A wood foundation is a great, economical choice and supplies are readily available at your local hardware store.
There are two options if you opt for a wood foundation:
It is important to know that we only recommend wood foundations for glass greenhouses 12x16 and smaller and polycarbonate greenhouses 16x20 and smaller.
Option 1 - Frost Level 11 Inches or Less
The first step is to dog out the perimeter of the greenhouse. Dig 3-4 inches deep and at least 12 inches wide. We recommend using concrete pier blocks in all corners to prevent frost heaves and to ensure that the greenhouse settles properly and remains level. Excavate all four corners to inset the pier blocks. Consider a concrete pier block that has a metal bracket so that you can mechanically connect your timbers for the best support. For larger greenhouses, place inset pier blocks every 6-8 feet for the best results.
Option 2- Frost Level 12 Inches or More
If you live in a colder climate where the frost level is deeper than 12 inches, you should consider using concrete tube supports as a typical outbuilding in your area. Bracing is not required unless the tubes are placed two feet or more above the ground. Again, it is critical that the pier blocks or tube supports are level, so the greenhouse installs properly.
Alternate the timbers in stackable joints and fasten them with galvanized connectors or deck screws for additional strength. For larger greenhouses, two rows of alternating stackable, joints are necessary. If you live in an area prone to high winds, consider tying down your wood foundation by driving a steel rebar into the ground and pin it to the wood or using a pier block with a metal bracket.