High tunnel (hoop house)
Steel bows on ground posts under one layer of greenhouse film, with sides that roll up: weeks added to both ends of the season, in the field's own soil.
Build
Tier 5 · Second industrial revolution
Food
Moderate — the layout must be square, and the film needs a calm day and a crew
Several days with a kit, after preparing the ground the season before
Crew 2–3 to raise the bows; more to pull the film
Home: Protected growing, Level 5 · Current, level 2 of 3 in its track. Serves 1 step.
Every build on this site has hazards that injure or kill — quicklime, carbon monoxide, molten metal, collapsing ground. Read the safety notice before starting, and read this build's own failure modes below.
What it is and why this shape
A high tunnel is the cheapest way to put a roof over growing ground: no heater, no fans, no concrete, and the crop grows in the soil. It shelters the crop from rain, wind and frost and holds the day's warmth, adding weeks at both ends of the season. Its weak points are wind and snow, which is why most of the work below is anchoring and bracing. Sizes and methods are from the Noble Foundation, the University of Nebraska and the University of Vermont.
High tunnel, 6.1 m wide: end elevation
Fig 1
Earth / subsoilTimberGlass
High tunnel, 14.4 m long: side elevation
Fig 2
Earth / subsoilTimberGlass
Keyed parts
| 1 | Ground posts | Steel tube 0.75–1.0 m long, one wider than the bow it takes, driven at least 60 cm; longer in sand. |
| 2 | Bows | Galvanised steel tube, 42–48 mm across, bent to a pointed gothic arch, 1.2 m apart: 13 bows. |
| 3 | Purlins | Tube along the underside of the bows: one at the ridge and one down each side. |
| 4 | Baseboard | 10–15 cm high along each side on pipe hangers; closes the bottom and lands the roll-up side. |
| 5 | Hip-board | Along each side at 1.5 m; carries the roof film and the top of the roll-up side. |
| 6 | Film and channel | One sheet of 150 µm (6 mil) UV-stabilised greenhouse film, held in aluminium channel by spring wire. |
| 7 | Roll-up side | A strip of film fixed at the hip-board, its lower edge wound on a pipe turned by a crank. |
| 8 | Corner braces | Diagonal tube from each end bow to the baseboard, at all four corners. |
| 9 | End wall and door | Timber or steel frame tied to the end bow, with a double door 1.8 m wide. |
| 10 | Snow props | Posts under the ridge purlin at every other bow, in winter only. |
Specifications
| Size | 6.1 × 14.4 m | Commercial tunnels are 6–9 m wide and any length. |
| Height | 3.4 m at the peak | 1.5 m at the hip-board. |
| Bows | 13, 1.2 m apart | Galvanised tube, 42–48 mm outside (1.66–1.90 in). |
| Ground posts | driven 60 cm or more | 0.75–1.0 m long; longer in sandy soil. |
| Film | 150 µm, UV-stabilised | Roof: one sheet about 8.0 m wide, 3 m longer than the house. |
| Roll-up sides | 1.35 m opening | From the baseboard to the hip-board; a hip at 1.8 m gives more air. |
| Roof at the peak | 30° | Steeper than 6:12 (26.6°), so snow slides. |
Bill of materials
| Material | Quantity | Notes and substitutes |
|---|---|---|
| Ground posts, steel tube | 26 × 0.75–1.0 m | Two per bow; each slightly wider than the bow tube. |
| Bows, galvanised steel tube 42–48 mm | 13 | Bent to the gothic profile; about 10.3 m of tube each, from post to post. |
| Purlins, the same tube or lighter | 3 × 14.4 m | Joined with swaged ends; fixed with cross-connectors. |
| Baseboards and hip-boards | 4 × 14.4 m | Treated timber, 25–50 × 150 mm, or steel tube; pipe hangers and self-drilling screws. |
| Corner braces | 4 | Tube, about 2.8 m, with clamps at each end. |
| Greenhouse film, 150 µm, UV-stabilised | 8.0 × 17.5 m | The roof sheet; plus two side strips 1.8 × 14.4 m and pieces for the end walls. Anti-drip and IR grades cost more and hold more heat and light. |
| Aluminium channel and spring wire | about 49 m | Along both hip-boards and round both end bows. |
| Roll-up pipes and cranks | 2 | One per side, the length of the house. |
| End walls | 2 | Timber frame, double doors 1.8 × 2.1 m, film or twin-wall polycarbonate. |
| Straps and ground anchors | 7 | Over every other bow, to cables through eye-bolts at the ground posts. |
Tools: Post driver · String lines and stakes · Tape measure · Spirit level and a laser or water level · Drill and driver · Pipe cutter and wrenches · Two step-ladders · Chalk line
Construction
Prepare the ground a season ahead
Choose full sun and ground that drains, near water. Build a pad a little above the surrounding ground, falling 1–2 %, with the narrow end toward the prevailing wind (University of Nebraska). Work compost into the top 15–20 cm of the soil now; once the tunnel is up the ground is harder to reach.
