The floor is "screenings" which is the tiny rocks and dust left over when crushing rocks for gravel. I laid a bed of it before I put in the GH foundation. The foundation is 6 x 6 Tamarac wood sitting on 16 x 8 x 4 concrete blocks, so the screenings base is about 12" deep. I did have 16 x 16 pieces of slate down to increase the heat sink, but I took them up (at least for now) since many of them will be covered by the raised beds. No point in wasting slate under raised beds. Not sure if I will put them back down or just leave the floor like it is now.