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Many backyard gardeners start their plants indoors in late winter or early spring, before the weather is warm. But the best way to jump-start plants for spring gardening and also extend the growing season into winter is with a backyard greenhouse. Our design is a 6-by-8-foot structure, large enough to accommodate dozens of plants but compact enough to fit into the smallest yard. It’s also easy to build, requiring only basic carpentry skills and easy-to-use tools. You can finish it over a couple of weekends.
The cost of the materials to build our greenhouse is about $1,200. That’s higher than some pre-assembled greenhouses, but our materials are more reliable than any “snap together” model; plus, this one is customizable. We include a potting bench, overhead plant hanger, and deck, but you can add or subtract features as needed. Whether you’re an expert gardener or a rookie, our greenhouse will expand your gardening potential and diversify the kinds of plants you can grow at home.
Specs and Materials
Cut the Timber Frame Foundation
Start by crosscutting two pressure-treated 4x6 timbers to 8 ft. long, and two other 4x6s to 6 ft. long. Mark both ends of all four timbers for a 1 3⁄4-in.-deep x 5 1⁄2-in.-wide half-lap joint. Set the depth of cut on your circular saw to 1 3⁄4 in., and make a shoulder cut across the timber precisely 5 1⁄2 in. from each end of the four 4x6s. Then set the saw to its maximum depth of cut and make the cheek cut in from the end of the timber .
Flip the timber over and make another cheek cut into the opposite edge. To complete the half-lap joint, trim away the last bit of waste wood with a reciprocating saw or hand-saw. Repeat to cut half-laps into both ends of all four 4x6 timbers. Lay out the 6x8-ft. timber frame foundation, overlapping the half-lap joint sat each corner. Measure the opposing diagonals to check that the foundation frame is square and make adjustments as needed; then secure each half-lap joint with two 3 1⁄2-in.-long structural screws .
Build the Roof Trusses
In this build, you’ll prefabricate the roof trusses that frame the green-house roof to simplify the framing process. Each truss is composed of two angled roof rafters and a horizontal collar tie. There are five trusses in all: front and rear gable-end trusses, and three intermediate trusses. Begin by cutting ten 2x4 rafters to 52 1⁄2 in. long. Miter-cut the top end of each rafter to 40 degrees; leave the bottom end square. Next, measure up 2 1⁄2 in. from the bottom of the rafters and cut a small notch known as a bird’s mouth cut. These notches allow the lower end of the rafters to sit flush on top of the side walls.
Also, measure down 25 1⁄2 in. from the upper end of each rafter and cut a 3⁄4-in.-deep x 1 1⁄2-in.-wide notch into the top edge of each rafter. Cut the 3⁄4-in.-deep sides of the notches with a jigsaw, and then chop out the block of waste wood with a hammer and 1 1⁄2-in.-wide chisel. These notches will accept a 1x2 strap after you install the trusses.
To assemble the trusses, butt together the 40-degree-mitered ends of two rafters. Then glue and screw a 1⁄2-in. plywood gusset plate over the joint between the rafters . (We used yellow decking glue throughout this project.) Fasten the gussets with 1 1⁄4-in. decking screws. Repeat for the remaining four pairs of rafters.
For each of the three intermediate trusses, cut a 1x4 collar tie to 60 in. Miter-cut each end of the collar tie to 50 degrees, then glue and screw it to the rafters with 1 5/8-in. decking screws . For each of the two gable-end trusses, cut a 2x4 collar tie to 56 in. long; miter-cut each end to 50 degrees. Set each collar tie in between and flush with the rafters, then screw it in place with 2-in. decking screws.
To the rear gable-end truss, add three small triangular 2x4 blocks. Place one in each lower corner, between the rafter and collar tie , and one in the upper corner tucked beneath the gusset plate. The blocks create a rough opening for the operable air vent. To the front gable-end truss, install two 13 5/8-in.-long vertical 2x4 blocks. Miter-cut the upper ends of the blocks to 40 degrees, and notch the bottom ends to fit snugly over the 2x4 collar tie. These two blocks provide fastening support for the polycarbonate panels.
Cut and Assemble the Air Vent Frame
Make the frame for the air vent from six 1x3s. Cut the top 1x3 to 8 1⁄4 in. long, and the bottom 1x3 to 36 3/8 in.; square-cut both ends of each piece. Next, cut two short side parts to 4 1/8 in. long, mitering their upper ends to 40 degrees. Lastly, cut the two angled parts to 17 1/8 in. long; miter each end of both parts to 40 degrees.
Assemble the air vent parts, then glue and screw a 1⁄2-in. plywood gusset plate across the top joints , and across the joint at each lower corner, with 1 1⁄4-in. decking screws.
