DIY Greenhouse: Plans, Costs and How to Build One

We sell greenhouse kits, so treat this section with appropriate suspicion - and then check the numbers yourself, because they are not what most people expect.

A DIY greenhouse is dramatically cheaper than a kit when the glazing is cheap. A PVC or cattle-panel hoop covered in 6 mil polyethylene film comes in around $4 to $8 a square foot of floor area, and nothing manufactured competes with that. It is genuinely the right answer for a lot of people.

The gap closes fast the moment you want rigid glazing. Twinwall polycarbonate sheet, H and U glazing profiles, glazing tape, ground-contact lumber, hardware and hinges add up quickly, and a self-built wood-framed twinwall house typically lands around $14 to $25 a square foot before you count your own labour. A delivered 10 ft-wide twinwall kit on a galvanized steel frame works out around $11 to $15 a square foot with freight included. At that point DIY is no longer a cost decision.

Cattle panel hoop. The best value in DIY greenhouses. Sixteen-foot galvanized livestock panels bent into an arch over a treated timber base rail, spaced about 2 ft apart, covered in 6 mil greenhouse film. It gives roughly a 9 to 10 ft span with real headroom, it is enormously stronger than PVC, and it goes up in a weekend with two people. The panels themselves last effectively forever; only the film is consumable.

PVC pipe hoop. Cheapest and fastest. Schedule 40 PVC over rebar stakes, hoops roughly every 3 ft, film clipped on. Fine for a season-extension tunnel in a sheltered spot. It has very little snow or wind capacity and PVC goes brittle in UV, so treat it as a few-season structure.

Wood frame with twinwall panels. The handsome option. A ground-contact treated sill plate, 2x4 wall studs at 24 in centres, a rafter pitch steep enough to shed snow, and 4 mm to 8 mm twinwall screwed to the frame with proper profiles and gasketed screws. This is a real carpentry project and the result can be excellent.

Lean-to against a south wall. Cheaper than freestanding because the wall is one whole side, and the masonry gives you free thermal mass that flattens the overnight low. Light comes from one side only, so it suits propagation and overwintering better than fruiting crops.

Recycled windows. Lovely, and much harder than it looks. Old sashes are inconsistent in thickness, rarely square, single-glazed, and heavy. Budget for a lot of framing and expect to fight air leaks.

Skip the traced blog diagrams and go to the sources that publish dimensioned drawings with a materials list and a load rationale behind them.

• State university cooperative extension services. Most land-grant universities publish greenhouse and high tunnel construction bulletins for their own climate, free, with real dimensions. Search your state's extension site for "hobby greenhouse construction" or "high tunnel plans".
• USDA Natural Resources Conservation Service. Publishes high tunnel specifications and, through its conservation programs, has offered cost-share assistance for seasonal high tunnels. Terms change, so check current details with your local service center.
• Agricultural engineering plan services. Several university-run plan libraries hold long-established farm building drawings, including greenhouses and tunnels.

Whatever plan you use, check it against your local ground snow load and design wind speed - a plan drawn for a mild coastal county is not a plan for a snow belt. Your building department publishes both figures for your address.

1. Choose the site: six or more hours of winter sun, no pooling, sheltered from the prevailing wind but not tight against a solid wall that creates turbulence.
2. Check local requirements - permit thresholds, setbacks, height limits - before you buy materials.
3. Set out the footprint and check square by measuring both diagonals until they match within a quarter inch.
4. Build the base: strip topsoil, lay fabric, install a level perimeter, fill with 4 to 6 in of compacted angular stone.
5. Build and square the base frame or sill plate on the finished base, and fix it down. Anchoring starts here, not at the end.
6. Raise the structural frame - hoops, studs or trusses - bracing as you go. Two people minimum.
7. Add the ridge, purlins or bracing that stop the frame racking along its length. This is the step people skip and regret.
8. Fit door frames and any vent openings before glazing, while everything is still accessible.
9. Glaze, working from one end and checking alignment continuously. Never glaze in wind.
10. Seal panel edges, hang doors, fit vent openers, and only then move anything inside.

Not allowing for thermal expansion of polycarbonate. Twinwall moves noticeably with temperature - roughly an eighth of an inch per 3 ft of length across a 50F swing. Drill fixing holes oversize, use gasketed screws with washers, and do not tighten them down hard. Panels fixed tight will buckle, craze around the screws, and eventually crack.

Installing polycarbonate upside down. The UV-protective layer is on one face only and the sheet is marked. Fitted the wrong way up, the panel yellows and goes brittle in a few seasons.

Leaving the flutes open. Seal the top edge of every twinwall sheet with solid tape and the bottom edge with vented tape, so condensation and dust can drain out but nothing gets in. Open flutes are why some twinwall greenhouses look cloudy after two years.

No anchoring. A greenhouse is a large light box, and wind over the roof lifts it. Fix the base at every corner and every 4 to 6 ft along the sides. This is the single most common reason a home-built greenhouse ends up in the neighbour's yard.

Not enough ventilation. Aim for roof vent area equal to 15 to 20 percent of the floor area, with matching low-level intake. Overheating kills more greenhouse plants than cold does, and it is far harder to add a roof vent after glazing.

Untreated timber in ground contact. Use lumber rated for ground contact for anything touching soil, or it will fail at the sill first and take the frame with it.

A roof pitch too shallow for snow. Snow slides off a steep pitch or a tight arch and sits on a shallow one. Wet spring snow is far heavier than people expect and it is what collapses home-built frames.