Flooring Tile - Preparing 2x6 T&G subfloor for slate

I am preparing my sub floor in my kitchen for slate. This is an older house, with 2x6 T&G boards on beams on a 4ft O.C. I am planning to put down 1/4 hardibacker, but I am not sure about applying the thinset directly to the 2x6. The floor is very strong, but there are some knotholes, and dips at the T&G lines. If I add a 1/4 CDX over the 2x6 to make a smooth surface, with this be enough?

4' center to center joists? :eek: I'de double check that.

You have alot of work ahead of you if want want the tile to last.

First, you will need to find out your true floor joist spacing, their unsupported span.

If your joists are truly 4' OC, there is no way you can install tile at all.

Also, you would need to install at least 1 1/4" of exterior grade plywood, in two sheets. After the two layers of plywood, you would then install your cement board in a bed of thinset and screwed.

Post and beam, yes. If thats the case, forget about it. No cement board or membrane I know of can be used on 48" centers.:thumbdn:

You cannot put cement board directly over the 2x6 planks. You cannot use 1/4" plywood, and definetly not cdx. This plan is not gonna work.

I am getting conlicting info here. The guys at the tile store are saying no problem.

This floor is very strong. This is an different setup I know, but 2x6 T&G on post and beam with a 4ft O.C. is not that odd. The beams are 4x10, and the 2x6 did have 3/4 particle board with vinyl on top. I have taken the particle board up, and am ready to install the base for the flooring.
If this won't work as is, what would be the best way to make this work? I can't see adding 1.25 plywood over the 2x6.

It won't work.

You will not find a industry standard (TCNA) or otherwise that approves it. Of course your tile store is telling you it's going to work, they want you to spend money and push you out the door.

Your floor joist structure needs to meet or exceed the L360 deflection rating.

edit.

Looking up L360 shows that the floor can not deflect more than 1/360th of the span. 48 inches / 360 gives .133, a little more than 1/8 inch. Pretty close to that now.

So you are saying the only way to make this work is to add new beams to shorten up the span, say to 2ft? Is 2ft fine?

It's better, but look at the message I sent you.