Building a DIY Greenhouse Foundation
Whether you are building your DIY greenhouse from the greenhouse kit, or from scratch, after designing your journey starts with preparing a suitable foundation on which to erect it. The foundation has to serve several purposes:1. A foundation provides a suitable way of anchoring the greenhouse frame to the base, to protect it from gusts of wind. Greenhouses are relatively light compared to their size, and may easily fall over or suffer wind damage if not properly anchored.
2. A foundation serves as the floor of your greenhouse. It should provide good drainage for excess water, a space to put benches, shelves and raised bed.
3. A foundation provides insulation from the cold seeping in from the ground, and from weeds, insects, pests, bacteria and fungus infecting your greenhouse.
The Size of Your Foundation
Before staring work on your greenhouse foundation, check your chosen greenhouse kit information for the size of the floor. Each greenhouse kit, even the ones that come with its own floor, has to have stable, level foundation on which to be fastened. If you are building from scratch or from a plan, make sure you double and triple check all measurements before starting construction. The old saying, “Measure twice, cut once” still applies here even though we’re talking about concrete.
Materials For Your Foundation
The two most popular types of foundation for greenhouses are wooden and cement or concrete slab foundations.
Building a Wooden Greenhouse Foundation
A wooden foundation is the most common type of greenhouse foundation because it is fairly easy to make and it is inexpensive. It is
suitable for all greenhouses smaller than 8’x12’, and greenhouses that are glass-glazed which require cement foundation due to their heavier weight.
To build a wooden foundation start by cleaning the terrain where you plan to erect your greenhouse. Remove all roots and rocks; taking care to make sure the ground is level, if it is not take the needed steps to make it so. Cover the area with a weed barrier or a ground cover sheet, this will keep weeds from entering the greenhouse, but will allow water to pass through for proper drainage.
Make the frame from 4″ x 6″ or 4″ x 4″ lumber, preferably of woods resistant to decay, such as cypress, cedar or redwood. If using pressure-treated wood, you will have to place a barrier between the wooden frame and the greenhouse frame, as pressure-treated lumber contains copper, which is corrosive to aluminum.
Use galvanized steel 2.5” deck screws to put the frame together. We highly recommend a high quality 18V cordless drill for
completing this work. If you don’t own one yourself and do not want to add the cost of the tool, many home centers offer daily tool rentals.
There are two ways of fastening your wooden frame to the ground:
a) You pound pieces of rebar through holes in each corner of the wooden frame into the ground, below the frost line. Make sure the hole through which the rebar passes is a tight fit, but not so tight that you splinter the wood while hammering.
b) You can make cement posts below the each corner of the frame and fasten the frame to the posts. Ask a helper at your local home store, there are several brackets made especially for this purpose.
Fill the wooden frame with three to four inches of gravel or crushed stone, and your foundation is ready to receive the greenhouse. The gravel and crushed stone offer a good system of drainage for your greenhouse floor.
It is a good idea to dig a drainage ditch around the greenhouse foundation, and fill it with gravel, to receive excess water. This will help in avoiding issues with hydro pressure building up under your greenhouse either from drainage issues or the sloping around the greenhouse itself.
Cement Greenhouse Foundations
A cement slab provides a much more stable foundation for a greenhouse. It is long-lasting, provides excellent drainage (if it is poured with the adequate slope), is a clean surface on which to place greenhouse benches or raised planting beds and it provides good barrier from cold, insects, pests and weeds. A cement foundation is necessary for glass-glazed greenhouses, as previously mentioned due to their weight.
A cement foundation should be poured so that it is an inch longer and an inch wider than the base of the greenhouse base. A three inches cement slab is considered sufficient for most greenhouses. Be sure to reinforce the cement with wire or fiber, this will aid in the strength of your concrete – failure to include these binding materials may result in cracking or failure of the cement. Special consideration must be taken in cold climates to make sure the base of your foundation is below the frost line to avoid “heaving”.
We highly recommend picking up a book on pouring cement slabs or consulting with a professional before conducting the pour.
The slope of the cement slab should lead to an outside drainage ditch at least four inches deep. You can make a slope towards the central drainage spot and install a drainage pipe, or make the cement slope to the one side of the greenhouse. Fill the drainage ditch with gravel or crushed stone.
To make fastening of the greenhouse frame to the cement foundation easier, it is a good idea to install a 2″ x 4″ wooden sill at the perimeters of the foundation. Attach the greenhouse frame with anchor bolts at one-foot intervals.
Some greenhouse kits have optional base frames. If you are not an experienced builder, you might find it helpful to purchase base that is already designed for your particular kit. Depending on the material, a base has to be fastened to the ground, usually to pre-poured concrete footers.
Both wooden and cement foundations are fairly easy to construct, but check the regulations in your neighborhood. Some zoning laws may force you to hire professional to install the foundation for your DIY greenhouse.