Low budget cold frame
Several folks have mentioned using cold frames over in David's seed starting thread. Since the new website makes posting photos easy I thought I'd do a thread on a very inexpensive design of cold frame that I use. This design uses thin wall PVC pipe which is easy to work with. The pictures are almost but not quite self explanatory. The pipe used is 1/2" SDR-13.5 schedule. Thicker wall pipe such as Sch 40 is unsatisfactory as it will buckle when you attempt to bend it to the degree necessary. The BOM for the pictured cold frame is as follows.
3 10' lengths of 1/2" SDR-13.5 PVC pipe. Lowe's price is ~$2 each
~1' of 3/4" SCH 40 PVC pipe
6 Tees, 1/2" slip. Lowe's price for bag of 10 is ~$3
1 sheet clear plastic sheet 6.25'X10' at least 3 mils, 4 is better. Lowe's price for 10'X25' [enough for exactly 4 sheets] ~$10.
20' light weight cord.
Proceed in this order.
Cut one 10' length into four equal 2.5' pieces.
Cut remaining two 10' lengths into four 5' pieces. One of these will be an extra.
With PVC cement assemble Tees onto each end of three of the 5' pieces. Exercise care to insure that the long axes of the Tees on each end are parallel. Hint: as an aid in getting these Tees parallel, prior to assembling the second Tee, insert but do not cement, the 2.5' pieces into the long axis of each Tee. This will aid in seeing that the Tee axes are parallel when assembling. Working on a flat surface such as a concrete floor is helpful.
Once the Tees have been assembled onto the three 5' pieces, insert and cement the four 2.5' pieces into the Tees to complete the frame. The frame is flat at this point.
Cut the cord into three equal pieces. Attach one cord to each Tee along one side of the frame.
The next step offers the most likely opportunity to screw up. The 5' pieces can only be bent so far before they buckle. There is no recovery from this. Experiment with bending that extra 5' length to understand what you can get away with. Also this next step is best performed on a flat surface adjacent to a vertical wall.
Draw or snap a chalk line parallel and 4' away from a vertical wall. Butt the side of the frame with the three attached cords against the wall. Push the opposite side of the frame towards the wall allowing the 5' pieces to assume the desired hoop shape and then affix the free ends of the three cords to the appropriate Tees to hold the desired frame shape. You likely can go a bit narrower than 4' depending upon what you determined with your prior experimentation with the spare 5' piece and how lucky you are feeling. The narrower you go the smaller the footprint of the resulting frame but the higher the center point and the steeper the slope of the side walls. Mine usually end up at around 3 3/4' width but remember that there is no recovery from over doing it.
Fabricate the plastic attachment clips as follows. Cut a length of 3/4" SCH 40 into approximately 3/4" lengths. You need a minimum of 10 but make some extras as they tend to go missing over time. By whatever means you have available cut each of these pieces lengthwise so that you get pieces that are a bit more than 180 degrees of arc. Discard the scrap pieces that are less than 180 degrees of arc. Smooth the sharp edges of the clips by whatever means is available to you.
Cut a sheet of clear plastic to 6.25'X10'. Center the plastic over the frame and attach with the plastic clips. You should end up with about a 6" skirt along the sides of the frame. In use, soil is shoveled onto this skirt to seal and hold down the cold frame. Soil is also shoveled onto the extra plastic at each end for the same function.
Use freshly purchased PVC. Aged pipe becomes stiffer and more prone to buckling.
The pictured cold frame uses a fan powered by a solar panel for forced ventilation but this is optional. You can just open up each end to the degree necessary to prevent over heating under the anticipated ambient conditions.
Pictures to follow.