Exterior Slate Walk

[MesaTileworks]

Can anyone point me to existing threads that discuss methods, products, and techniques for putting slate on an exterior area? I’ve poked around the archives and found a few discussions but not come across anything comprehensive.

I’ve just been asked by a friend to scope out a repair of a slate walk where some pieces are coming loose, and while I’ve done some work with slate, I’ve never done it outdoors.

[jadnashua]

Do you want to do this embedded in sand, or onto a solid surface with thinset?

[John Bridge]

Matt,

Slate tiles go down pretty much like other tiles. In your area freeze/thaw can be a problem with exterior installations. If the tiles were bonded direct to the sidewalk, that and lack of expansion joints could be causing the delamination.

[CaliGrown]

John’s said it straight to the point. Slate and some other natural stones tend to flake or shale off in layers over time. Slate tiles are either gauged or ungauged so anticipate an imperfect finish with some deviating thicknesses. Thinset and go amigo!

[MesaTileworks]

The slate pieces are apparently popping off clean with no morter remaining on them (morter remains on the slab ramp to which they were originally installed). So it would seem that the problem was in the bond between the mortar and the slate. My friend would like to just clean the thinset off the substrate and re-install the slate where possible.

Does any kind of underlayment need to be laid between the concrete ramp and the slate? Is there any particular kind of mortar that would be preferable for this kind of exterior application? Any other tips for what might have gone wrong with the original bond and how to avoid the same thing happening again?

[Kyras]

This is classic stone guy stuff. I say stone guy as opposed to tile guy. Tile guy thinks about bonding, stone guy thinks about bedding.

Here is the story. I'm standing inside in the A/C looking out the window. Just outside, in the 99 degree direct sun, the stone guys are setting a deck of 2" thick bluestone (slate). Those guys are roasting out there, with giant cardboard sombreros. The stone is hot. The sand is hot, water is hot, mortar mix is hot. They do a really nice job, with coping and all. It's a huge deck.

A month later, I am standing with the owner and the super. I was called to look at the cracking joints in the bluestone. Now, back when I did work for the gubmint, the Navy inspector had this long steel rod and he would go "ping" all over your floor looking for loose tiles. He was a real hardass. So, I get the steel rod (breaker bar) and you know what it sounds like, it's hollow. Once in a while you hit a nice solid, bonded spot, but 90 percent is hollow sounding. The stone is not bonded.

About that time, the stone guy boss shows up and explains oh no that is two grades of stone, the number one and the number two and you hear the difference...I just let him talk and try to save his butt and that's fine, but it's a lie. I was tempted to walk over and pick up a piece of coping with my bare hands, but I let it go. He was going to come by and touch it up, which is the last I saw of it.

Sadly, he did not listen to me. I tried to explain. Here is what I would do differently. Cover all materials. Keep everything wet and shaded, work under a party tent of possible. The main thing is the bridge bond. Bluestone is dense and not porous like bricks. Mortarmix won't draw in and bond it. Wetting the stone would help, just before it is set, to remove dust and help the bond. Better yet, a slurry mix of cement and water painted on the back of each piece just before setting. Or a handful of pure cement dusted over the wet mortarmix just before laying the piece in. Or thinset. It's a bridge bond because the mortar can't stick to the dry stone all by itself, it needs help.

This, or some variation of it, is what went wrong with your stone. Most stone guys are content with bedding the stone, they don't really care about bond that much. Why? I don't know, I'm a tile guy.

To reset it, use thinset, almost any kind will do the trick. Very wet. Wet the stone, wet the whole area, give it a few minutes to soak in, remove standing water, set the piece, stand on it, do the Twist. Try not to make a tripping hazard, that is the main thing.

[jadnashua]

If you go with thinset, you might want to bite the bullet and go with A118.15 stuff that has the highest and widest range of temperatures and bond strength AND, burn a coat of it onto the back of the stone just prior to setting it in place.