How to Make a Simple Greenhouse

Home gardeners start searching seed catalogs in the dead of winter---it's a primeval urge to get spring going. The first greenhouses were built to house tropical plants brought back from the tropics by seafaring explorers. Today, greenhouses come in all shapes and varieties from the "crystal palaces" of the Victorian conservatory to the humble cold frame used to start salad crops in city rooftop gardens. Of all greenhouse forms, the "hoop house" is one of the simplest and, with the help of modern plastics, easiest to build.

Things You'll Need

• Cypress, redwood or preserved lumber:
• 4 4-by-4-inch-by-2-foot pieces
• 4 2-by-6-inch-by-12-foot pieces
• 4 2-by-4-inch-by-7-foot pieces
• 4 2-inch-by-4-inch-by-6-foot
• 2 2-by-4-inch-by-3-foot pieces
• 2 1-by-4-inch-by-12-foot pieces (for each door)
• Polyvinyl chloride (PVC) "schedule 80" or stronger:
• 14 ¾-inch-by-10-foot PVC pipe
• Six ¾-inch-by-2-foot PVC pipe
• Five matching PVC cross-joints
• Two matching PVC T joints
• PVC cleaner and cement
• Carpenter's square and level
• Hammer and screwdriver
• Drill
• Miter or back saw
• 34 ¾-inch galvanized electrical metal tubing (EMT) straps
• 14 to 20 galvanized strap brackets
• Two galvanized door hinges (four if building two doors)
• Non-corrosive screws and nails
• Staple gun and heavy-duty staples
• At least one tall helper
• Optional:
• Substitute for wood foundation: 14 ½-inch PVC tubing stakes
• Substitute for ridge poles: 1 ¾-inch-by-10-foot section EMT roof ridge for electrical wiring
• Door handle and latch
• Shade tarp

Instructions

1. 

   Locate a site for your greenhouse; a sunny, well-drained location is just right for spring, but a summer greenhouse needs afternoon shade, such as from overhanging trees, to avoid overheating. Its foundation should never sit on boggy ground.

2. Set wooden corner posts in concrete piles. Dig below the frost line (you can find out how deep this is when you get your building permit), line with gravel, fill it with concrete, and insert a 4-by-4 corner post in each. There should be 11 feet 8 inches between the front and the back post edges and 12 feet on the sides. Make sure at least 4 inches of post remains above the pile. Set the tops of the posts elevations for a level foundation. Set a floor of pea gravel or mulch for your greenhouse, and lay out two 2-by-6-inch-by-12-foot pieces for the ends of the hoop house.

3. Make a support frame with 2-by-4-inch-by-6-foot "stiles" (sides) and a 2-by-4-inch-by-3-foot "header" (top) between them in the center of one end, attaching them to the base and each other with metal or wooden brackets screwed on the inside. Mark angles for 2-by-4-inch-by-7-foot braces by laying them under the end framing from the corner of the base to the top of the door frame, marking the resulting angles, then cutting with a miter or back saw and attaching with inside-mounted brackets.

4. Make a door with two 1-by-4-by-68-inch stiles, two 1-by-4-by-30-inch rails and one cross-brace to steady it (again joining boards with brackets). Hang this door on the center support frame with two strap hinges.

5. 

   Attach one side base to two corner posts with non-corrosive 3-inch wood screws or nails, checking with a carpenter's level before finishing and countersinking screws or nails. Nail or screw the front base to the corner posts, bracing it with some temporary supports; the front base should cover the ends of the side bases. Repeat around the frame, checking for level as you attach each board.

6. 

   Fit 2-foot sections of PVC between the 10-foot "ribs" of the house, ending with T-joints at each end. Trim the end sections if the total ridge length exceeds 12 feet, 4 inches, then glue all but the end ("T"-joint) ridge sections and ribs with using PVC cleaner and cement and lay flat to dry. Fit the ends again to make sure the ridge measures 12 feet, 4 inches, then glue the end T-joint sections.

7. 

   With a helper, attach one side of the hoops, starting in the center of one side. Space double EMT straps 2 feet apart, starting in the center (the end straps should be mounted at the corners on the front and back of the foundation) of each side, then slide the PVC rib ends into the straps, bending the other side the ribs down after finishing the first side. Staple plastic covering to the end frames, then staple the top sheet to the sides, fold the corners over and staple to the support frames on each end.

Tips & Warnings

• Add a second door to the back of your greenhouse for ventilation on warm days. As an alternative to a permanent structure, make a temporary hoop house by sinking stakes into the ground to hold the ends of the hoops and building just the end structures of your house. The whole thing can be broken down at the end of the season for storage or moved from season to season. Permanent greenhouses need thick polyvinyl to last---some deluxe kits include corrugated polycarbonate panels. Buy a 20-by-24-foot sheet and 4-by-7-foot sections of 4- or 6-mil polyvinyl sheeting to cover the greenhouse. Cut the cover poly a bit larger so it can be tucked under edges at the base of the greenhouse. Mound pebbles or mulch against the foundation for a good seal against wind and to dissuade small creatures. Add a retractable tarp to shade your greenhouse, according to your climate. Galvanized brackets (for joining doors and support frames) can be expensive---you can also use wood "corner blocks" and "keystones" cut out of scrap lumber (as long as it's exterior grade) to brace your framing.

• Most municipalities require a building permit to build a permanent structure, and it's wise to check even for temporary structures. Building inspectors can be great sources of information. Countersink nails and screws, round edges on wood and cover sharp edges with silicone caulk to avoid tears in expensive plastic. Take care when using PVC solvents, and always wear eye protection when using power tools.

Resources

• 12-by-14-foot hoophouse instructions
• Frame greenhouse
• Cold frames