How to Build a High-Tunnel Greenhouse
Construct a durable high-tunnel hoop greenhouse using ground posts, metal pipe arches, purlins, and tensioned UV-resistant greenhouse plastic.
- Read the tools, materials, safety notes, and steps below for free.
- Tap Start Camera Coach when you want hands-free voice help.
- Allow camera and microphone. Point your phone at the work; the coach watches your progress and tells you the next step.
🧰 Required Tools & Equipment
- ✓ Post driver or sledgehammer
- ✓ Cordless impact driver
- ✓ 6-foot step ladder
- ✓ Torpedo level
- ✓ Measuring tape
- ✓ Utility knife
📦 Materials & Supplies
- ✓ Galvanized steel ground posts
- ✓ Bent metal conduit hoops
- ✓ Top rail ridge purlins
- ✓ Cross-connector purlin clamps
- ✓ Self-tapping metal screws
- ✓ 6-mil UV-resistant greenhouse poly film
- ✓ Aluminum lock channel with spring wiggle wire
- ✓ 2x6 pressure-treated baseboards
⚠️ Safety Checkpoints & Warnings
- • Check for underground utilities before driving steel ground posts into the soil.
- • Only pull and secure greenhouse film on a calm day with wind speeds under 5 mph.
- • Maintain 3 points of contact on the ladder when installing the ridge purlins and arch apexes.
📋 Step-by-Step Guide (5 Steps)
Square Footprint and Drive Ground Posts
Mark the perimeter corners using stakes and string, checking square by confirming equal diagonal measurements. Drive the galvanized ground posts straight down along the perimeter lines at uniform spacing (typically 4 or 5 feet apart), leaving 12 to 18 inches exposed above ground.
- • Two parallel rows of vertical steel posts driven to identical exposed heights
- • Level bubble centered against each post on both axes
- • Driving posts at an angle instead of true plumb
- • Failing to check diagonal measurements, resulting in a trapezoidal footprint
Erect and Fasten Arch Hoops
Slide the ends of each bent metal hoop over the corresponding paired ground posts. Push the hoop down until fully seated against the ground post stops, then fasten each connection using self-tapping screws driven through pre-drilled holes.
- • Continuous tunnel of semicircular metal ribs aligned in height and curvature
- • Hex-head screws fully driven through both pipe walls at each base connection
- • Not seating the hoop fully into the ground post sleeve before fastening
- • Stripping self-tapping screws by over-torquing metal joints
Install Ridge Pole and Purlins
Run the top rail purlin along the underside apex of the hoop arches from front to back. Clamp the purlin to every arch using cross-connector clamps and drive self-tapping screws through the clamp brackets to lock the frame rigid.
- • A straight, horizontal steel ridge purlin locked under the peak of all hoops
- • Cross-connectors firmly seated with screw threads engaged
- • Installing the ridge pole above hoops where it will rub and tear the plastic film
- • Tightening clamps before all hoops are straight and plumb
Attach Baseboards and Lock Channels
Fasten pressure-treated baseboards along the exterior base of the ground posts using pipe conduit straps and structural screws. Screw aluminum wiggle-wire lock channels along the exterior face of the baseboard and around the perimeter of the outer end hoops.
- • Continuous wooden baseboard flush to the ground across all side posts
- • Aluminum channel mounted flat with screws spaced every 12 inches
- • Leaving sharp aluminum burrs inside the lock channel that can puncture poly
- • Placing lock channel screws too far apart, causing channel pull-out under wind load
Pull and Lock Greenhouse Film
On a low-wind day, pull the 6-mil poly film evenly over the frame from one side to the other. Lock the film into the apex and end-arch channels first using spring wiggle wire, working out wrinkles, then secure the side baseboard channels while maintaining firm, even tension.
- • Taut greenhouse skin with a drum-like surface and minimal bagging
- • Spring wiggle wire fully seated inside aluminum channel grooves along the perimeter
- • Roughly 6 to 8 inches of excess film trimmed neatly past the wire channel
- • Attempting to pull poly in winds exceeding 5 mph, risking tears or structure damage
- • Pulling unevenly, creating diagonal stress wrinkles across the roof skin
Credit: inspired by an original tutorial by This Old House — watch the original on YouTube.
Don’t have everything?
- No Post driver? Heavy sledgehammer with a wooden block capping the post pipe
- No Wiggle wire lock channel? Wooden batten strips screwed over poly onto 2x4 framing with foam tape backing