Foundations: Wood Frame vs. Wood Post
Greenhouse gardening is addictive, and most people who build a greenhouse will have more than one greenhouse in their lifetime. But, for your first DIY greenhouse, it is a good idea to start small. Do not invest too much money until you know that you will be able to invest the necessary time, and that your interest in greenhouse gardening will not disappear to be replaced with a new hobby. It may be a good idea not to make a permanent foundation for your first greenhouse. You might find out that you have to move your greenhouse to a new location, or remove it altogether. Finding the perfect location is not easy, you might decide that another spot has more sunlight, or your neighbor might plant a tall tree that is blocking the sunlight for your greenhouse plants. A permanent cement foundation is near impossible to remove, so it is better to start with more temporary, wooden foundation, or just with wooden or concrete posts.
Keep in mind that a wooden foundation is not suitable for large greenhouses, larger than 12’x16’ and those glazed with glass.
Greenhouse Wooden Post Foundations
Small greenhouses, particularly those covered with polyethylene, do not need a solid foundation, but they do need to be anchored to
the ground. Greenhouses are fairly light, in ratio to their size, and can be easily overturned by a strong gust of wind. Quonset type greenhouses are shaped similarly to airplane wings, and wind can actually lift them off the ground or their foundations. A strong anchor is necessary to avoid this.
Depending on your soil, you can construct wooden or cement posts, to which you can anchor your greenhouse base frame. Wooden posts are the cheapest and the easiest anchors, but they do not last long in comparison to concrete, particularly if your soil is not free draining and the posts are often sitting in water.
To place wooden posts for your greenhouse, mark the site on which you wish to erect your greenhouse. Place temporary stakes at the corners and dig holes at least three feet deep, but below the frost line (you area may require a different depth). Place some gravel or crushed stone at the bottom of each hole. Use 4x4s of treated pine, or some other type of wood that does not decay easily, such as redwood or cypress. Place the precut posts in each hole and back fill the hole with soil.
For more weather-resistant and stronger footing for your greenhouse, pour concrete posts and attach your greenhouse via concrete bolts and fasteners.
Greenhouse Wooden Frame Foundation
A wooden foundation will be stronger and longer lasting than just wooden posts, but will costs slightly more and it takes a little longer to make. The advantage over wooden posts is that wooden foundation also provides a good floor for your greenhouse and suitable drainage for excess water underneath. The greenhouse frame is anchored to the entire wooden frame of the foundation, providing much stronger anchoring and a more stable structure.
Prepare the site for your greenhouse by clearing it of debris, tree roots and rocks. You might want to remove the layer of grass if other wise left undisturbed, but it is not necessary if you are covering the site with a weed barrier cloth. The cloth is a good idea as it prevents weeds, pests, insects and bacteria from entering your greenhouse.
Place the weed barrier cloth over the cleared site and place a wooden frame the size of the greenhouse base on it. Make sure that the wooden frame is square and level. The frame is usually made of pressure-treated pine, but more expensive lumber such as redwood, cedar or cypress can also be used. Use two layers of 4x6s of 4x4s and attach them with galvanized steel deck screws. Check again if your frame is square and level.
Anchoring The Greenhouse Frame
There are several ways to attach your wooden frame to the ground. The easiest and most effective is anchoring with earth anchors, one for each corner of your greenhouse wooden frame. You can also use rebar, attached to the wooden frame, or placed through holes in the wood and hammered into the soil underneath to at least three feet, or below the frost line in your area. REMINDER: When doing any earth work or driving stakes into the ground you must be 100% sure of what is under the earth as you may cause damage to electrical power lines, or even worse, injury yourself.
Once your wooden frame is anchored, fill it with three to four inches of gravel or crushed rock. It will ensure good drainage and provide solid footing inside of the greenhouse. It is also a good idea to dig a drainage ditch on the outside of your wooden frame, and fill it with gravel, to provide drainage for the rain water away from the structure.
While both wooden posts and wooden frames provide solid anchoring for your greenhouse, if you opt for the posts, you will have to come up with some way to drain excess water, and to provide clean floor for your plants and yourself. Posts might be a good solution if your greenhouse kit comes with its own floor. Wooden frame ensures simple but effective drainage and clean and comfortable floor for your DIY greenhouse.
Photo Resource: http://www.substiwood.com/deck_post_foundation.htm