Tile over Roth radiant panels [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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03-08-2007, 05:21 PM
I'm in the process of designing, and plan to build, a house for myself. I'm planning on using in-floor radiant heat throughout, hardwood floor in living rooms and bedrooms, and tile in kitchen, baths, entry, and utility.

My currently planned floor structure is, 9.25" I-joist, 16" o.c., longest span ~12', additional support under interior walls, showers, and tub. I used Rosewood Forest Products joist and beam calculator for the sizing. Deflections calculate less than L/480 even with 400# point load in the center of the span. You know, like two people in the kitchen at the same time, or in the shower! :-)

Flooring layers will be 7/8" T & G ext plywood, 1" Roth radiant heat aluminum covered foam panels screwed down, 1/4" CBU (haven't decided which yet) fastened with thin set & screws (or nails) per mfr instructions, Ditra laid down per mfr instructions, and then the tile.

I have several questions I haven't been able to find answers for yet. Roth installation instructions say consult your flooring contractor. I plan to be the flooring contractor!

Have any of you installed a combination like this? If so, how did it work out?

What will be the thickness of the flooring layers above the Roth panels, assuming 3/8" for the tile, and 1/4" for the CBU? How thick is Ditra? I need to know this so that I can match, or adjust, the hardwood floor thickness.

What kind of screws or nails should I use to fasten the CBU? The ones readily available are on the order of 1 1/2" long, which aren't long enough because of the extra 1" of the Roth panel. I've read several comments about the countersink and corrosion issue with screws in CBU, so I know I can't grab just any 2" to 2 1/2" screw off the shelf. Am I stuck with predrilling and countersinking the CBU for lack of proper screws or nails? How about the #8 x 2 1/2" Power Head wafer screws from McFeelys web site?

Any other comments & suggestions will be appreciated. I hope I'm not in over my head!

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03-08-2007, 05:46 PM
I just found a CAD dwg of Ditra. It's 17/64" thick. But I still need to know what the installed thickness will be, approximately, accounting for the thinset layers.

Also, it's Roseburg Forest Products, not Rosewood.

John Bridge
03-08-2007, 06:36 PM
Hi and welcome. :) We'll ask for a first name. :)

You're getting a little out of the tile area and mostly into the flooring area. I'm not familiar with the heat you're using, but I know the folks at Warmly Yours will be happy to help you out no matter which product you choose. Click the ad at the bottom of the page.

The flooring structure sounds good for ceramic tile, not for stone tiles. Don't know what you have in mind there.

03-08-2007, 07:47 PM
Name's Mac. It's in my user name, but I wouldn't expect you to know that! The last three are my last name.

I thought later I should have said hydronic heating. The Roth panels are a sandwich of aluminum plate on foam insulation with formed channels into which PEX tubes are pressed. Warmly Yours is electric heat, not quite the same. The tiles will be ceramic, most likely, although I was looking at some travertine tiles at HD today that look interesting.

I found some more info on screws, wafer head and others. To revise my questions a bit. How much can a screw head stick up above the CBU surface w/o causing a problem laying the tile? Would screws made for Hardiplank siding be suitable for CBU? I would think so, but maybe someone knows better.

The other question about layer height. I figure the total height, including the radiant panel, of the solid material is ~1 3/4". How much will 3 layers of thin set add to this, 1/8" each? Or is a thin set layer thicker, or thinner? At 1/8" each the total height will be about 2 3/8" which means I will probably need to add a 1/2" underlayment under the HW to bring it to the same level as the tile.

Any info about this will be appreciated. Thanks.


03-08-2007, 08:48 PM
Mac, Ditra is not 17/64" thick. It's only half that. Ditra with it's layer of thin set bonding mortar, combined call it 5/32" thick.

The thickness of the other thin set layers you mention is going to depend upon a number of variables, trowel size and the angle at which the trowel is held are only two of them. But to generalize I'd say a 1/4" sq. notch trowel will give you slightly less than an 1/8" build-up. Maybe 3/32" is a better number.


03-09-2007, 01:07 AM
Thanks Mike2,
Boy, did I ever suffer a brain fart on this one. The CAD dimension of 1/8" = 8/64" + 1/64" adds up to 9/64", doesn't it? Somehow my brain computed 1/8 = 16/64.

Your numbers look good to me. I will probably put 1/4" Hardibacker under the hardwood also. That way the level of both floor coverings should easily be within 1/4" of each other, which should be OK using transition pieces.

03-09-2007, 09:48 AM
I would worry about the deflection of the Roth panel itself. I've installed them under hardwood but not tile and I'm no expert on the subject but a layer of styrofoam under 1/4 cbu gives me the willies. I would think you'd be better off using something like Viega climate panels

03-09-2007, 06:22 PM
why not warmboards from the start of the design?

