Slope and Drainage for Your Greenhouse

There is no passionate gardener who does not have at least a small DIY Greenhouse on his or her wish list. Whether to extend the

French Drain How To – DIY Greenhouse

growing season, to grow some exotics or to get a head start with seedlings during the long winter months, greenhouses are the natural extension of our outdoor garden. But, considering the cost of even a small greenhouse, for most of us, a DIY greenhouse is an only option.

Decisions, decisions

Building your own greenhouse starts with making some important decisions. The first and the most important is: how much money are you going to spend. Unless you limit yourself to what you can afford, your greenhouse will become a money pit and a bone of contention between you and your family and none of you will be happy.

The next important decision is: what are you going to grow in your DIY greenhouse? This decision will affect the next one: what kind of foundation to build. The greenhouse foundation can be as simple as leveled earth to full cement floor with built-in drainage system. Whether you decide to leave the soil bare and plant your tomatoes directly in it, or pour nice cement slab with anchors for the greenhouse frame, you need to ensure proper drainage of water within and outside of your DIY greenhouse.

Drainage inside and out

Gardening is a messy business. Even if you are very careful, there will be excess of water that you should not let pool on the greenhouse floor. Stagnant water is dangerous both for plants and people. It is a fertile environment for dangerous pathogens and, in the summer,  it can breed mosquitoes. It makes the floor slippery and dangerous and makes the air inside of the greenhouse chilly in the winter.

Draining water outside of your DIY greenhouse is even more important. Just one inch of rainfall brings 27,154 gallons of water, which has to go somewhere. Making sure that the excess water goes where you want it to go prevents some expensive potential problems in the future, such as crumbling foundation of your home, flooded basement or significant erosion of your yard.

Start with a slope

Whether you are using soil as your DIY greenhouse foundation or you are pouring concrete, you need to ensure that the gravity will do its business by providing a slope for the water to drain.  A drop of 1/8 to ¼ per linear foot is the norm. If you are pouring concrete slab, make sure that its surface is sloped away from the building, if your greenhouse is attached to it, or slightly domed in the middle, if the greenhouse is freestanding,  to shed water.

If the terrain on which you are building the foundation for your DIY greenhouse is sloped, use it to your advantage. Level the terrain ensuring the standard 1/8 to ¼ per linear foot slope.

Inside drainage

In places where soil is permeable, the drainage is adequate and a solid floor is not necessary. You might wish to leave a part of the greenhouse floor bare to plant in it, and other part (down slope!) covered with about four inches of crushed stone or gravel to provide drainage underfoot and a clean and dry space for your gardening benches.

Don’t neglect to plan walkways through your DIY greenhouse. You can use any type of stepping-stones or a brick path filled with sand.

If you are pouring a concrete floor, install one or more French drains, and make sure that the floor slopes toward them. A trench drain along the sidewalls is more expensive, but it makes building the floor easier and drainage faster. Trench drains are mostly made of PVC and are sold in sections of three to eight feet. They have anchor tabs, which make them easy to attach to the concrete.

If you are building a large DIY greenhouse, check the local regulation about the drainage of excess fertilizers and pesticides. Even with the small greenhouse, you should think about the chemicals draining into your ground water and be careful not to allow too much spillage.

Outside drainage

The water from the roof of freestanding greenhouses is normally drained through a grass or gravel covered swale. A swale is a ditch that collects the excess water and allows it to slowly penetrate the soil. The swale should have sides with a one to five feet vertical to horizontal slope.

Greenhouses that are close or attached to your home need a system of gutters, downspouts and drain pipes to manage runoff. This piping system directs water runoff to a swale or to a pond or drainage area.

Don’t forget to check the local regulations about the water runoff to the neighbors’ properties. This is particularly important if you are located near a protected or a wetlands area, which can be seriously adversely affected by the pesticides and fertilizers runoff. As a passionate gardener, you do not want your DIY Greenhouse to hurt the nature around you.


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