Bluestone over concrete patio

Question about laying bluestone over a concrete patio....We are having a stone fireplace and pergola built, and are trying to decide what to use to cover our existing concrete patio. The original concrete slab is in perfect shape and very well-laid, and we have expanded it to the size and shape we want with new concrete (at the same time the footing for the fireplace was done). We have a contractor lined up to lay concrete pavers, but now we are considering bluestone. The contractor says he would want to dry-lay the bluestone over the concrete and use polymeric sand in the joints. I'm not sure how he would secure the edges of the patio. He would really prefer laying the pavers. We are uncertain about how to lay the bluestone properly, and thinking maybe we should just do the pavers. We love the look of natural stone and don't mind paying the extra cost for materials versus pavers, but if we have to pay a stone mason to lay it all in mortar, etc, then the cost would be prohibitive. Can we really dry lay the bluestone on concrete? If so, how would this best be done?

ANSWER

I would lay the bluestone over the concrete patio and set it in mortar.

My thoughts have changed in this last year. It's not that any method is terrible, but logically thinking about things and knowing how water, freeze-thaw conditions, etc. work, I have come to the following conclusions.

1. If you lay pavers or any material over concrete and set it in sand, water can seep down onto the concrete. Although everything may be pitched properly, it is really not the best situation for allowing water to either drain in the right direction or percolate down to the soil. It cannot percolate down to the soil since it is on top of concrete. Once the water seeps into the sand joints, it cannot drain as freely as if it were running over the stone surface.

2. Polymeric sand is a material that hardens. People like it because they feel it is more permanent than sand or stone dust. Consider the following.

I recently had my own patio re-laid due to drainage issues that were there when I purchased my home. The contractor wanted to use polymeric sand, but I said no. I am not sure where you live, but in NJ this past year the snow and cold weather was the worst it's been in a long time. There was a lot of freezing and thawing of the ground. My patio heaved, but then resumed it's original level after the snows disappeared. If I had used polymeric sand, I would take an educated guess that there would have been multiple cracks in it. It's a hard material, but not that hard.

So these are the reasons I would recommend laying the bluestone over the concrete. It will be more expensive than dry laying it and somewhat more expensive than pavers, but if it's an option financially, I am sure you will be happy in the long run.

See the following page with additional links.

Bluestone Patios