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Small Garden Greenhouse: Part 1

Author Photo
By Tom Larson | Apr 30, 2008

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Figure 1: The completed greenhouse.
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Figure 2: The key to the torsion box's strength is the honey-comb like internal framework that’s glued to both external surfaces. The yellow and blue pieces need only be fastened to the red and outer pieces sufficiently to hold them for gluing. Getting screws into the internal cross members is facilitated by rotating the outer cross members (Box B). Locating the internal frameworks is as easy as marking the outer 2-by-4 frame at 12-inch intervals and centering the pieces. Once the two plywood skins are glued to the completed frame, you will have a rigid, durable and relatively lightweight foundation for the greenhouse. See Box A detail in Figure 3 and Box B detail in Figure 4.
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Figure 5: Upper left corner detail of the sidewall layout (Box A in Figure 7). Note that the diagonal (dashed) line crosses the plywood’s edge at 5'6" and the red stud's cut angle follows.
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Figure 6: Lower left corner detail of the sidewall layout (Box B in Figure 7). Note that the red stud is taped to the base's side flush with the top, while the blue bottom plate is located on the surface.
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Figure 4: By judiciously rotating loosely fastened cross members, the internal frameworks can be assembled for gluing. (Box B in Figure 2.)
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Figure 3: Corner detail of the torsion box frame (Box A in Figure 2). Note that the 45-inch-long 2-by-4 is to the right, while the full 8-foot-long piece is to the left. The 2-by-4 frame should be squared up with one piece of plywood before installing the internal frameworks.
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Figure 7: The completed base makes a perfect surface for laying out the greenhouse's sidewalls, and makes it easy to mark angled pieces for cutting. See Box A detail in Figure 5 and Box B detail in Figure 6.
Composite Illustrations By Nate Skow

When I decided to build my first greenhouse, I wanted a place for my vegetable plants to continue growing after they had become too large for the lighted shelves where I started them. I wasn’t sure I wanted a greenhouse at all and didn’t, therefore, want to put much money into one to use for a trial. Buying a kit or a complete structure involved more than I wanted to spend, so I decided to build my own.

Since I wasn’t sure where I might locate the greenhouse, I made it light and sturdy enough to move around, and covered it with an elaborate array of laminated arc-shaped beams supporting two layers of hardware store plastic. These plastic-covered laminations have survived winter storms, thunderstorms and some small hail, but creating the beams was tedious and time consuming so I designed a more conventional structure for this project. I also chased pieces of the cheap plastic I used as a cover (the first time) all over the neighborhood one winter, so I recommend using film made for greenhouses – it will last for several years.

When I leave my plants in the greenhouse at night, a small thermostatically controlled electric heater keeps the temperature above 50°. During the day, heat is always a hazard so ventilation is essential. On a cold but sunny day, the temperature in the greenhouse tops out at about 140° with the door closed. I try to keep the temperature below 80° by propping the door open, but during my first house’s second spring, I accidentally left the door shut at noon. After lunch and a brief nap, I returned to plants that looked as if they had been sprayed with brush killer the day before. Opening the door and watering didn’t revive them. To help prevent that happening again, I’ve included a window and a heat-activated opener in this design. Commonly available radio-transmitter systems have alarms to alert you if the temperature inside the greenhouse gets too hot or cold. I’m going to set mine up this spring.

The wall and roof framing used in this greenhouse is appearance-grade 1-by-2 pine. The base is untreated construction grade plywood and pine 1-by-4s and 2-by-4s. The skids are 10-foot, treated, construction-grade 2-by-4s. If you intend to water by general spraying, you should consider using treated or naturally rot-resistant lumber, such as cedar, redwood or walnut heartwood. Other possibilities are Osage orange, mulberry and black locust. I’m not concerned about rot because I moisten flats by pouring water into a corner I have lifted, and pots I water just enough to keep the soil damp.

I used a 4-foot-by-8-foot piece of wood lattice as the shelf in my first greenhouse; I replaced it a year later with plastic lattice. Both proved too flimsy. In this design I used 3/4-inch thick cedar lattice. I like lattice because it allows some light to shine through, and spilled water doesn’t puddle on its surface.

Materials List

• 1 4-foot-by-8-foot sheet of 3/4-inch lattice (plant shelf)
• 1 30-inch wooden screen door
• 2 4-foot-by-8-foot sheets of 3/8-inch plywood (top and bottom of base)
• 3 8-foot 2-by-4s (perimeter of base)
• 7 8-foot 1-by-4s (interior of base frame)
• 5 8-foot 1-by-3s (support frame for plant shelf)
• Total 8-foot 1-by-2s = 40 (sort carefully at the lumberyard or buy several extras)
12 8-foot 1-by-2s (back wall)
8 8-foot 1-by-2s (end walls)
11 8-foot 1-by-2s (rafters)
6 8-foot 1-by-2s (plates)
3 8-foot 1-by-2s (window frame)
• 1 10-foot 1-by-2 (ridge pole)
• 2 10-foot treated 2-by-4s (skids)
• 2 3-inch eye bolts with nuts and washers
• 2 tubes construction glue
• 1 heat-activated opener
• 120 linear feet of batten tape
• 10-foot-by-30-foot greenhouse plastic
• Approximate numbers of screws:
100 11/2-inch exterior screws
50 2-inch exterior screws
100 3-inch exterior screws
20 3 1/2-inch exterior screws

