Brian Post, a stoneworker and landscape architect from Chester, Vt., replies: The goal of a patio foundation is to provide a stable base that won't settle or heave. Settling occurs from poor compaction or erosion. Heaving is caused by freezing water and tree roots. Drainage and compaction are the keys to avoiding those things and ensuring a patio's (or a stone walk's) longevity. Using clear (also called "clean") crushed stone for the lower layers is key to a solid foundation. Avoid using rounded stone-crushed stone should be angular and jagged so the pieces lock together. Don't use a material with fines (this includes sure-pack, crusher run, plant mix, and quarry process). The fines prevent drainage.
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