12x16 Greenhouse with Twinwall Glazing

Build both 16-foot side walls flat on the ground: full-length top and bottom plates, studs and 4x4 end posts on layout, fastened with 3" nails or screws.

If the greenhouse sits on concrete or bare ground, the bottom plates must be treated lumber — or add a sacrificial treated plate below. Untreated pine against a slab wicks water and rots invisibly.

Rack each wall until its diagonals match, then nail the 24" plywood strips along the base with 1-3/4" screws or 8D nails — the plywood is what LOCKS the square you just created.

Stand the walls and brace them with 2x4s at an angle to stakes or the slab. They stay braced until all four walls are tied together.

Build the front wall leaving a 36" rough opening for the storm door — or whatever YOUR door's instructions call for. Check the actual door's rough-opening spec before framing; storm doors are unforgiving about openings.

Same as the front, minus the door opening. If you want powered ventilation later — and a 12x16 greenhouse in summer will make you want it — this is the wall to plan a vent or fan opening into now.

Stand the front and back walls, level and plumb everything with the 4-foot level, and fasten the corners together. Add plywood at the corners and temporary diagonal bracing until the roof goes on.

Get plumb right here. Every truss and rafter cut assumes these walls are vertical; a half-inch lean becomes a gap at the ridge.

Assemble the two end trusses — 30° top chords, 60° bottom chord ends, king post — with 1-1/2" pocket holes and the exterior 2-1/2" pocket screws. Build the second directly on top of the first so they match exactly.

Screw the trusses to the tops of the end walls from underneath, angling screws into the side wall tops. Run the eave 2x4s along the side wall tops and fasten the trusses to their ends.

Mark the ridge board's rafter layout on the ground — it's a one-minute job flat and a ladder acrobatics job in the air — then set the ridge between the gable trusses. A temporary block under each ridge end (or a metal hanger) holds it while you fasten.

The rafters are cut to length with parallel 30° ends per the cut list; now cut the birdsmouth foot on each so it seats on the eave. Cut one, test it in place, and use it as the pattern for the other thirteen — never measure fourteen rafters individually.

Fasten each rafter at the eave and the ridge, and screw up from under the side walls into the rafter feet. At the ridge some rafters will land opposite each other — toenail those at an angle. Work in opposing pairs so the ridge stays straight instead of being pushed sideways rafter by rafter.

Check the whole structure for square and rack it true with temporary bracing where needed. Then add 2x4 blocking at the front, back, and sides wherever the twinwall's edges and fasteners will need backing.

This is the step where the glazing manufacturer's instructions take over from the plan — read them now, not on the roof.

Screw the 1x4 purlins across the rafters, working from the bottom of the roof up, two 1-3/4" screws per crossing. These are what the roof glazing screws into, so their spacing must match your sheet manufacturer's span rating.

Framing done — now the twinwall. Your sheet's instructions win every argument with this plan: UV-coated face out, ends sealed with the proper strips, manufacturer-specified fasteners, snug but never dimpling the sheet.

On the front and back, start at the center and work outward so any shortage lands at the corners where trim covers it. Cut the door opening and gable angles with a utility knife or fine blade. Join sheets edge-to-edge with H-channel — and know going in that getting sheets INTO the channel is a fight; the source build's fix was rubbing the channel edges with a bar of soap, and it works. Sides run vertical per manufacturer's direction.

Measure the actual as-built roof length, including the overhang the gutters need, before cutting roof sheets. Seal the top edges per the manufacturer, then screw the sheets to the purlins with sealing-washer fasteners. Do not overdrive — a dimpled washer is a future leak.

Work from the bottom up: metal corner trim on the vertical corners and eaves, ridge cap over the peak. The trim isn't cosmetic — it covers every cut sheet edge and closes the corner gaps you deliberately left in the glazing step.

Install the storm door per its instructions, then the gutters and downspouts — 32 feet of roof sheds a lot of water, and directing it into a rain barrel waters the greenhouse for free.

The source build later added cedar siding over the plywood knee walls, and a back-wall vent for airflow. Both are worth it; neither is urgent. Grow things.