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Collapsible Greenhouse
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Give your plants a chance with this portable starting house
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By Don Ross; Photography by Roger Yip
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The shelves are attached to the supports with dowels (top), each shelf drops easily in place
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Tip top
The top of the greenhouse needs to shed rain. I designed a cambered, wing-like profile to create the essential slope, while leaving the front, back and end panels the same height and with square corners-a much easier building job.
The ends of the top are best cut from a length of 2x8 red cedar. The longitudinal frame members are 2x2 red cedar, just like the rest of the frame. For no other reason than aesthetics, I cut a leaf pattern through the end pieces. Beauty is also why I used 4"-long galvanized finishing nails rather than the ubiquitous and all too obvious deck screws to hold everything together. Predrill holes slightly smaller than the nail itself to avoid splitting the cedar. The centre frame piece must be notched to accommodate the 2x2 stringers, and these are best fastened with the 31⁄2" deck screws for maximum strength. No one will see them anyway. As with all other frame members, use your carpenter's square to be sure the top comes together with 90° corners.
The entire top frame is a little wider than the rest of the structure to shed rain. To continue the flow of the curved top, I wrapped the poly around a piece of pine quarter round, and nailed it with 15⁄8" galvanized nails to the front edge of the top. This arrangement adds something to the drip edge and means a batten isn't needed to protect that area of the plastic. With the quarter round in place, stretch the poly over the top and staple it along the lower back edge of the drip edge.
At this stage, leave the plastic larger than necessary all around the top for a good grip. The idea is to pull the sheet as taut as possible, back to front and end to end. A tight membrane keeps the roof frame strong and allows water to shed completely. Just be sure to round the top edge of the back stringer and the upper back corners of the end pieces smoothly before you put on the plastic. Sharp corners will puncture the poly. Finish up by reinforcing all stapled edges with more wooden battens.
Assembly begins
The wall panels of this greenhouse are held together by hinges that have removable pins so the structure comes apart for off-season storage. Install the hinges on a large, flat surface. Tap out the hinge pins before assembly, anchor the hinge plates with screws, then apply a little white lithium grease to the pins before putting them almost all the way back in. This strategy makes it easier to remove the hinge pins at the season's end.
With the frame of the structure together, you can easily add the other pieces. The positioning of the shelf support pieces on the end panels and on the centre supports of the unit is arbitrary. You can put them anywhere you like, or add more shelves if you wish. The end supports are permanent. The centre supports, on the other hand, are removable to allow for seasonal storage, and sit atop blocks affixed to the back and front central vertical frame members. The blocks are positioned so that when the cross supports are placed on them, the end and central supports are at the same level. This set-up is key. Dowels keep the centre supports in place.
The inside front-to-back distance is 221⁄4", and this becomes the length of the central horizontal pieces. Drill a 1⁄4"- diameter hole, 1" deep and 3⁄4" in from the ends of the underside of each cross support. Tap a 2" length of hardwood dowel into each of these holes. Next, drill a 5⁄16"-diameter hole at least 1" deep into the top centre of each support block, before these are mounted with screws to the vertical members. The pegged cross supports in the blocks add more strength to the structure as a whole; they also support the shelves.
Click here to read about sprucing up your greenhouse.
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