Gravel vs sand base for bluestone patio
view from patio toward rest of yard
I live in a brick row house in Pittsburgh, PA that has a narrow but deep back yard with multiple levels (the first level is just behind the house and is currently a gravel patio, then you walk up some steps to the second level). I would really like to replace the gravel patio with a bluestone patio. We are planning to do a dry laid installation of the bluestone, but I've gotten conflicting advice about what type of base to use. Your site and others seem to suggest first having several inches of 3/4" gravel followed by about 1 inch of gravel dust for leveling. However, I spoke to another person that suggested that mixing up the existing gravel with the soil underneath, compacting that, and covering it with 2" of sand would make a better base that would cause less heaving in the end.
The main difficulty with this patio is that it's sort of sunken, i.e. it's at a lower level than all areas around it (see photos... ignore the item covered by the tarp), and it's surrounded by concrete on all four sides (both neighbors have concrete patios), so we expect it to get a lot of moisture and there is no place for the water to go other than down into the soil. The area we plan to cover is about 125 square feet. Any advice would be appreciated!
Do you have a water problem there now? The water need somewhere to go or you are going to have a problem. Perhaps a drain that goes out to the street? You need drainage. It's something that may not be important now, but wait for that big storm!
As far as the installation goes, I never heard of mixing soil with gravel. I don't think that person is knowledgeable. Have you seen this page on my website?
Without a doubt, this is how it should be installed. https://www.landscape-design-advice.com/building-a-patio.html
But you also need to address the drainage.