A greenhouse may be built upon a foundation of timbers, block, brick, or concrete. Elevating the foundation has the advantage of gaining height and headroom in some greenhouses. The greenhouse foundation should be stable, level, and square.
A popular material for the floor is ¾" crushed rock, but the floor may be concrete, dirt, mulch, brick, or tile. The installation of perforated drains beneath a porous layer of flooring helps to assure good drainage.
When building a greenhouse, it is useful to insulate the foundation. This will prevent the loss of heat through the ground and permit the ground to act as a "heat sink." The installation typically consists of 1-2 inches of polyurethane or polystyrene insulation 1.5-2 feet deep around the foundation.
Information provided by: Stressbaby