Make the Greenhouse Door
To make the 24 5/16-in.-wide x 76 3⁄4-in.-tall greenhouse door, start by cutting two 1x3 vertical stiles to 76 3⁄4 in. long. Next, cut the three horizontal rails to 19 5/16 in. long; cut the head rail and midrail from a 1x3, and cut the bot-tom rail from a 1x4.
I joined the door frame parts using a plate joiner to cut slots for compressed-beech biscuit splines. Position the top edge of the midrail 37 1⁄4 in. down from the head rail. After cutting corresponding slots in the parts, I applied glue to the slots, inserted the biscuits, and clamped the frame together . If you don’t have a plate joiner, glue and screw 1⁄2-in. plywood gussets across the joints with 1 1⁄4-in. decking screws. Let the glue cure overnight.
Frame the Walls
All four walls of the greenhouse are framed with 2x4s. Cut the horizontal top plates and sole (bottom) plates for the two sidewalls to 8 ft. long. For the rear wall, cut the top and sole plates to 65 in. Next, cut five 2x4 studs for each sidewall, and four studs for the rear wall, to 65 1⁄4 in. To assemble the walls, drive two 3-in. decking screws through the top and sole plates and into each wall stud .
To frame the front wall, which includes the doorway opening, you’ll need ten 2x4 parts: Cut the sole plate to 65 in. and then cut two double-2x4 top plates. Each double top plate consists of an 18 5/8-in.-long 2x4 topped with a 17 3⁄4-in.-long 2x4. Next, cut two wall studs to 65 1⁄4 in. long, and two trimmer studs to 75 3⁄4 in. (The trimmer studs form the rough opening for the door.) Finally, cut a 27 3⁄4-in.-long 2x4 temporary header to span across the top of the trimmer studs. Screw the parts together with 3-in. decking screws. Once the wall is installed, remove the header. Use a paint pad to apply a coat of solid-color stain to all surfaces of each part . Allow the stain to dry overnight.
Attach the Polycarbonate Panels
Doing this to the wall frames prior to erecting them will save you time and trouble. Begin by peeling off the clear protective film from the back side of each polycarbonate panel. Lay the polycarbonate across the walls, starting in the center of the wall frame. Make sure all seams align with the center of a stud, but leave a 1/8 -in. expansion gap between the panels. Fasten the polycarbonate panels to the wall frames with 1 1⁄4-in. decking screws spaced about 16 in. apart.
Slide the polycarbonate panels into place at each end of the wall frame , and then use a router fitted with a flush-trim bit to cut away the overhanging polycarbonate . A router provides the quickest and most accurate way to trim the polycarbonate, but if you don’t own a router, lay the polycarbonate panel in place, mark where it overlaps the wall frame, and then trim it to size using a circular saw or jigsaw.
Install Foundation and Walls
Set the timber frame foundation on the ground and use a 4-ft. level to confirm that it’s level across its width and length. If necessary, dig out or build up the underlying soil until the frame is level. Next, bore 1⁄2-in.-diameter holes, spaced about 24 in. apart, through the timber frame. Use a small sledgehammer to pound 1⁄2-in.-diameter x 18-in.-long steel reinforcing bars through the holes and into the ground . The bars will keep the foundation from shifting. Cover the ground within the timber frame with landscape fabric; then add 3 in. of gravel or bark mulch to create the floor.
Install the prefabricated walls, starting with one of the sidewalls. Stand the wall on the timber frame foundation, aligning the sole plate flush with the outer edge of the 4x6 foundation timber . Secure the wall to the foundation by driving 3-in. decking screws down through the sole plate and into the 4x6 below . Space the screws about 24 in. apart. Repeat to install the opposite sidewall.
Raise the rear wall, sliding it between the two sidewalls, and confirm that the walls are plumb. Screw the rear wall down to the foundation with 3-in. drywall screws, and then screw through the studs at each end of the rear wall and into the sidewall ; space the screws 16 in. apart. Repeat to install the front wall . Then use a handsaw or reciprocating saw to cut out the 2x4 sole plate that runs across the doorway threshold.
Install the Trusses
Add the roof trusses by tilting the rear gable-end truss into place on top of the rear wall . Align the truss flush with the outer edge of the wall, and drive a 3-in. decking screwup through the top plate and into the bird’s-mouth cut in each rafter. Then install the three intermediate trusses, making sure to position each over a wall stud . Fasten each intermediate truss as you did the gable-end truss: Screw up through the top plate at a slight angle and into the bird’s-mouth cut .
