Help! Water under newly laid flagstones

[s219]

I found that when using mortar to attach stone, it needs to be on the wet side. From what you describe, I think your mortar was too dry. I'm guessing that you spread it all out at the same time, then laid the stone on top of it. The first few stones are probably fine, but the last few are the ones that are coming off easily.

The moisture isn't important, but there was no reason to put plastic over it.

In the last couple of years I've gone from dealing with the short working time of mortar to the much better and stronger bond of using regular thinset mortar that everyone uses for tile. I also back butter each tile, or stone. This controls the mess, and there is never a dry spot in the mortar. If the surface is porous, like tiling over brick or block, I skim coat that too.

Do you have a SDS rotary hammer with a flat chisel bit? I would use that to clean off the existing mortar and start over fresh.

We used a recipe specifically recommended for laying flagstones, and it was mixed as we went -- each batch was good for about 2 stones (~ 8 sq.ft). It was about as wet as normal mortar I'd say. We sponged down the slab and stone before setting, to prevent them from pulling moisture from the mortar too quickly. Did everything by the book as far as I can tell.

I talked to a mason today, and he said if the mortar bed is solid and well-attached, which I confirmed it is, then either use modified thin set or a product called Laticrete "Latapoxy" to reset the stones down on the existing mortar bed. The Latapoxy is expensive, but very certain to work, so I may go that route.

 

[s219]

I visited the tile supplier we used when we built our house and looked at their Laticrete products. Ended up buying two 50# sacks of 254 Platinum thin set, which is Laticrete's best thin set. Was $39/bag with contractor pricing. Main reason I went with this was working time -- it's rated for 2 hours pot time, whereas their epoxy thin sets are only 45 minutes. I knew I'd need more time than that.

I pulled up all the flagstones this morning, wire brushed the underside of the stones and the top of the mortar bed, and blew everything off real good with an air nozzle. Mortar bed is very solid. I re-bedded all the stones in their original positions with thin set. I buttered both the mortar bed and the back of the stones, and then used a 1/4" square notch trowel to spread a good layer of thin set over the mortar bed. This thin set was very sticky/tacky, which I was glad to see. I debated what size trowel to use (normally it would be 1/2" when laying big tiles) but decided to go thinner in this case since I wanted the stones to easily align horizontally with their imprint in the mortar, and I didn't need bedding from the thin set.

I mixed the first 50# bag and got through 13 stones, roughly 45 square feet. That is about half the coverage stated on the bag, so I knew I was being liberal with the thin set, which is good. Once it got warmer/sunnier in the afternoon, I did two separate 25# batches from the second bag, and that was a smart decision.

So far, so good it seems. I gave the stones a knuckle knock tonight and got the quiet solid thunk you're supposed to get. Of course there are heavy thunderstorms on the way tonight, so I tarped over the patio, using a bigger tarp this time so there would be less chance of leaks. I am hoping to grout the joints on Thursday, assuming the stones are still stuck down by then. I told my wife if they don't stick down this time, we're getting a real nice wood deck over a masonry base instead of a patio, and I'll re-purpose the bluestone for something else.

 

[GLyford]

Concrete is also a nice base for pavers, if you want more options. But I hope the thinset works...

 

[CADplans]

Many people set those type pavers directly in a sand base,,, with no bond whatsoever,,,
Why would you worry if they do not bond down??

 

[s219]

Many people set those type pavers directly in a sand base,,, with no bond whatsoever,,,
Why would you worry if they do not bond down??

I do like the idea of a sand (stone dust) base, and would have done that if the patio was at ground level. My front walk was done that way. With a sand base, water under the stone is not an issue since it can drain, so minor movement of the stone is no big deal. But due to slope, this patio had to be built up with block walls (about 2' high in the downhill corner) so we poured a slab on top. Once you're laying flagstones over the top of a slab, you have to keep the stones mortared down, well grouted, and immovable. Otherwise water can get below them in the winter, freeze, and heave them. I think the problem is that once you're in the position of having to lay them over a slab, it becomes a more complex project.

In retrospect, I could have probably put in a compacted base of stone dust inside the block walls and laid the flagstone on that, then swept polymerized sand into the joints and called it a day. That would have been a lot easier for several reasons. In retrospect I would have avoided the slab and all the complexity that comes with it.

 

[GLyford]

well, at least this way your pavers won't frost heave.

 

[s219]

Here's an update -- the thin set seems to have worked very well. All stones are solidly stuck down, probably better than mortar could have done by itself. I grouted yesterday, just before rain showed up again, and it came out very nice:



I used concrete coloring (charcoal) to make the mortar a little darker -- mixed it to the point where the wet mortar was similar in color to the wet bluestone, so I'm hoping they are close in color when dry as well.

I tented over the patio with a tarp to provide protection from rain and some breathing space, and got about 9-10 hours of dry protection before heavy storms hit and the tarp gave out. We got about 3.5" of rain in 2 hours, it was just plain ridiculous. Grout still looks good this morning, so no harm done.

I'll give it another couple days and then give a light scrub with muriatic acid and water to get all the grout haze off the stone. After that, I need to start sticking on the stone veneer to the base, and also decide what to do about stairs.

I have to say, if this was a deck I would have been done in a couple days, rain or shine. I really didn't understand how much more weather-dependent and weather-vulnerable masonry work is. I would think twice about doing this kind of project during a rainy spring/summer. This would make a better project for the fall around here. Live and learn I guess.