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Greenhouse How To

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BLACK+DECKER B+D Contributor 188 Projects

A greenhouse can be a decorative and functional building that adds beauty to your property. A greenhouse also can be a quick-and-easy, temporary structure that serves a purpose and then disappears. The wood-framed greenhouse seen here fits somewhere between these two types. The sturdy wood construction will hold up for many seasons. The plastic sheeting covering will last one to four or five seasons, depending on the materials you choose, and it is easy to replace when it starts to degrade.

The 5-ft.-high kneewalls in this design provide ample space for installing and working on a conventional-height potting table. The walls also provide some space for plants to grow. For a door, this plan simply employs a sheet of weighted plastic that can be tied out of the way for entry and exit. If you plan to go in and out of the greenhouse frequently, you can purchase a prefabricated greenhouse door from a greenhouse materials supplier. To allow for ventilation in hot weather, we built a wood-frame vent cover that fits over one rafter bay and can be propped open easily. 

You can use hand-driven nails or pneumatic framing nails to assemble the frame if you wish, although deck screws make more sense for a small structure like this.

Step-by-Step How to Build a Greenhouse

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    Prepare the installation area so it is flat and well drained; then cut the base timbers (4 × 4 landscape timbers) to length. Arrange the timbers so they are flat and level and create a rectangle with square corners. Drive a pair of 8" timber screws at each corner, using a drill/driver with a nut-driver bit.

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    Cut 12 pieces of #3 rebar to length at 24" (if necessary), using a reciprocating saw or hacksaw. Drill a 3⁄8"-dia. pilot hole through each timber, near both ends and in the middle. Confirm that the timber frame is square by measuring diagonally between opposing corners (the measurements must be equal). Drive a rebar spike through each hole, using a sledgehammer, until the bar is flush with the timber. 

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    Cut the sole plates, caps, and studs for the two kneewalls. Mark the stud layouts onto the plates and caps, spacing the studs at 24" on center. Assemble each kneewall by driving 3" deck screws through the sole plates and caps and into the ends of the studs. 

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    Install the kneewalls onto the timber base. Set each wall onto a side timber so the sole plate is flush with the ends and side edges of the timber frame. Fasten the sole plate to the timber with 3" deck screws. 

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    Begin the end walls by cutting and installing the end sole plates to fit between the side plates, using 3" deck screws. Cut the ridge support posts to length. Install one post at the center of each end sole plate, using screws or nails driven at an angle (toenailed). Check the posts with a level to make sure they’re plumb before fastening. Note: The front post will be cut later to create the door opening. 

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    Set the ridge pole on top of the support posts and check it for level. Install temporary cross braces between the outer kneewall studs and each support post, making sure the posts are plumb before fastening the braces. Double-check the posts and ridge for plumb and level, respectively. 

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    Create a template rafter by cutting a 2 × 4 at about 66" long. Hold the board against the end of the ridge and the top outside corner of a kneewall cap. Trace along the face of the ridge and the cap to mark the cutting lines for the rafter. Cut along the lines, then test-fit the rafter and make any necessary adjustments for a good fit. 

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    Mark and cut the remaining rafters, using the template to trace the cutting lines onto each piece of stock. Tip: A jigsaw or handsaw is handy for making the bottom-end cuts without having to over-cut, as you would with a circular saw.

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    Install the rafters, using the deck screws driven at an angle into the kneewall caps and the ridge. The rafters should be aligned with the studs and perpendicular to the ridge. 

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    Mark the two door frame studs by holding them plumb and tracing along the bottom edge of the rafter above. Position the studs on-the-flat, so the inside edge of each is 16" from the center of the support post (for a 32"-wide door, as shown). Install the studs with angled screws. Cut and install two studs on the rear end wall, spacing them evenly between the kneewalls and support post. 

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    Complete the door frame: Mark the front support post 78" (or as desired) up from the sole plate. Make a square cut at the mark, using a circular saw or cordless trim saw (inset), then remove the bottom portion of the post. Cut the door header (from the post waste) to fit between the door studs. Fasten the header to the door studs and remaining post piece with screws. 

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    Begin covering the greenhouse with the desired cover material (6-mil poly sheeting shown here), starting at the end walls. Cut the sheeting roughly to size and secure it to the framing with wood tack strips fastened with wire brads. Secure the sheeting at the top first, the sides next, and the bottom last. Trim the excess material along the edges of the strips with a utility knife.

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    Attach sheeting to the edges of the sole plate on one side of the greenhouse, then roll the sheeting over the top and down the other side. Draw it taut, and cut it a little long with scissors. Secure the sheeting to the other sole plate (using tack strips), then attach it to the outside edges of the corner studs. 

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    Create the door, using a piece of sheeting cut a little larger than the door opening (or purchase a door kit; see photo below). Secure the top of the door to the header with a tack strip. Weight the door’s bottom end with a 2 × 4 scrap cut to length.

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    Option: Make a vent window. First, cut a hole in the roof in one rafter bay and tack the cut edges of the plastic to the faces (not the edges) of the rafters, ridge pole and wall cap. Then build a frame from 1 × 2 stock that will span from the ridge to the top of the kneewall and extend a couple of inches past the rafters at the side of the opening. Clad the frame with plastic sheeting and attach it to the ridge pole with butt hinges. Install a screw-eye latch to secure it at the bottom. Make and attach props if you wish.

  16. Plastic door kits, available from greenhouse suppliers, include self-adhesive zipper strips and are easy to roll up and tie for access or ventilation. You can also create your own roll-up door with zipper strips and plastic sheeting purchased from a building center.

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