We find all landscapers have their own preferred products and methods. The following is the rationale we use as we are asked this question a lot. On our projects, we typically use ½” clear below a flagstone patio, we do this for a few reasons: 1.) Clear stone does not require compaction and cut flagstone is not always a perfect thickness, and therefore, it normally requires some minor adjustments to the granular layer. Levelling with screenings or Granular M means if the top material is loose, it could compact and result in minor settling. Using clear stone, we find we can set the flagstone with less risk of future settling. 2.) the clear stone offers a drainage layer. 3.) ½” clear is easier to grade and work with than a larger diameter stone below a patio. However, for stair treads, a larger diameter stone is recommended depending on conditions. Drainage is a very important aspect of setting stair treads. If the area is susceptible to water, use larger stone, it will allow for water to flow. We have seen stair treads wash out because of a too-small drainage stone used.