Hi guys, I am new to this forum. Great info. I have a question regarding the sub floor requirements/recommendations for installing marble floors (had I known ahead of time I would have installed a sheet of vinyl) Anyhow, I am almost done building my own home. I am planning to install marble (wife's idea)on engineered 2x4 web joists 16" oc, 12'6" span, glued and screwed 5/8ths T+G plywood. I was hoping to just lay down the ditra but heard that is not enough, was going to do the lath and morter but heard that doesn't work, was told to put down another 5/8 but not to screw into the joists but that won't work has I have radiant pex below. I do not want to make my floor any thicker than neccessary as most of the other flooring has been installed. Any help, ideas or rope is appreciated. Thanks.

Can you make contact with the joist manufacturer to determine the deflection rating? It's supposed to be L/720 for natural stone. There's no way to tell with a joist span calculator.

As for the subfloor, you must have two layers of ply for stone. The extra layer helps with deflection between the joists, and further uncouples the stone installation from the joists. 5/8" ply would be the bare minimum in your case, as the two layer must add up to 1 1/4". To avoid damage the radiant heat pex underneath, use screws that are no longer than the thickness of both layers of ply.

Most truss floor joist systems are engineered for about L/480 unless there was a plan specification made at the design stage.
Truss floor systems are great for long spans but flex more than a conventional floor system. That makes for a risky natural stone installation.
As Kman suggested, check with your builder to see if they can get you the engineering specifications.

If you were to use Schluter Ditra, minimum specification from them is a 3/4" Ply or OSB subfloor with a minimum 3/8" ply underlayment.
Because you only have a 5/8" subfloor your going to have to have a minimum 1/2" underlayment layer.

Remember these are minimun specifications and you are dealing with a truss joist system.

Thanks for the info guys. I really don't want to add that much material to the floor, 1/2 in ply + ditra + tile. My transitions will be too high. Of these two options which would be best 3/8 plus ditra or wire mesh mortar bed? Thanks again.


Dave

A mud bed would need to be 1 1/4" thick, so the ply/ditra option would be best.

Best to use a lot of screws with 3/8" ply. It doesn't like to lay down by itself. You'll also want to use screws no longer than 1" to keep from hitting your heat tubes underneath.

With 3/8" ply you don't meet Schluter's installation requirements. The original subfloor is not thick enough for a stone/Ditra installation.

There would be NO warranty from Schluter.

I would suggest you switch to a porcelain tile if you don't want to build to industry specifications.

Yep, that original subfloor has to be 3/4".

It's kind of odd, though, that their specs call for a minimum 3/4" ply subfloor, with min. 3/8" on top of that, but the joist spacing can be up to 24".

I don't quite get the logic of that either, especially on truss floor systems. But if that's what they say and cover for warranty purposes then I guess you might be ok.

I certainly wouldn't do it to minimum standards for any of my own jobs.

Well guys I rolled the dice. I talked to my joist mfg and he said deflection on a 13 ft span is negligible. I also contacted my marble supplier who has been in the business for almost 30yrs. He as stated the his installers use mesh with just enough mortar to cover it and they have had no issues. He said that I will have no issues using 3/8's ply either. I have glued and screw plywood 2in screws on the joists and 1 in in the field. The floor is extremely solid. I know it goes against what most have said but I feel very confident in the strength of my floor. This is not a cost cutting issue as I have built a Sip house with triple pane windows, R-70 ceilings etc. I hope I am right. My 4 yr old daughter jumped on the floor and gave me the thumbs up. That has to count for something eh? Thanks for all the feed back. I will be sure to write back with good or bad news. Cheers.

I'm jumping in here...

Your house sounds mighty fine, good on you for the windows and insulation in the ceilings. Wish mine was as well built!

Just a comment on the plywood - is this a second layer that you've just put down? If so, according to my research when I did my floors, you want to avoid screwing the second layer into joists. Also, install in an overlap pattern over the first layer. All sheets should run lengthwise perpendicular to the joists, and most importantly, on the second layer of plywood, ensure the ends of the plywood don't land on a joist, to avoid tenting (run the end of the plywood past the jost by 1/4 to 1/3rd of the spacing between the joists. Screw every 4" along edges, and every 6" in the field. Allow 1/8" between sheets for expansion, and 1/4" along walls, etc. At least that's what my research has instructed; the experts here on the forum can certainly correct me where I'm off base...
Best of luck, let us know how it works out...
See:
http://www.ilevel.com/literature/F1203.pdf
and
http://www.tileusa.com/publication_main.htm