Sizing a hoop house
A hoop house, also called a high tunnel or polytunnel, is a greenhouse made of arched hoops covered with plastic film. It extends the growing season by several weeks in spring and fall, and protects crops from rain, wind and frost.
The arch
The calculator treats each hoop as half an ellipse, with half the width as the horizontal radius and the height as the vertical radius. Its length is estimated with Ramanujan’s formula for the perimeter of an ellipse. When the height is exactly half the width, the arch is a semicircle and its length is π × width ÷ 2.
Example
A tunnel 10 ft wide, 7 ft high and 20 ft long, with hoops every 4 ft:
- hoops: 20 ÷ 4 + 1 = 6 hoops;
- each hoop is about 19 ft long, so about 114 ft of tubing;
- with 1.5 ft of extra film on each side to bury or fasten, the cover is about 22 ft wide;
- to also cover both ends, the film must be about 20 + 2 × 7 + 2 × 1.5 = 37 ft long;
- floor area: 200 ft².
Greenhouse film is sold in standard widths and lengths, so round up to the next size.
Choosing materials
- Film: use greenhouse film, typically 6 mil, UV-stabilized and rated for 4 years. Ordinary construction plastic breaks down in sunlight within months.
- Hoops: galvanized steel tube or bent conduit is sturdier than PVC, which sags in heat and can crack in cold.
- Spacing: 4 ft is common for small tunnels; 6 ft is the usual maximum. Use closer spacing where snow falls.
- Anchoring: set ground posts or stakes and fasten the film with wiggle wire and channel, or bury the edges in a trench.
Ventilation
A sealed hoop house overheats quickly on sunny days, even in winter. Roll-up sides, end doors or vents are essential. Aim to keep the inside below about 85 °F (29 °C) for most crops.
Orientation
For winter growing, a long axis running east to west gives the most low-angle sun. For summer crops, a north-south axis gives more even light along the beds. Check that no building or tree shades the tunnel in winter.
Before building
Ask your town about permits: larger or permanent structures may need one, and some homeowner associations restrict them. Remove snow from the roof promptly, since wet snow can collapse a light hoop house.