Determining the size of your greenhouse
After determining the location of your greenhouse, you will need to establish the dimensions. The chart and diagram in Figure 1 will help establish the height and width. The length of pipe A will establish distance B which will be the width of your structure. Dimension C will be the height of the structure (above the side walls). For example, using the chart below, if the length of the roof side pipe (A) is 7 feet, the width of your structure (B) will be 11' 4" and the height (C) will be 5' 7".
|Roof side length||Width||Height|
It is important that the layout of your project be squared. To do this you may use the 3, 4, 5 rule. If one leg of a triangle is 3 and the second leg is 4, the triangle will be a right triangle when the diagonal is 5. In the figure below, the distance from D to E is 3 feet and D to F is 4 feet. The corner will be squared when the distance between F and E is 5 feet. Repeat this for one other corner and your layout will be squared. To check your work, measure from corner to corner (G to I) and again from corner to corner (D to H). The distance will be equal if your layout is squared.
After determining the location of your greenhouse, drive a wood stake into the ground to mark one corner of the structure. Use the string line to mark the general outside perimeter of the structure and use the 3, 4, 5 rule to square these sides. Drive wood stakes in at each corner to establish the length and width of your structure and check to make sure the base is square.
Using a hammer or heavy mallet, drive a metal rod into the ground at each corner and at approximately 5 foot intervals along each of the long sides of the perimeter to loosen the ground. This will make it easier to drive the PVC pipe into the ground for the side wall construction. Using sections of PVC pipe that are 6 to 8 inches longer than the desired height of your side wall, drive them into the ground along both sides of the length of the structure where you used the metal rod to loosen the soil. These will be your side wall supports and will be connected to the roof bows that will be constructed later. Install the 1x4 inch wood perimeter footing boards and secure to the side wall support with pipe screws. Use a level to check the vertical level of each pipe support as you attach the 1x4 inch wood strips to the side wall supports.
Leveling the greenhouse:
Run a string line around the perimeter of the structure toward the top of the side wall supports. Use this to determine the lowest point of your structure and the starting point for leveling the side walls. Secure a string line approximately 1 inch below the top of one of the side wall supports on the wall with the lowest point. Stretch the string line to the next corner. Place a line level in the middle of the string and with the line taunt; adjust up or down until the line level indicates a level line (figure 4). Mark each side wall pipe with a pencil where the string line touches the pipe (figure 5). Continue this around each side of the structure. Cut the pipe with a pipe cutter or saw where you marked each one during the leveling process.
|FIG 4||FIG 5|
To add strength to the side walls you will need to add bracing. Attach a piece of PVC pipe to one of the corner pipes and extend it diagonally down to the wood perimeter footer board on the long side wall (figure 6). Use screws to attach the pipe brace to each of the wall supports that it crosses. Do this for each corner.
Building the bow:
Cut two 7 foot lengths of 1 inch PVC pipe. Connect one length of pipe to a 90 degree elbow as shown (figure 7). Connect a 45 degree elbow to the other end of the pipe as shown (figure 8). Use the second 7 foot length of pipe and connect to the open end of the 90 degree elbow. Add another 45 degree elbow to the remaining end of the pipe. Connect the 45 degree elbows to the side wall uprights using the PVC couplings to complete each section of the support structure (figure9). After assembling the necessary number of bows and connecting them to the sidewall supports; your structure will begin to take shape as shown (figure 10). The connections can be secured with self tapping screws or glued together to make the sections more permanent.
|FIG 7||FIG 8|
|FIG 9||FIG 10|
Installing the purlin:
Use sections of pipe along the roof ridge of the structure to provide a support surface for the plastic and add stability to the bow assemblies. Attach each bow to the purlin with screws using a tape measure to check that each bow is spaced the same distance as the sidewall supports (figure11). Use screws to secure the purlin connections where the sections are joined with PVC couplings. Install purlin supports along each side of the roof structure using screws to attach the purlin to each bow section. It's a good idea to cover all the screws with duct tape to prevent them from ripping through the plastic when you're ready to cover the structure.
Framing for a door or fan opening:
In the center of the end wall, use two vertical 1x4 inch boards to frame the size opening you desire. Level each one vertically and space them to allow room for the size door or fan opening required. Mark and trim each upright to match the slope of the bow. Secure them with screws to the footer board and PVC pipe. Add a header board at the top of the door opening for support and to provide a surface to secure the plastic covering.
Covering the greenhouse:
Cut the plastic to an approximate length to cover the structure by measuring from the ground on one end, across the length of the structure, and down to the ground on the other end. Drape the plastic over the structure making sure it is distributed evenly so there is sufficient plastic to cover both sides and both ends of the structure. Staple the plastic in several places to the footer board on one side of the structure to secure. Cut a strip of batten tape the length of the structure. Place the batten tape along the footer board, over the plastic, and staple through the tape and plastic to the footer board. The batten tape will help to prevent the staples from tearing through the plastic. On the opposite side, pull the plastic tightly over the structure and secure to the footer board using staples and batten tape. Trim all but a few inches of excess plastic along the edges. Roll or fold the remaining plastic tightly against the footer board and staple through this and the batten tape to finish the edge.
Both the door and fan ends are finished by starting at the outer ends of the side walls and pulling the plastic tightly towards the center to eliminate any wrinkles or sags. Using staples and batten tape, secure plastic to the footer boards and any uprights used to frame the door and fan openings. Any excess plastic in the middle can be cut away after securing the edges along the door and fan openings.
These are a few photos to show the inside of the structure. At the front is the opening for the screen door. To the rear is the space for the fan installation. The greenhouse is light enough that four people can lift it from the inside and walk it to another location if necessary. This greenhouse is still in service at Zilker Botanical Garden.
The instructions and the tool list remain the same. The prices for the supplies are current and are for an 11' x 20' structure.
|Hammer or small sledge hammer (3 or 4 lb)|
|String (strong carpenters braid nylon string)|
|Electric drill (with Phillips head bit or nut driver)|
|Felt tip marker|
|Steel pin (or pipe) to drive holes for PVC|
|PVC pipe cutter (or hack saw)|
|Saw (circular power saw or hand saw)|
|Staple gun with staples|
|4 ft carpenters level|
PVC pipe (1" schedule 40, gray, 10 ft lengths
(Electrical not plumbing pipe)
|20@ $2.42 =$48.40|
|PVC couplings (same as above)||20@ $.35 = $7.00|
|PVC pipe 90 degree elbows (same as above)||5@ $1.10 = $5.50|
|PVC pipe 45 degree elbows (same as above)||10@ $.97 = $9.70|
|1"x4"x12' treated wood||4@ $4.97 = $19.88|
|1"x4"x8' treated wood||7@ $3.00 = $21.00|
|Poly film (plastic)||24' x 32' approx $33.00|
|Screws (self tapping)|
|Wood screws 3" and 4" deck screws||Aprox $10.00|
Black nylon tape (Batten tape) to staple poly film in place
|Wooden screen door with hardware||32" approx $20.00|
A film crew soon came from Texas A&M and videoed our group constructing another greenhouse in a backyard in less than three hours. Even with a few technical glitches, we were able to complete the project in the allotted time frame.
I decided to enter our project for award consideration in the Texas Master Gardeners Association competition. We won second place in the large county category. We were also presenters at the TMGA Conference that year teaching fellow Master Gardeners how to build a good, inexpensive greenhouse. We had so many requests for the instructions that I copied them to CDs and we were able to more easily and economically distribute them to anyone who asked.
This is the "demonstration greenhouse" that my husband made for us to take to presentations.