When adding a greenhouse to your garden or landscape, you will need to make sure you have a level area to install or construct your structure. Footings, or foundations, for your greenhouse should share a few simple characteristics: the foundation should be anchored to the ground, it should be well-drained and it should be relatively free from gaps where weeds or grasses might enter.
A wood foundation provides an inexpensive foundation and is relatively simple to build. The design consists of a wood foundation with an aluminium frame. Choose woods that resist decay and mould, such as cedar or redwood. If you're using a pressure-treated wood, be sure to include a barrier material between the wood and aluminium greenhouse frame, as the chemicals in pressure-treated wood are corrosive to aluminium. A wood footing will also require a ground cover or weed barrier, which you can purchase at your local home and garden store. Avoid using tarp, as it is not designed to allow for drainage.
A concrete slab foundation sits atop the ground, generally over a moisture barrier and layer of compacted gravel. The slab should be longer and wider than the greenhouse itself by at least 1 inch on all sides. The depth of the slab, or thickness, should be 3 inches for a greenhouse. To ensure proper drainage, a concrete slab will need a centre drain system with piping that leads well outside the footing of the structure. A concrete slab is poured directly into a wood frame and allowed to set. Once the concrete is dry, the wood framing is removed and the structure is built on top of the slab.
A concrete wall foundation has a footing that sits below the frost line, thus creating a stronger and more substantial foundation than the wood or slab types. In most cases, this type of footing will require a building permit, as you will need to dig a trench and place forms in the ground. Once the trenches are completed, concrete is poured in to form a below-grade foundation. The structure is then attached with bolts. This is a more permanent option and works well for larger greenhouses, particularly those with all glass walls and ceilings.