31 How deep does concrete need to go? If you require a permit, your city by-laws may state that you need to set the greenhouse foundation beneath the frost line depending on the properties of your soil. You may need a mini excavator to dig to the necessary depth. Foundations in some colder climates where extensive digging is required can incorporate concrete sona tubes to be the portion that extends down to the frost line. What do you need for drainage? Most contractors will suggest installing a drain into your concrete floor with a bit of a slope to the drain for easy run off. This is a very simple way to clean your greenhouse when necessary.