Built similarly to a barn, the freestanding gambrel roof greenhouse has considerable structural strength and is excellent in regions with heavy snowfall. Erected in sections, it lends itself readily to either a film plastic or rigid plastic covering.

One of the most time-consuming aspects of building this type of greenhouse is cutting the plywood gussets. To speed this process, cut one of each pattern as shown in the diagram, and use it as a pattern for all the others. Also carefully note the cutting diagram for the legs and rafters. After they are all cut, glue and nail the legs and rafters together with the gussets.

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For a 10- by 12-foot greenhouse, the frame is constructed by sinking 4 by 4s, 4 feet long, into the ground. Leave enough of the post exposed so that you can nail on the I by 12 frame. (This greenhouse can be placed on a traditional foundation for more permanence.)

To level the posts, drive them in about 30 inches and cut off the excess after you have put the frame in place and adjusted it until it is level. Place the posts 6 feet apart down the sides and 5 feet apart across the front and back. Across the top of the 4 by 4 posts goes a 2 by 4 sill. Mark the sill where each leg and end suppert will go.

To frame the greenhouse, put the pre-constructed end walls in place first and brace them securely. Put the two 1 by 4 purlins on the ridge, then put in the two center supports and hold them in place by nailing them to the purlins. Double-check that everything is square before nailing. The structure is further strengthened with 1 by 4 purlins at the middle and lower edge of the roof.

As an alternative to rigid plastic, two layers of film plastic can be used for glazing. You can maintain the thermal layer of air between them with a small squirrel-cage fan. Install the fan at one end near the ridge line by mounting it to a plywood panel suspended from the end rafter. Cut a hole in the plywood for the fan to draw in air, and use flexible plastic tubing and a plastic pot with a hole cut in the bottom to direct the air between the two layers of plastic.