Depending on the greenhouse chosen and whether it is a seasonal or an all year around growing project. These two factors should be the basis for the type of foundation to build.
If it is seasonal, then a topical perimeter base made out of landscape timbers or a 6-8″ wide concrete base wall also called a grade beam is sufficient. Make sure it is properly anchored into the ground. This is assuming that the interior floor of the greenhouse will remain dirt or gravel.
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ANCHOR YOUR FOUNDATION
can be accomplished by digging concrete corner piers 18″ to 30″ deep – 12″ diameter. Inserting qty 1 – 10″-12″ galvanized anchor bolts embedded into the center of the concrete pier. Secure a six inch wide landscape tie to the concrete pier, making sure to expose the threaded part of the anchor bolt minimum 6″-7″ above the level of the concrete.
If using landscape ties the concrete pier should be flush with the dirt. The landscape tie placed on top of the concrete flush with the dirt. Anchoring the landscape tie with properly placed holes drilled in the center of the tie, minimum 6″ from the end corner of the tie.
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Top plates for greenhouse frames should be standard Douglas fir. Always install and secure a 2×6 or wider, green top plate on top of the concrete grade beam.
GREENHOUSE FOUNDATION FLASHING
Although it is perfectly OK to anchor the greenhouse frame on top of the wood sill plate, as a rule its best to install sill plate flashing. Covering the entire width of the sill plate and bent over the outermost edge 2″ down with an outer drip edge.
This creates a smooth even surface for sealing the gaps with quality silicone caulk between the greenhouse base frame and the sill plate. This also prevents moisture from leaking and soaking onto the wood sill plate. This flashing feature provides a clean looking greenhouse base appearance protecting the wood sill plate and provides a much longer life span.