5 A concrete foundation is more permanent, of course, and depending on the greenhouse you select, will become a part of your landscape for generations. Other common foundation materials include cinder blocks or bricks. Believe it or not, we have also seen a terracotta foundation that looked pretty great! Pressure Treated Wood Foundation A hobby greenhouse (under 120 sq. ft.) can be fastened to 4" x 4" or 4" x 6" pressure treated wood foundation. For a larger greenhouse, 6" x 6" beam or 8" x 8" wood timber is your best choice. Wood timbers can be stacked to increase height inside the greenhouse and create a more permanent look. Keep in mind the higher the greenhouse, the more air inside that you will need to heat. Another option is to use wood that is rot resistant and readily available in your area. Cedar is a great choice for a greenhouse foundation. Pressure-treated wood foundation, and upper right, concrete foundation.