Construction notes

The strength and rigidity of the torsion box that forms the base of the greenhouse require only that the frame pieces fit snugly together and provide a level base for the plywood until the glue sets. The completed base weighs about 140 pounds so you might want help moving or tipping it to attach the plastic and skids to the underside. The upper frame is amply strong for its intended purpose, but it is best not to use it as a lever to tip the base.

The upper frame’s joints are “crowded” with screws. If you hit a screw while drilling a pilot hole, adjust the angle of the bit until it clears.
Greenhouse plastic is damaged by oil-based paint; use latex coatings only.

Base construction

First, make a frame by cutting two 45-inch 2-by-4s and fastening them to two 8-foot 2-by-4s (shorter ends between the longer sides) using two 3 1/2-inch deck screws at each corner as shown in Figure 2. Next, mark three 93-inch-long 1-by-4 stringers shown in red and fasten them inside the frame at the points marked on the ends using two 3-inch deck screws at each joint.

Measure the distance from the inside of one side to adjacent red stringer (10 1/8 inches in our box). (Because of minor deviations in the thickness of lumber, a direct measurement is best.) Cut 14 1-by-4 blocks to that length. Measure the distance between two of the red stringers (11 1/4 inches in our box) and cut 14 1-by-4 blocks to this second length.

Fasten seven of the shorter blocks to the first of the red stringers using one screw at the marks. For now, leave the other end of these blocks unfastened (the end that would connect to the outside 8-foot frame). Next fasten seven of the longer blocks between the second and third stringer using one screw at the center of the stringer. Rotate these two rows of blocks (in yellow) 90 degrees as shown in Figure 2. Fasten the remaining blocks (in blue) in place using two screws at each joint. Then rotate the other two rows of blocks back into position and fasten the first row to the frame. Drill a hole in each end 2-by-4 for a 5/16-inch eye bolt 2 inches long. Locate these holes just far enough off center to miss the red stringer centered on the inside. Put the eye bolts in the holes and put washers and nuts on the inside.

Next, fasten 3/8-inch thick plywood sheets to both sides of the frame using 1 1/2 -inch screws in each corner and at the center of each side. Remove the screws on one piece of plywood and set it aside as shown. Run beads of construction glue on the top edge of all the frame pieces. Lower the plywood onto the frame; be careful that the plywood drops directly down onto the frame. Replace the screws. Place weight on the plywood. Remove the weights after the glue has cured. Turn the box over and repeat the process with the other piece of plywood.

End wall framing

The completed base will now become a place to lay out the wall frames. Set it on a couple of sturdy sawhorses. (Don’t have sawhorses? See “Sturdy Sawhorses” in the January/February issue.) See Figure 7 for details on the following. Make a mark on the base’s edge 7 feet 6 inches from one corner. From the adjacent corner make a mark at 5 feet 6 inches. Temporarily fasten (with tape) two 8-foot 1-by-2s (future end wall studs shown in red) with lower ends 1 3/8 (long dimension in 1-by-2 cross section) inches from the lower edge of the base as shown. Lay a straight edge from one mark to the other and draw a line that extends across each of the temporarily fastened future end wall studs.

Temporarily attach an 8-foot 1-by-2 (future end-wall bottom plate shown in blue) along the base opposite the diagonal line you just drew – be sure that it fully overlaps the two end-wall studs already attached to the base’s sides (Figure 6). Measure 16 inches from each of the base’s edges and mark the bottom plate. Position two 8-foot 1-by-2 studs (shown in yellow) on the 16-inch centers using the bottom plate as a guide, mark them for cutting using the diagonal line to determine the angle.

Lay a fresh 8-foot 1-by-2 (shown in green) with its lower edge on the 5-foot 6-inch and 7-foot 6-inch marks. Mark the 1-by-2 at both edges of the temporarily attached end wall studs (shown in red) – this will be the end-wall top plate.

Cut the four red and yellow studs and green top plate where you have marked them. Cut blue bottom plate to length (50 3/4 inches) and fasten the red end wall studs to its ends using 3-inch deck screws. Drill pilot holes for all screws used to fasten the frame together. Fasten the upper ends of the red studs to the ends of the green top plate. Fasten the two yellow center studs between the upper and lower plates. Repeat this process to build the other end wall frame.

Look for In the Shop‘s Small Garden Greenhouse Part 2 in the July/August issue.


An avid gardener and woodworker, Tom Larson combines these passions in his workshop and garden in Sioux Falls, South Dakota.

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