10 Prepping Your Site option 1 Dig out the top three or four inches of topsoil for the perimeter of the greenhouse (at least 12 inches wide). Now dig out all four corners to inset concrete pier blocks to ensure that the green- house remains level and does not shift or settle due to soft ground or frost heaves. Place inset pier blocks every 6-8 feet. option 2 If you live in a colder climate where the frost level is deeper than 12 inches, you should consider using concrete sona tube supports below grade as you would do with typical building structures in your area. Again, it’s critical that the pier blocks or sona tube Three: Building a Wood Foundation