Not Level? No Problem. How to Build a Greenhouse on a Slope

Get your land to work with you for your plants. Is your yard uneven or sloped but you want to build a greenhouse? It may not seem like the “perfect spot,” but a sloped site for a greenhouse offers a bonus that a level site does not—the ability to use gravity to harvest rainwater.

The following is an excerpt is from The Bio-Integrated Farm by Shawn Jadrnicek & Stephanie Jadrnicek. It has been adapted for the web.

(Photographs courtesy of Shawn Jardnicek unless otherwise noted.)

Leaving Level Ground

When I first started building greenhouses I always followed the standard recommendations to properly site the house: Pick a level spot, and carefully grade the land so water flows off and away from the greenhouse. After all, if water flows back into a greenhouse, it creates a boggy mess. If water flows through a greenhouse, it removes precious heat as it passes from inside to outside.

Here is the framework of the first greenhouse I built on a sloping site—a steep garden on my father’s property. The greenhouse slopes in two directions: 8 percent from back to front and 2 percent from right to left. Water runs from the highest corner (top right of the greenhouse) to the lowest corner (bottom left). Terraces outside the greenhouse and raised beds inside placed on contour slow the flow of runoff and allow water to infiltrate.

My levelheaded thoughts shifted when my father requested I build him a greenhouse. I agreed to the venture, picking out the perfect spot for the greenhouse near his home. It was close enough to the house to run electricity and plumbing easily. Using an angle finder, I checked to make sure the distant trees wouldn’t shade the greenhouse in the dead of winter when it would most need the sun. Certain I’d selected the perfect spot, I happily announced the results of my site analysis to my father. However, like many design problems, the perfect spot presented me with a perplexing problem that forced me to shift my perspective on the “right way” to do things.

Sometimes it’s bureaucratic red tape that compels a change of mind, such as a required setback from the edge of the property. Or the problem might be a safety issue—for instance, an overhead power line. Whatever problem arises, solving it pushes you into unfamiliar territory. With my father’s greenhouse this unfamiliar territory happened to be a steeply terraced garden.

How could I build a greenhouse on a steep slope?

To make matters worse, the site sloped in two directions, dipping overall to a single point at one greenhouse corner. It seemed like an impossible obstacle, but a viable solution slowly emerged. With a little extra effort we figured out how to lay the base of the greenhouse all in one plane (albeit a sloping plane).

With that worked out, the next problem to solve was how to deal with rainfall. After water flowed off the roof and hit the ground, it would naturally flow down the slope, which could cause erosion. We compensated by connecting terraces to the sloped greenhouse. The terraces slowed the flow of the runoff and allowed water to infiltrate rather than erode the landscape.

While the system was not ideal, it worked. And it brought to my attention an important question. If I could build a greenhouse on sloped terrain where water runoff created a problem, could I somehow use the slope and runoff to my advantage? In short, could the problem become the solution?

Installing a Sloped Greenhouse System

My job at Clemson University posed the perfect scenario to test greenhouse systems. We wanted to build a new greenhouse and move several of the older existing greenhouses into a suitable area closer to other farm buildings. The lessons I’d learned building my father’s greenhouse convinced me it was possible to build greenhouses on a slope to catch more winter sunlight and use the slope to harvest rainwater.

The chosen site sloped gently to the west and connected to the parking lot, office, postharvest area, and market building. The site needed a complete grading and a water-management system to keep the parking area and buildings dry. As I worked on the greenhouse plans, I spent a few extra days developing a comprehensive plan to catch and use the water shedding from the buildings and parking area as well.

Tilted Surface Compared to Horizontal depicts the percentage increase in sunlight received on a tilted surface in comparison to a horizontal surface. Solar Radiation on Tilted Surface shows that as slope increases, the amount of direct solar radiation a site receives also increases. Analysis performed using calculator on www.pveducation.org.

Sloping the land toward the south catches sunlight and water, but too much slope causes erosion. So I chose a slope of 1 percent. This gentle slope—descending 1 foot over a length of 100 feet—catches a little more sunlight. Yet it’s not steep enough to cause seeds and soil to be washed away.

To create the slope, we harvested soil from areas slated to become water-holding ponds. Because of the land’s natural western slope, we would have had to move an enormous amount of soil to create a perfect south-facing slope. To compromise, I chose to create a two-directional sloped platform, grading the land to the southwest and maintaining the existing western slope. Because we were building five greenhouses over a large area, we built cascading ponds to accommodate the two directions of the slope. The dam of each pond became a pathway for walking into or between greenhouses.

We positioned the ponds on the south end of the sloped land to capture reflected light in the greenhouses during winter. Rainwater running off the greenhouses flows downhill into the ponds. The ponds are sized appropriately for our climate; thus, rainwater alone is sufficient to replace natural evaporation and keep the ponds full.

Setting the Orientation

The angle of sunlight becomes lower in the winter, and this effect is greater the farther north a property is located. An east-west orientation is always best for any size or shape of greenhouse because it maximizes solar energy capture. The east-west orientation prevents low-angle solar rays from reflecting off the greenhouse plastic instead of penetrating into the interior.

However, placing multiple greenhouses in an east-west orientation is difficult because each greenhouse can cast a shadow on the adjacent greenhouse. The effect of winter shadow increases as you travel north, necessitating even greater distances between greenhouses.

The 40-degree latitude is the point at which an east-west orientation is the only choice that makes economic sense. If you’re building multiple greenhouses on a site that is south of 40 degrees latitude, you can position them in a north-south orientation without losing an economically significant quantity of solar radiation. The north-south orientation allows much closer spacing without worries about shadows, and that’s what we opted to do at Clemson.

A finished greenhouse platform slopes 1 percent to the south from B to D and A to C and 1 percent to the west from B to A and D to C. Water flows into a reflecting pond on the south side. Greenhouse bows will attach to the metal stakes along the edge. Long-lasting landscape fabric installed on the beds prevents weed intrusion.

When building greenhouses on a sloped platform, orient the platform toward the south. That way, the greenhouses will absorb more solar energy, and the rainwater collected on the south side will reflect more light into the greenhouse during winter.

The orientation of a greenhouse in regard to slope has an impact on how water will flow toward a pond. If your greenhouse is located in an east-west orientation on a south slope, then you’ll need to channel water flowing off the north side of the greenhouse around the greenhouse and into the pond. If your greenhouse is in a north-south orientation, water flows off the greenhouse sides and straight downhill into the pond.

Building the Platform

Building a sloped greenhouse platform requires planning and grading. In a nutshell, the process is as easy as determining the direction and percentage of the land’s slope, calculating how much soil is necessary to alter the slope, then setting grading stakes and actually moving the soil.