Hello.

I am about to lay chinese slate 16" tile on a hydronic concrete floor 1-1/2" thick that was poured onto 3/4" tg. plywood floor supported by 2 by 12's on 16" centres with no spans longer than 10'.
I do not want to use a membrane for the obvious cost reasons.
The concrete is now 5 months old.

Any tips or advice would be highly appreciated.
Thanx . in advance

Dale

How many square feet are we talking about, Dale?

roughly 1200 square feet

Are you sure its was a concrete pour or was it gypcrete?

If it was gypcrete then you defiantly need a membrane.

I would highly recommend a membrane anyway over a heated floor no matter what the topping was.

Yes ,I laid the tubing and poured the concrete ,a friend finished it.
what sort of price should a guy pay for a membrane.
It seems the membrane is more pricey than the tile.

If I dont use a membrane what sort of problems might occur.
cracked tiles
cracked grout lines?

You could have problems, such as the ones you mentioned.

Its up to you whether you want to risk a slate installation of that size over a heated floor.
I couldn't tell you what membranes may cost in your area. Where I'm located they run about 1.55 a foot or more.

Any cracking in the concrete pour?

There is some small cracking across door openings and also across the middle of a couple roooms.
I thought there would be a lot more .
The cracks are about 1/32 of an inch wide.
The drywallers got primer on the perimeters of all rooms.
I was thinking of sanding the floor surface .

on a quote I got for the tile job the contractor wanted 3.50 a square foot just for the membrane and 5.50 to lay the tile.
what sort of membrane do you recomend.

Heated concrete almost always cracks. The cracks will propagate into your tile. If you have had the heat on in the slab for a while, it has probably cracked as much as its going to and you could just run about 18" wide strips of antifracture membrane over just the cracks. The membrane I usually use is a peel and stick tar based compound with fleece on the surface. The name escapes me at the moment. Ditra is better, but also thicker and more expensive.

Another option is a highly modified thinset such as Custom's megaflex. But at $50/bag that could be even more expensive than membrane.

So maybe I shouldnt tile, till I run the the heat through the concrete for a few months first?

The concrete is already fully cured (generally accepted cure time is 28 days) So I don't think you need a few months. A couple of weeks of heat and a cool off and see what happens.

Because the coefficient of heat expansion is very similar in concrete and stone, your main concern is the shrinkage cracks.

I'm sure that one could make arguments for or against the membrane. I have always dealt with fairly green slabs, so I just put it on automatically. I understand the cost issues, but $1500 on membrane is not that huge when compared to the cost of tile, labor and materials to re-do a cracked floor.