Set out a square rectangle
Stake the four corners at 6.1 × 14.4 m and run string lines at the height the post tops will stand. Square the corners with a 3-4-5 triangle and check that the diagonals are equal, as in setting out a block. Mark the tops of the corner stakes so the strings can be reset.
Drive the ground posts
Drive a post at every bow position, 1.2 m apart down both sides, on the outside of the string line, to at least 60 cm; longer posts in sandy soil (Noble Foundation). Use a spacing template, keep each post plumb, and bring all their tops to the string.
Stand the bows and end walls
Set each bow into its pair of posts and bolt it. Frame the end walls with posts set 60 cm into the ground, tamped or in concrete, and tie them to the end bows; put a door header at 2.1 m.
Fix the purlins, baseboards and hip-boards
Fix the ridge purlin and a side purlin down each side, on the underside of the bows, with cross-connectors. Snap a chalk line along each side and fix the baseboard (10–15 cm high) and the hip-board (at 1.5 m) to the posts and bows with pipe hangers.
Brace every corner
Fit a diagonal brace from each end bow down to the baseboard, at all four corners, and link each end bow to the next bows with pipe bracing (University of Nebraska; Noble Foundation). An unbraced tunnel racks sideways in wind.
Fit the channel and pull the film
Screw aluminium channel along both hip-boards and around both end bows. Wait for a calm morning. Unroll the roof sheet along one side, then pull it over the bows with a crew on both sides and on ladders, never letting the wind get under it. Fasten it in the channel with spring wire, starting at the middle of each side and working out, pulling it taut as you go.
Hang the roll-up sides
Fix a strip of film along each hip-board, wind its lower edge onto a pipe fitted with a crank, and let it rest on the baseboard when down. Where the sides meet the end walls, leave a fixed strip of film at least 45 cm wide so they overlap (University of Nebraska).
Close the end walls
Cover the end walls with film or twin-wall polycarbonate, which lasts 10 years or more, and hang the doors. Keep them shut in wind.
Prepare for wind and snow
Strap over every other bow to cables through eye-bolts at the ground posts. Before the first snow, put props under the ridge purlin at every other bow, and plan to clear snow early and evenly from both sides (Noble Foundation; University of Vermont). Do not keep a crop from frost with a ventless heater: it fills the closed tunnel with carbon monoxide and ethylene (Cornell e-GRO).
Tolerances that matter
| Dimension | Target | Why it matters |
|---|---|---|
| Bow spacing | 1.2 m on centre, from a template | Uneven spacing leaves slack film between bows, which flaps and tears. |
| Ground posts | at least 60 cm deep, plumb, tops level | Shallow posts pull out in wind; uneven tops twist the bows. |
| Diagonals | equal | An out-of-square frame will not take the film evenly. |
| Film | taut, no flapping edge | Flapping film wears through at the bows in one windy season. |
| Drains | 45–60 cm clear of ground posts | Digging nearer loosens the soil that holds the posts down (University of Vermont). |
How this fails
Film pulled on a windy day. The sheet becomes a sail. Wait for calm.
Posts too shallow, or in sand, pulled out by the first gale.
No corner braces. The frame racks sideways and the film splits.
Snow left to build up on an unheated tunnel until the bows buckle.
Roll-up sides left open in a storm, or flapping loose: the wind gets inside and lifts the house.
A ventless propane or paraffin heater run overnight for frost protection: carbon monoxide for the grower, ethylene for the crop.
How you know it worked
VerificationThe diagonals are equal, every bow sits 1.2 m from the next, the film is taut with no flapping edge, both roll-up sides wind fully up and down, and the tunnel has come through its first gale and its first snow without damage.
Scaling and variants
Longer: add bows at 1.2 m; tunnels 14.6, 29.3 or 43.9 m long use standard 30.5 or 45.7 m rolls of film (University of Nebraska). Wider: 9 m tunnels take heavier bows and more purlins. Warmer in winter: a second layer of film, held apart by a small inflation fan, cuts the heat lost through the cover by a quarter to a third (FAO: U of 6.0–8.0 W/m²K for one layer, 4.2–6.0 for two) and helps shed snow (University of Vermont). A multi-span house joins tunnels at their gutters.