Site Preparation for DIY Greenhouse Foundations
Once you have decided on the perfect site for your DIY greenhouse, you have to prepare the site and make it suitable for a building. You should know at that point what kind and what size greenhouse you are building and whether you plan to make your greenhouse permanently anchored to the chosen site, or you think that you might move it at some point. If there is a possibility of moving it, you should not build a permanent foundation like one made of cement, but one that is more temporary and can be easily removed or moved.
A greenhouse foundation serves several purposes: it provides stable anchor for the greenhouse base and the frame, it keeps the greenhouse floor protected from weeds, insects, pests and bacteria, it helps to ensure good drainage of excess water and it makes greenhouse floor easier to use and walk on.
Finding or Creating Flat Terrain
If the site of your future greenhouse is fairly flat, your job is easy. All you have to do is clean the site of rocks, tree roots and any rubbish. At that point you have to decide if you will install your greenhouse directly over the existing grass, sand or earth, or you will be building a foundation. If the ground is not flat you have two options. You can either level the ground and then build your foundation (through use of machines or elbow grease) or you can add blocks under your foundation, or adjust you concrete pour so that the foundation is level even if the ground is not. Do not build a greenhouse on a crooked foundation!
Slope Concerns for Your Foundation
If your terrain is on a slope, you will have to do some digging, to provide at least reasonably flat ground for the greenhouse. Keep in mind that the rain will be running down the slope and may enter the greenhouse unless you provide good drainage around and away from your structure. Cold is also a factor as the bottom of a hill frequently becomes a cold pocket, which could result in issues with your plants or additional heating costs.
Once you leveled the ground to prepare it for the greenhouse, you have to make the decision on the type of foundation.
Building a Greenhouse Without a Foundation
If you plan on simply anchoring your greenhouse over existing turf, you will have to plan on making a barrier to prevent weeds from entering the interior of the greenhouse. The greenhouse environment is not only optimal for your plants, but will make weeds thrive. A weed barrier is a fairly cheap solution, and it will allow water to go through for proper drainage. Even if the soil inside of your greenhouse is free-draining, plan on digging a drainage ditch on the outside of the greenhouse walls, to take excess water away from the greenhouse’s frame. A four-inch ditch filled with gravel or crushed rock should be enough to do the job, but your installation may require a larger ditch depending on the location of where you are building.
Without a foundation, you have to come up with a way of anchoring your greenhouse to the ground. Greenhouses are fairly light in ratio to their size, and could easily be blown away with a strong gust of wind unless you provide solid anchor. Pouring cement footers below the frost line would provide solid anchoring points. Forms for these footers can be purchased at any home improvement store. In order to determine where the frost line is for the area in which you are building, consult local contractors or a farmer’s almanac.
Wooden Greenhouse Foundations
Wooden foundations are easy and cheap to make, and provide a solid base for most types of greenhouses. Keep in mind that greenhouses with glass glazing and those larger than 12’x16’ need cement foundation to handle the weight and loading of those types of structures.
Preparing the site for a wooden foundation is fairly easy – besides removing rocks and tree roots, you can make it on top of any type of ground. Make sure that it is fairly level, though.
There are several ways of making the wooden foundation, but all of them involve covering the site with a weed barrier, placing a wooden frame the size of the greenhouse base on top, and filling the frame with about three inches of gravel or crushed stone.
The wooden frame provides an easy way of anchoring your greenhouse base, but you need to fasten it to the ground to make it strong and stable. You can make cement footers below the wooden frame corners, or you can drill holes in the lumber at the corners and hammer a piece of rebar to the soil underneath, to the depth below the frost line – the main thing is to make sure your foundation is secure in it’s place.
Cement Greenhouse Foundations
Preparing the site for making a cement foundation is basically the same for wooden foundation: making the terrain level and clean of debris. Pour cement slabs about four inches thick, with a slight slope to ensure good drainage. Make a drainage ditch at the lower side of the slope, to collect excess water.
Whether you are building a foundation or not, you have to provide a suitable site for your greenhouse, where there is at least six hours of sun daily, protected from strong winds and with adequate drainage. Investing a bit of time to plan and prepare the site for your DIY greenhouse will pay off later, when your greenhouse kit arrives and you are ready to have some real fun.