expansion joints, and very little documentation of any type. In the area of "Floor Covering Installation Systems" the only installation guide is for Planiseal - which is actually part of their "Concrete Restoration Systems" line.

I could certainly be talked into looking at any particular product line (though I would prefer it if Master Wholesale in Seattle carried the line), but what I really need is a complete installation method for everything from the joists up to the cementatious topping.
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Old 05-25-2014, 07:30 PM   #4
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Hi Keith

I've read the first post three times and I'm wondering if you can confirm a couple of things:
The finished colored cement coating is going over the entire bath floor including the shower floor?

Does that shower have a curb or are you recessing the floor joists in this area?

Does the finished bathroom floor height have to match up with the flooring heights in other rooms?

How thick is the cement coating?


Quote:
I have the TCNA handbook and understand that I should segment the area into 8' sections with control joints
Could you provide the specific number of the TCNA method that you are talking about for this?
Also I'm not familiar with slab heat. Is this a hydronic system?
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Old 05-25-2014, 08:56 PM   #5
centerisl
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Hi Jim!

Hey, you're local! I could almost see Tiger Mountain if it weren't raining!

Yes, I'd like to cement coat the entire floor.

No shower curb is planned. We were planning on adding another 3/4" plywood layer to build height to allow us to taper 1" towards a trench drain in the shower - we were also thinking of a tapered recess on the 3 joists in the shower area. They happen to sit on a load bearing wall about 5 feet in from the outer wall, plus they're 12" sitting on ply (where the old tub was thinset) sitting on 10" - there's quite a bit of structure under there!

There's an entrance to the bathroom from the master closet - currently carpet there, and a height transition could be accomodated.

Quote:
How thick is the cement coating?
Yes, that's part of the big question. Is it cement over cement? Mortar? HardiBacker? I'm pretty impressed with the flexural strength of the HardiBacker - I'd love to get (2) 1/4" layers down but that's almost impossible to mechanically fasten through.

What are your thoughts? Where would you put the crack isolation?

PS: SlabHeat is a higher capacity radiant electric system by the folks who do SunTouch. Not only do they support use in showers, they allow a vertical run to a heated seating area! You can't use the grids, but you can use the metal strapping to fasten it down.
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Old 05-25-2014, 09:29 PM   #6
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Hi Keith

I'm surprised at how infrequently I can see Mt Rainier from our area. Maybe it's nature's way of making sure we don't take it for granted?

I have a hunch that your shower set-up is going to narrow down some of the options because your going to have to decide on a shower system and linear trench drain. Is the size of the shower open for a little bit of flexibility? Ordering a custom linear drain can be pricey and take a considerable amount of time. I just received one that took 6 weeks to make.

The other, more important, question that I have is that I'm still confused on the shower area. Is it going to be recessed into the joists? Are you limited on how thick your mud bed can be? Do you need a low profile trench drain? A drawing would be extremely helpful for this.

It's confusing but I'm thinking of a couple of different ways of doing what you want but the way the shower is set up and the waterproofing system could veto what I have in mind. I need to know how much thickness you have to play with for the mudbed.

Also, I'm pretty sure the 2 layers of hardi are frowned upon. I think they would rather you do one layer of half inch.
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Old 05-26-2014, 08:50 AM   #7
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Quote:
Could you provide the specific number of the TCNA method that you are talking about for this?
EJ171. I'm assuming that the radiant heat is closest to "exposed to direct sunlight" - not that I have a lot of "direct sunlight" experience in the PNW!
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Old 05-26-2014, 09:30 AM   #8
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Quote:
Is the size of the shower open for a little bit of flexibility? Ordering a custom linear drain can be pricey and take a considerable amount of time.
I was actually planning on a stock 36" drain - towards the outside wall because we have so much space under there for running plumbing. In fact, that's one of the reasons to not use tile in the shower - to make it easier to handle the compound slope and to reduce (slightly) the overall slope because we won't have mortar joints to retain water. One of the drains I would consider is the QuARTz by Aco - the 34" drain is under $200, here is the installation diagram.

