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How to Build a Pole Barn Greenhouse

Daisy Peasblossom Fernchild

This design combines cob building, New Alchemy Institute bio-shelter principles and a rocket stove with pole construction. The long, back-sweeping north side is based on classic New England salt box houses, which were designed to encourage cold winds to sweep up and over a building rather than battering against it. Cob and rocket stoves go well together; rocket stoves utilize a heat sink which smooths out heat distribution. The heat sink can absorb solar heat on clement days.

Maximizing sunlight while creating options for ventilation are essential considerations for any greenhouse.

Step 1

Plan the location of the greenhouse. Considerations are solar exposure, ventilation, heat retention and a convenient water supply. The greenhouse should be placed where it can face south and get east and west sunlight. The north side of the building can be a larger building, hillside that can shelter the building from wind or a long sloping roof which shelter a storage area or office while causing north winds to move over the building rather than against it.

Step 2

Select the poles. The poles for the central ridge of the greenhouse should be tall poles that will help support the roof. The front poles can be very short, since they will support the front ledge of the framework for the glazing, as can the poles for the north edge. Poles used in the ends of the greenhouse will support the front frame for the glazing and the roof for the office or storage area.

Step 3

Select the type of glazing that will be used in the greenhouse. Options include glass, plexiglass or heavy duty, clear, plastic sheeting. The type of glazing will affect the type of support that must be incorporated into the structure to hold it. To ensure maximum exposure to sunlight during daylight hours, the main part of the glazing should be mounted on a south-facing slanted roof.

Step 4

Prepare the ground. Clear any large obstructions from the building area. With a compass, position a string that stretches east and west to indicate the north wall. Fasten it in place using pegs. From either end of the north wall, position strings to indicate the east and west walls, then stretch one between the ends of the east and west walls to indicate the south wall.

Step 5

Place two batten boards -- boards nailed across the tops of two stakes -- at each corner, about 2 feet out from your initial indicator string. Tie a string to a stake, stretch it across the tops of the batten boards above the indicator string.

Step 6

Check for squareness by measuring your layout from corner to opposite corner. If your two measurements match, your string markers are square. If the measurements do not match, check the corners with a carpenter's square, starting with one corner on the north wall and moving the strings until the corners appear to be 90-degree corners, then check the south corners. Measure corner to opposite corner again. Repeat the process till the distance from the northeast corner to the southwest corner and the distance from the northwest corner to the southeast corner are the same.

Step 7

Divide the length of the east wall by two. Place a batten board east of the marker string, centered on the middle of the wall. Place a batten board west of the center of the west wall. Tie one end of a string to a stake and drive it into the ground west of the west batten. Pull the string over the west batten then across the east batten. This marks the location for the central poles.

Step 8

Mark the positions for all poles. Dig post holes for each. Holes for support posts should equal one-third of the length of the central posts. Set the posts in place, holding them roughly in position with rocks and other aggregate. Make a collar for each post of 2-inch by 6-inch lumber. Screw the boards together around the post. Use a level to make sure the posts stand upright. Use double-headed nails to attach bracing boards to hold them in place.

Step 9

Mix and pour concrete around each of the posts. When the concrete has cured, nail 2-inch by 6-inch boards to the both sides of the central posts and to the outer edge of the north and south posts. Cut 2-inch by 6-inch boards angled on the top end and with a notch in the bottom end to form roof rafters. Nail 3/4-inch plywood on the north-facing rafters. On the south-facing rafters, nail 1-inch by 2-inch boards 3/4 inch down from the top of the 2-inch by 6-inch rafters. These will serve as supports for the glazing frames.

Step 10

Dig a trench 2 feet deep by 3 feet wide south of the central posts, and one 2 feet deep by 3 feet wide north of the central posts. At one end of the north trench, construct a rocket stove mass heater. Place a layer of firebrick 3 feet long in the end of the trench in the windward end of the trench -- this will often be the west side.

Step 11

Stand firebrick on edge to form the sides of the firebox and reburner chamber. Leave a connecting passage between the firebox and reburner chamber. Also, create an ash clean-out access at the bottom edge of the firebox. Place a round piece of stove pipe above the reburner chamber and turn a barrel over the top of it, leaving a 2-inch space between the top of the stovepipe and the barrel.

Step 12

Attach stovepipe to the barrel or just under the edge of the barrel and along the length of the trench. At the far end, attach two elbows to bring the pipe back to the barrel assembly, add another elbow to a stove pipe that will lead to an insulated pre-fab chimney on the north roof. Backfill the trench with fireproof material. Lay two rows of concrete block north of the trench and across the ends to the front of the south facing trench and across the front of it, forming a bin. Fill the bin with fireproof aggregate. Leave a 40-inch space at the end opposite the stove in which to place a doorway.

Step 13

Attach poultry netting to the north side of the central posts and use it as a base to create a cob wall between the storage space and the greenhouse. Surface the rubble on the office side with finished cob to create a seat. On the greenhouse side, use corrugated sheet metal painted black to create a waterproof surface over the heat sink bin. This will act as a solar collector or work bench. In some seasons, it can be used as a surface on which to place flats of seedlings.

Step 14

Build a plant bench at the front of the greenhouse. Use 2-inch by 4-inch lumber to make square shapes. Stand the squares on edge and nail 2-inch by 4-inch boards across them, leaving a 1/2-inch gap between the boards for drainage. Attach siding to the east and west ends of the office space and create an exterior door in the east wall. Make supports for the glazing in the east and west ends of the greenhouse part. Use 1-inch by 2-inch firring strips to create frames for the glazing. Place the framed glazing in place. Use butterfly catches to hold the frames in place.