The actual per square foot load of a hot tub is not that great, so it will generally work on decent soils with limited improvement. For example, a four foot deep hot tub exerts about 250 pounds per square foot. Compacted angular gravel or road base can support easily 2000 psf.
The key is to remove unacceptable soil first, before you place your support layer. Unacceptable soil would include organic soil, uncompacted fill, wood, debris, or other waste material, soft clay, and soft silt. Once you get down to decent mineral soil, meaning sand, gravel, hard clay, glacial till, or dense silt, you can backfill to grade with structural fill, compacting in six inch lifts as you go.
Of course, you can read the manufacturer's recommendations on base preparation, as previously suggested. You can also discuss foundation requirements with the building inspector, if a permit is required. I second the opinion to skip the plywood, it is unnecessary, and likely to rot over time. Similarly, if you feel you need a barrier for the fill, make sure the PT you use is rated for ground contact, most PT sold at the big box stores is NOT rated for ground contact.
The key is to remove unacceptable soil first, before you place your support layer. Unacceptable soil would include organic soil, uncompacted fill, wood, debris, or other waste material, soft clay, and soft silt. Once you get down to decent mineral soil, meaning sand, gravel, hard clay, glacial till, or dense silt, you can backfill to grade with structural fill, compacting in six inch lifts as you go.
Of course, you can read the manufacturer's recommendations on base preparation, as previously suggested. You can also discuss foundation requirements with the building inspector, if a permit is required. I second the opinion to skip the plywood, it is unnecessary, and likely to rot over time. Similarly, if you feel you need a barrier for the fill, make sure the PT you use is rated for ground contact, most PT sold at the big box stores is NOT rated for ground contact.
