I have a large (600-700 sq ft) outdoor concrete patio that's probably 25-30 yrs old but in good condition (no noticeable cracking). I'd like to tile over with 16" slate. Concrete is the kind with pebbles showing on the surface, and it has wide expansion joints wedged with ancient 2x4s. I've read all the related threads on the forum (very helpful, thank you) but would like to make sure I'm doing the right thing here. Current plan: Take out wood, put in pre-formed expansion joints and fill rest of joint with some kind of mortar. Tile on top of concrete (sections are 8' sq, so i should be able to fit 6 16" tiles across and down and not tile across an expansion joint), using caulk instead of grout along the existing expansion joints. Joint concerns: Will pre-formed expansion joints work here? Is it ok to fill in the extra space in the expansion joints with mortar? How do i space the tiles to make this look ok - would normally do 1/4" but from what i've read the expansion joint should be at least 1/2" (live in northern california, pretty moderate climate)...in which case would it be to strange to have 1/4" spacing everywhere but where the expansion joints are? Where do i buy pre-formed expansion joints and the right kind of caulk? Other concerns: Is slate ok for this use? I already broke my sample tile, which doesn't seem to be a good sign. But it's my cheapest natural stone option, and porcelain tile seems too slippery. Do i need to scarify the concrete? What's the thinnest mortar layer i can get away with (very little vertical clearance bc house is on slab)? Have to confess i'm not really a DIY-er...I've got a landscape contractor that's doing this as part of overall landscape project. But he's not a tile installer by trade, and if I hadn't been concerned enough to research this, he'd have filled in the expansion joints with rebar and mortar and installed the tile on top without a second thought. Need to be doubly sure of details so I don't send landscaper on wrong path. Many many thanks.

Hi Elsie, Welcome. :) The only thing I qestion is the idea of pre-formed expansion joints. I don't know what you have in mind. The plastic type that might be found in a pool deck? In any case, I wouldn't go that route. You should remove the 2x4 wood, insert about 3/8 in. thikness of corregated cardboard all the way down to the bottom of the slab (about 3- 1/2 in. Keep it to one side of the space or the other so it will be straight. Then "bond" additional cement mortar to the wide side of the joint. When you finish you'll have only a 3/8 in. joint, entirely filled with cardboard. You can tile up to both sides of this joint and fill the joint in the tile with caulking. The cardboard below will keep the caulking from going in deeper than necessary. Eventually the cardboard will rot away, which is fine. The pea gravel surface should be thoroughly cleaned and skimmed with modified thin set before you begin laying tile. This will make things go much quicker than having to mix and spread all of the thin set when setting tile. It will take mucho thin set to do this job. Don't underestimate the cost factor. Don't ask me exactly how much it will take. I don't know. :D

Thanks, John, you are a lifesaver. Really appreciate the fast response and the site in general. Fantastic resource - I didn't know anything about tile installation until yesterday, and now I (hopefully) know enough to avert what would have been a faulty installation. Elsie

Hang around, Elsie. Others may have something to offer as well.

Maybe this calls for a "Schluter." Dilex, their brand of preformed expansion joints. http://www.Schluter.com

Elsie, maybe you could try using quarry tile instead of slate or porcelin? It's very low cost ($2-3.00 PSF for Metropolitan quarry basics) super durable (PEI 5), comes in some nice natural looking colours, different sizes and shapes, and offers some grip for outside uses. I'm planning to do a patio next spring with it (over concrete slab) after we thaw out up here in MN. Just my 2 cents worth, Colleen ;)

the schluter dilex products look like they would be great, but is it worth the extra cost if i'm tiling over? thanks for the suggestion on the quarry tile - i'll look into it. i actually have a great price on the slate - $1.20 per square foot from a place in san jose (terico) that sells at wholesale prices with a $200 minumum.

The schluter thingies were just a suggestion to get a professional looking expansion joint. You can do as John suggested, but be sure to leave a grout line above the expansion joint. Fill that grout line with a polyurethane expansion sealant (not silicone caulk). You can get color matched sealant so the expansion joint won't be noticable. Just don't ignore the expansion joints. They will come back to bite you!

Hi guys, What about doing the standard expansion joints as you suggest and then using an isolation membrane and avoiding the expansion joints in the tile field?

Hi again - If I can I trouble you all with another question...contractor seems to be comfortable that he can do the expansion joints as John recommended. But he says we need a thicker mortar base to set the tiles in so we can bridge slight differences in height between concrete sections. Says thinset is too thin. Any thoughts? I'm confused by the difference between different bonding elements. Also, not sure if isolation membrane can substitute for expansion joints in the tile as suggested by Kirk? Oh, one more...do I use sanded grout? Any tips on grouting slate? I hear it's tricky. Thanks, Elsie

The stone tiles are apparently "ungauged," meaning thicknesses vary dramatically. Sounds like the guy knows what he's doing. Sanded grout or plain old mortar. Mortar is gray. Dirt is gray. Life can be good. :D

The tiles are gauged. It's the evenness of the ground he's worried about. But sounds like we're ok either way. Wise words on grout. We don't want to fight mother nature here. :)

So no comments on the isolation membrane?

Sorry, Kirk. Was going to answer and got distracted. No, can't do that. The expansion joints have to be honored no matter what. It is used primarily for cracks, and none of the membranes are guaranteed to prevent cracking of the tiles. Not for expansion joints at all.

Ok so you can't install tile like a floating floor ie Pergo.? I ask this because my brother has some cracks in his interior slab that have telegraphed through. Also in many slabs I don't even see expansion joints and they get tiles.

Kirk, Builders are builders. "Control" joints should always be used by they take time. Pergo is a different deal. Tiles have to be bonded to something. Ditra is a very good system for over cracks in concrete, but even Schluter won't guarantee it. Concrete is a pain. ;)

So except for this situation, where there is no room to build up, a decking of lath and mud with an isolation membrane from the concrete slab would work? Would that have enough mass and stiffness to avoid cracks in the tile? Or should there be a layer of plywood, then lath, then mud or CBU and then tile? I think a lath reinforced mud bed would be the most rigid.

If you have room to increase the height of the floor by a minimum of an inch and a quarter, the floating mud bed will work except in the most extreme of situations. No plywood. Moisture barrier laid right on the slab, then lath with mortar over it.

OK John, So by moisture barrier do you mean Felt, 6 ml poly or someting else like Laticrete? What holds the lath in place? In cement slabs we try to put the reinforcement in the middle of the slab rather than the bottom, but really the idea is to keep it within 2 inches of thetop surface. Does the same idea apply here? Would a bottom layer and a middle layer of lath be an improvement? Thanks for all the time and info.

We ought to be doing this over in the Hangout, but ... The tile industry, meaning the TCA, recommends the lath in the middle, but I disagree with that. As you know, the lower stress on a slab is tensile and the upper is compression. You don't need a lot of reinforcement is something is going to compress; you need it if it is going to stretch (tensile). I recommend keeping the wire or lath near the bottom. Over a concrete slab it really doesn't matter that much. It is not attached, though. The weight of the mud and gravity hold everything in place. Moisture barrier can be poly or tar paper. Tar paper takes up more space, so I like poly.

Most of the cracks in concrete come from roots lifting or walls pushing down on dirt that gives way, causing the top to crack. I think that is why they put it near the top.

The only builder I work for pours the slabs six inches thick. Two layers (grids) of rebar, two inches up and two inches down. :)