The Noble Research Institute Poly Pipe Hoop House
Sometime around 2000, I began demonstrating the use of high density polyethylene (HDPE) pipe in hoop house construction. My interest in using poly pipe was primarily to find a lower cost alternative to steel pipe. Also during this time, many first time hoop house growers were using PVC pipe as a construction material only to have their houses destroyed in storms. In addition, a tremendous supply of used poly pipe, sometimes in lengths of several thousand feet or more, lay abandoned around oil fields across Oklahoma and Texas. If the pipe proved suitable for hoop fabrication, it would substantially reduce the cost of house construction.
From the beginning, we chose 2-inch-diameter SDR-11 poly pipe for our houses because it is a common size used in the oil field and is readily available at oil field supply companies. It is also used in geothermal energy systems. This pipe has a wall thickness of ¼ inch making it extremely durable. Because it comes in a roll, a pipe bender is not required to fabricate hoops. Because polyethylene is a product of natural gas, the cost of poly pipe will reflect the cost of natural gas. Typically, the cost of SDR-11 poly pipe is less than an equivalent diameter galvanized steel pipe and more than an equivalent diameter PVC pipe.
Since poly pipe does not have the structural strength of steel pipe, we limited the width of our first houses to 10 feet. These houses performed admirably and did not exhibit the same deficiencies common to similarly sized free span PVC houses, namely hoop fracturing caused by over-flexing during high winds.
Our next structures were 14-foot-wide free span models. After four years and several modifications, we are pleased with the results. To reduce costs and make it easier to attach and detach the poly film covering, we elected to use rope straps positioned over the top of the structure between the hoops as an alternative to poly fastener (Wiggle Wire) or lath to secure the film to the frame. As an added benefit, rope straps stabilize the hoops without the need for pipe purlins.
During early testing, we noticed that water tended to accumulate in the folds of the poly film when the poly was raised for venting. Standard operating procedure calls for closing the house during stormy weather. On one occasion, we failed to close the house during a storm and the house collapsed under the weight of water that collected in the folds of the cover. In response to this design flaw, we developed a method of venting the house without causing excess folding of the poly film. This venting method, called a split vent, enables the house to be vented along the sides of the structure without the need to raise the poly film along the sides of the structure. The split vent also enables venting without exposing tender vegetation to cold air, a common occurrence during late winter in houses equipped with roll-up vents.
Many people ask if 2-inch poly pipe could be used to construct hoop houses wider than 14 feet. My answer is possibly. I know of wider structures, but they are generally equipped with interior columns to provide support against collapse and purlins to stabilize hoop movement. Before constructing a free span structure greater than 14 feet wide using 2-inch SDR-11 poly pipe, build a small structure and evaluate its performance under your growing conditions.
The Noble Research Institute poly pipe house is a hybrid, incorporating features of both permanent and completely portable structures. With the exception of the ground posts, every component of the structure can be disassembled. While the structure is designed to be portable, it is also suitable as a permanent structure. Equipped with the Noble Research Institute portable end wall or a comparable end wall, the Noble Research Institute poly pipe high tunnel hoop house offers the market gardener an alternative, cost competitive, season extension structure with a long service life.
Plans for constructing a 14-foot-wide x 36-foot-long poly pipe high tunnel hoop house are now available at no cost by contacting the Noble Research Institute. The plans will also be available on the Noble Research Institute website in early 2012.