Designed for an existing raised bed setup, our client wanted a removable cover to use during the colder months to protect plants and extend their growing season. Although the manufacturer offered a soft cover kit, they only made one size that didn’t fit this kit. So, when asked if we could make one, we said, “yeah, we can do that.” (paraphrasing)  

Upon inspection, the screened upper portions of the boxes were not very strong and racked side to side with minimal effort. Therefore, we decided to extend the frame lower and mount it to the sturdy sides of the lower boxes. To ensure better strength and weather resistance, we constructed the frame using 1/2 inch EMT conduit (Electrical Mental Tubing). Because the cover was going to be a plastic film (sheet), we wanted to avoid sharp corners, providing a smooth transition at the angles of the roof. The three parts of the rib assembly were shaped with a tubing bender to prevent kinking, then connected with couplings at straight locations where the tubing butted together. We also added collar ties across each rib and purlins connecting all the ribs to increase strength and rigidity. To join the collar ties to the ribs, we flattened the ends and formed a saddle-shaped tongue smoothly fitting against the underside of the frame ribs. Next, we mounted the purlins beneath the ribs and collar ties and joined them with a carriage bolt (smooth end up) where they intersected. Once assembled and mounted, the frame was very rigid and robust.

The removable cover we hand-made, using UV-resistant plastic (made for greenhouses), is rated to last approximately four years. Since the client will only use the cover during the colder seasons, it’s reasonable to expect they will get double that or about eight years of extended growing seasons. All 12 window openings (10 low, 2 high) and separately made screens and cover flaps have edges reinforced with a fabric bias tape to prevent tearing. Each window flap has two straps of premium hook and loop tape to keep them in a rolled-up position. We added 2-3 hook and loop patches to hold the flaps closed when unrolled. The door flap also has two retaining straps to keep it open and a pair of rugged door zippers to keep it closed. The sides and roof are a single sheet sewn to the front and back walls along two seams. The seams and all other attachments to the plastic film have bias tape added for reinforcement and tear resistance.

An interesting fact worth sharing: We considered using PVC conduit, but EMT seemed the better choice because of its greater strength without added bulk. However, we later discovered that PVC conduit might react with the plastic film of the cover, causing it to degrade rapidly and fail well before its expected lifespan. The C in PVC stands for Chlorine which can outgas from the pipe, destroying the UV stabilizers added to the plastic film and breaking down the polymers of the film itself.

If you have read this far and looked at the photos carefully, you may have noticed that the straps for holding the door flap open are missing. Yes, we should have added those, an error we realized after completion. Fortunately, the door on the planter boxes swings outward and keeps the door flap out of the way, so the client told us not to bother adding the straps.