Set the front gable-end truss on top of the front wall and tilt it into place . Drive 3-in. drywall screws up through the top plate and into each rafter, and then secure the truss with two 3-in. metal L-brackets. Screw the brackets to the top of the trimmer studs and into the 2x4 collar tie .
Sheathe the Roof
Before attaching the polycarbonate roof panels, set a 1x2 strap into the notches cut into the top edge of each rafter. Secure the straps by driving a single 1 5/8-in. decking screw through the 1x2 and into each rafter . Then install a 2x4 block between each pair of rafters  to close off the space between the top plate and the underside of the polycarbonate roof panels.
Fasten the polycarbonate panels to the rafters with 1 1⁄4-in. decking screws, spaced 16 in. apart . Continue installing the panels across the rafters on each side of the roof, leaving a 1/8-in. gap between each panel. Once all the roof panels are in place, make the ridge cap, using a pressure-treated 1x8 and 1x10, each 99 in. long. First, use a circular or table saw to rip the 1x10 down to 8 in. wide. Then bevel-rip a 10-degree bevel into one edge of the 1x8. Screw the two boards together and set the ridge cap over the peak of the green-house roof. Secure it in place by driving 1 5/8-in. decking screws down into the rafters .
To help create a weathertight roof, apply a continuous bead of silicone sealant to the seams between the polycarbonate panels. Then cover each seam with a 1x2 batten . Fasten the battens with 1 5/8-in. trim-head screws, spaced 16 in. apart. Repeat the process to apply battens to the vertical seams in the greenhouse walls.
Finish the Door and Vent
Add 1x4 exterior casing around the doorway opening, and then hang the door with two self-closing hinges . Mount the hinges 6 in. from the top and bottom of the door. Then install the automatic vent opener by screwing it to the air vent frame and the 2x4 collar tie .
Build the Potting Bench
For the potting bench’s three mounting brackets, cut two horizontal 21 1⁄4-in.-long 1x4 bench supports, and one 25 3⁄4-in.-long 2x4 diagonal brace. Trim one end of the six 1x4s to 45 degrees, and both ends of the three 2x4s to 45 degrees. Glue and screw one 1x4 to each side of the 2x4 to create a 45-degree diagonal brace. (You’ll use 1 5/8-in. decking screws for the potting bench.) Slip each mounting bracket onto a stud in one sidewall. Position the brackets 35 1⁄4 in. above the timber frame foundation, and fasten them by screwing through the horizontal 1x4s and into the side of the wall stud . Then drive a single screw through the lower mitered end of the brace and into the edge of the wall stud.
Since there’s no way to attach a mounting bracket to the studs at each end of the sidewall, screw a 1x4 cleat across two studs in the front wall and rear wall. Place each cleat at the same height as the mounting brackets: 35 1⁄4in. above the foundation. These support the 1x4 potting bench slats.
Make the surface of the potting bench from four 96-in.-long 1x4 slats. Set the slats across the mounting brackets and cleats, spaced 1⁄2 in. apart, then fasten them with 1 1⁄4-in. decking screws .
Add the Overhead Plant Hanger
Complete the interior of the greenhouse by installing an overhead bar for hanging plants and baskets. Cut a length of 1⁄2-in.-diameter metal conduit to 94 in. Drill a 1⁄2-in.-diameter x 1-in.-deep hole in the 2x4 collar tie in the front and rear gable-end trusses. Position the holes 12 in. from the end of the collar ties. Slip the conduit into the holes, and then secure it to the underside of each 1x4 collar tie with a single conduit strap .
Build and Install the Pressure-Treated Deck
For the deck frame, cut two 2x4 x 72-in. band joists, and five 2x4 x 20 1⁄2-in. floor joists. Screw the floor joists between the two band joists with 3-in. galvanized decking screws, spaced 16 1/8 in. apart, to create the deck frame. Set the frame in front of the doorway and fasten it to the timber frame foundation with four 3 1⁄2-in. structural screws. Support the outer two corners of the deck frame with concrete blocks or pressure-treated posts to hold the deck level.
Cut four pieces of 5/4-in. x 6-in. pressure-treated decking to 72 in. long. Lay the decking across the floor frame, spaced 1⁄2 in. apart . Fasten the decking to the deck frame with 2-in. decking screws. Now, with your greenhouse complete, bring in the plants and watch them grow!
Joe is a former carpenter and cabinetmaker who writes extensively about remodeling, woodworking, and tool techniques. He has written eight books and is a contributing editor to Popular Mechanics. He also appears on the Today’s Homeowner TV show, and co-hosts the weekly Today’s Homeowner Radio Show. Joe writes from his home in Roxbury, Connecticut.