Quote:
The other, more important, question that I have is that I'm still confused on the shower area. Is it going to be recessed into the joists? Are you limited on how thick your mud bed can be? Do you need a low profile trench drain? A drawing would be extremely helpful for this.
I can do a drawing as soon as someone gives me a suggested buildup of that area. Again, we have 22.75" inches of structure below the (current) plywood to play with if we have to set it down.

Quote:
It's confusing but I'm thinking of a couple of different ways of doing what you want but the way the shower is set up and the waterproofing system could veto what I have in mind. I need to know how much thickness you have to play with for the mudbed.
The Aco specifications say "Mortar base '2' plus tile and adhesive should be no less than 1-1/2", and that seems typical (though I am not planning to use tile). Preslope is minimum 3/4" whether we use mortar or slope an additional layer of 3/4" plywood.

Quote:
Also, I'm pretty sure the 2 layers of hardi are frowned upon. I think they would rather you do one layer of half inch.
I'm not clear on the difference between the 1/4" and 500 boards, but I noted that the 1/4" is 1.9lbs/sqft and the 500 is 2.6lbs/sqft, so 500 <> (2) 1/4"
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Old 05-26-2014, 09:42 AM   #9
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Quote:
I'm surprised at how infrequently I can see Mt Rainier from our area. Maybe it's nature's way of making sure we don't take it for granted?
One of the features we are adding is an egress window on the wall opposite the shower (which will be my wife's makeup area). Attached is the view from the floor below. We are also adding a (Milgard Ultra fiberglass)window below the awning window in the shower, which will result in a nice view from the "throne" out over the lake and (on a good day!) to the Cascades.
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Old 05-26-2014, 09:51 AM   #10
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Keith, if you'll visit our FAQ you'll find a brief tutorial on how to post and properly attribute quotes here on the forums. Very simple once you see it.

Old PNW Indian name of DFBB Jones (Drinks Fire-Water By Bucket) usta say he could predict the weather by looking at Mt. Rainier. "If cannot see Mt. Rainier, is raining. If can see Mt. Rainier, will rain soon."
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Old 05-26-2014, 10:32 AM   #11
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Quote:
Keith, if you'll visit our FAQ you'll find a brief tutorial on how to post and properly attribute quotes here on the forums. Very simple once you see it.
Found it! But that isn't an attributed quote.
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Old 05-27-2014, 07:44 AM   #12
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Keith, unfortunately we don't have a topping that is designed for use over plywood. The potential for cracking with any cementitious topping is too high, unfortunately. To me this seems like the perfect place for tile, if you do decide to go that route we can offer you a variety of system options.

Best of luck with your install, your view is beautiful!
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Old 05-27-2014, 08:33 AM   #13
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Quote:
The potential for cracking with any cementitious topping is too high, unfortunately
That seems to be a common belief, but I can't figure out why that would be the case. Is it a flex issue? Certainly the expansion / contraction of cement over plywood would be less complex than tile over cement over plywood.
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Old 05-27-2014, 08:51 AM   #14
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Wink

UPDATE: While Jim hinted at a "couple of different ways" of attacking my problem, maybe it's time for me to consider relaxing one or two of my design parameters.

After talking it over with my wife, we are willing to punt on heating the shower floor. I've always been a bit nervous about line voltage in a wet area anyway, and was planning a separate wiring loop for isolation.

And if cement over ply (even with an intermediate layer like HardiBacker) is truly problematic, I could get talked into tile on the floor. Or I could tile just the shower area.

If I go with tile over radiant in the dry area, then I guess I'd just use RH135-13. Though I'd still be tempted to experiment with cement!
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Old 05-27-2014, 09:42 AM   #15
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Yes, the challenge here is the flex of the floor under the topping. Cement toppings aren't very forgiving when it comes to floor settling or flexing. If you're willing to consider tile, the heated floor is very doable and you have a variety of cement SLU's that would work as part of the system.
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