DIY Greenhouse Foundation Footing Layout
Most people like to place their DIY greenhouses on some type of foundation or footing. Foundations serve to provide a stable anchor to which to attach the greenhouse. It can be solid, be a poured slab of concrete, it can be a concrete wall, a wall made of concrete blocks, or it can be made of pressure treated pine or some other durable lumber like cedar, cypress or redwood. Whatever type of footings you decide on, it must fit the dimensions of your chosen greenhouse, it must be “square” (verify by running a tape diagonally from corner to corner), and should be more or less level (use a longer level for accuracy). Make sure that the earth where you plan to make the foundation for your greenhouse is compacted and stable, so that you do not have the unpleasant surprise of settling ground or seasonal movements later on. Such movements can cause damage, including broken glazing if severe enough.
Concrete Wall Foundations
Concrete wall foundations are a good choice for larger greenhouses and those with heavy, glass glazing. It is similar to the poured slab concrete foundation, but cheaper. While poured slab provides not only the foundation, but also the floor for the greenhouse with a concrete wall you have to decide on the type of floor later, to ensure good drainage and safe movement through the greenhouse.
Footings are areas, which are normally shaped as squares or rectangles, which are made of poured concrete. They are called “footers” because they act as the feet of the structure, acting as its point of contact with the ground. Footers have two basic desired functions. First, they spread the load of a building over a wide area so that neither the soil nor building will move. And second, for areas where frost is a concern, to prevent the building from moving during the spring thaw. The concrete wall to which you will be attaching your greenhouse to has to be set on a solid footing below the frost line, one for each corner of the greenhouse and the foundation wall. Start by marking the area where the greenhouse will sit with wooden stakes and battens. Dig a trench all around to below the frost line. To determine where the frost line is in your area we strongly suggest searching on Google, talking to your local code enforcer or looking in the farmers almanac for your area. The footing should be twice as thick and twice as wide as the wall (12-16 inches). The footing must be square and level. From each corner of the wooden form, mark the dimensions for the each footing with strings. Where strings meet, pound in another stake (use plumb bob to mark the exact spot.) Remove the strings, mark the area with paint and dig a hole for the footings. Your concrete footing can also be used to support wooden greenhouse foundation rather then placing such a foundation directly on the ground. We strongly recommend this approach to guard against your structure moving during the spring thaw. After your footing hardens, pour the concrete for the walls. The wall should be at least six inches above the soil level. After the wall is dry, dig a ditch for the drains around the perimeter to drain excess water away from your foundation. Fill the ditch with gravel or crushed rock to facilitate easy drainage. You can also start the work by marking and digging the holes for the footings, and then digging the trench for the wall and pouring
the wall later. It is a good idea to attach an insulation board on the inside or the outside of the wall to prevent against the cold seeping through the concrete foundation.
Building Your Greenhouse Sill Plate
To make it easier to attach the greenhouse base frame and walls to the concrete, it is a good idea to fasten a wooden sill plate to the top of the foundation wall or to the edge of the concrete slab. Precut 2″ x 4″ pieces of treated pine are commonly used for the sill, but cedar, cypress or redwood may also used although they are more expensive. The sill also functions as a buffer between the concrete and the greenhouse frame helping to reduce heat loss from the greenhouse.
There are many ways to attach the greenhouse base frame to the sill, but the most commonly used by DIY-selfers are concrete anchor bolts. You need to fasten bolts within one foot of each corner, and additional anchor bolts about four feet apart.
The sill plate makes it much easier to attach any type of greenhouse frame, whether your DIY greenhouse is made of aluminum, wood or PVC.