03-10-2007, 12:04 AM
I was planning on using Warmboard when I got started on the design, but I have one concern with them. It's the same issue as Viega and other aluminum channels over plywood, and that's heat transfer. They are great on the upper side, but not so, imho, on the bottom side. There's limited insulation value below the tubes, and of course by design, the floor will be fairly warm. That means that with an all wood system below the tubes a fair amount of heat will be radiated down into the crawl space, both through the subfloor and the joists. I want to keep as much of the heat going up as possible, not down. My first plan was to use Warmboard over blue board over the joists, then I found out about the Roth panels. The crawlspace will be insulated and part of the conditioned envelope, so there will no insulation below the subfloor.
Interestingly, I've made a somewhat guesstimate that even with the Roth panels over the 7/8 ply, if I put an aluminum foil radiant barrier under the subfloor, I can cut the heat being radiated downward by about $10/mo during heat season. In this case the foil serves as a very low emitter of heat. Wood is fairly good emitter of heat.

03-10-2007, 09:30 AM
"""""The crawlspace will be insulated and part of the conditioned envelope, so there will no insulation below the subfloor."""""

No insulation below subfloor!!!!!
unless you want the crawl space at 70F, BIG mistake!!! I'd use at least R19 under floor, I put 8" under floor in crawl space.

go over to heatinghelp.com

do a 'search' for warmboards. that is a good point about warmboards, though the vast majority think its the best for floors. also do a search for aluminum foil barrier, about R2 at best.

03-10-2007, 10:00 AM
Let me be a little bit clearer. The crawlspace will be sealed and insulated around the perimeter. I'm using ICF walls which start at the footing. They form an insulated wall from ground to roof. Also, if you think about it, which surface area is most likely the smaller, the floor area of the house, or the area of the crawlspace wall? Let's insulate the smaller area. Also, the ground surface temperature in the crawlspace is higher during the winter than cold ventilating air sweeping under the floor will be. How many vents get closed in winter, and how tight are they? Why not let the higher temp surface, the ground, be the heat sink below the floor? And yes, the crawlspace will be 70F, well actually more likely around 65 or so. It will be ventilated into the house space.

This is newer energy efficiency thinking that runs contrary to years of normal construction practice, but it's becoming the new mantra in building. The attic will be sealed and part of the conditioned envelope also. Foam nsulation will be on the attic side of the roof.

The foil has essentially no R value. Aluminum is a super conductor of heat but a poor radiator of heat. I want the underside of the floor to not be much higher temp than the air that will be moving under it so there will be very little heat lost by conductance. I expect the ground surface will be 55 to 65. With the floor underside being at say, 75 to 80, heat will radiate to the ground. The foil will cut that down significantly.

03-10-2007, 10:24 AM
what is your heatloss for the crawl space?

I'm all for insulating and heating 'used' spaces.

are you regularly going to use the attic and crawl space?

why heat what you do not use!

Ok, one more question, whats the heatloss of the crawl space and the space above?

which roth system are you thinking of using?

03-10-2007, 01:06 PM
Mac...as the names says...not a pro here but have a suggestion for you. While your planning, you could encorporate some load bearing walls under the areas you plan to tile. It would stiffen the floor and you could maybe get up to natural stone. Roth says you can use tile over their product but I have no experience with that. Anyone who works on your floors will have to be very careful about piercing the tubing. Good Luck. Randy.

03-10-2007, 01:12 PM
Mac...where did you learn about that energy efficiency system. I live in Maine and we can use all the help we can get in the winter. And your right, it does run contrary to everything I've ever learned. But I'm not an old dog yet. :scratch: Randy.

03-11-2007, 12:39 PM
crawl space:

I hope you are insulating the crawl space floor or you are exact right, the ground will be a heat sink!!!! so much for being energy efficient!

BadgerBoiler MN
10-31-2007, 07:59 AM
Kudos to Mac,

I see that you have done your homework on the thermal envelope. Typical soil is a very good insulator, though I prefer slab-on-grade radiant floors. Insulating below sub-floors never hurts but has more to do with response time and conditioned spaces below than heat-loss.

You may find your wood floored areas lagging behind your tiled floors. If they are in the same air space the tile will satisfy the thermostat before the wood warms up. This is especially true in mild weather.

You will also find that design water temperatures will be higher for wood in design conditions and might require insulation below the floor to respond quickly enough for sharp drops in outdoor temperature.

As to your tile question, the Roth panels are a fine idea but we always use 1/2" cement board below tile placed on top of would sub-floors.


10-31-2007, 08:18 AM
Welcome, MA. :)

That's a pretty old thread you're replying to there, though.