Flagstone On Concrete?

PlasterMaster

I have a freind across the street that is DEAD SET on putting flagstone on his old conrete patio deck. There is nothing wrong with his concrete slab (about 8 years old and no cracks), he just wants flagstone glued down on it. I told him I was sure it can be done but wasnt sure how to go about doing it or what products around here in Chicago are he can purchase to adhere the flagstone to the concrete. Any help would be greatly appreciated.

Thanks,

PlasterMaster

Mike Finley

Flagstone on concrete pad, hell yeah, nothing better.

Use mortar.

PlasterMaster

Should he prep the existing patio deck before laying mortar on it and lay the flagstone? Like possibly rolling it out with Acryl 60 first or something else? And advice/instructions would be greatly appreciated.

Mike Finley

I'm out of ammo, I'll leave the details up to the masonry pros who will come along.

I have only done this once, and only for myself, I poured a 6" pad and made sure the surface was really rough, let it cure under plastic and water for 6 days then applied the flagstone directly to the pad with mortar, grouted the joints.

Tscarborough

If he rolls it with Acryl 60, he will have about fifteen minutes before it turns into a bondbreaker instead of a bonding agent. A true bonding agent should be used (Acryl 60 is an bonding additive, meant to be added to cementious products to improve the bonding capabilities of the mix). A bonding agent can be painted on and will re-emulsify when hit with mortar, providing an enhanced bond between the layers.

He should make sure the slab is clean, and if it is old, he could acid wash it. The mortar bed should be 100% coverage, and no less than 1/4" at the shallowest point. The mortar for the bedding does not require compressive strength, so it should be an air entrained, high bond-strength mortar. Check your local suppliers for the best product. The grout will probably be the same material, although the desired physical properties are a little different. Strength is still not the issue, but for the grout you want as little shrinkage as possible, high bond strength, but also a dense and minimally absorbative mix.

denick

Plaster,

This is probably the old fashioned way. We live in climates that are similar and we have done many jobs like this and to my knowledge they are still there. Some 40+ years.

The concrete needs to be clean. If its fairly new it could just do with a scrub brush and a cleaner. Yes you could pressure wash. If he wanted to rough it up its not a bad thing to do. Just take a hammer and chisel and rough it up 1/2" deep dimples would be great. You can use an electric hammer. then wash out the loose pieces and dust.

First thing that should be done when setting flagstone in mortar is WASH the bottoms of the stone before you even start anything. Unless you get flagstone out there that are clean and not covered with dirt and dust. Even a light film is not good.

My answer is for natural cleft stone not a sawed dimensional stone that could be set in thin set.

If you are talking about the type of flagstone we have it is irregular in thickness from end to end of each stone and from stone to stone. All you need is portland cement as a bonding agent. Wet an area that can be worked in a small amount of time. Mix just portland cement and water to the consistency of stiff paste and paint it on the concrete where you will set the next few stones. Place your mortar for that stone and set the stone. You can dampen the back of the stone and paint it with the same paste if you are worried about the bond.

You need a good mortar mix and how you grout is important.

How do people grout flagstone would probably be a good thread.

__________________
Nick

"Every day I remind myself that my inner and outer life are based on the labors of other men, living and dead, and that I must exert myself in order to give in the same measure as I have received and am still receiving" Albert Einstein

Mike Finley

Tell me about it! That was the worst part of the entire project!

jvcstone

The portland slurry is a good way to insure bonding. Kind of a PITA to do though. I used a latex bonding agent to do the same thing the last couple of flag on slab jobs (including my own first floor) with excellent results. Just brushed it onto both surfaces with a water brush.

I almost always use the same mix for bedding and grouting, and do it as I go.
JVC

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John VanCamp
www.jvcstoneworks.com