Build a post-collapse greenhouse as a hooped frame set in the ground, sized from the length of the arch material you have, braced and anchored against wind uplift, and arched steeply enough to shed snow, with doors at both ends and vents that open. A semicircular arch of material length L spans about 0.637 times L and rises 0.318 times L, so a 4.9 metre panel gives a span of about 3.1 metres. Never burn fuel inside without a flue and ventilation, and never enter a tunnel that is sagging under snow.
A post-collapse greenhouse is a protected growing space built from salvaged or improvised frame material and transparent cover, set up to survive local wind and snow without factory parts.
The Arch Fixes The Size, And Wind And Snow Fix The Strength
The carrier guide (Four-Season Growing & Season Extension) names bent metal conduit, cattle panels or heavy wire hoops for the frame; salvaged greenhouse plastic or several layers of clear sheeting for the cover; framed end walls with doors at both ends; and a long axis east-west. Its greenhouse plastic (6 mil) is described as lasting 1-4 years.
Worked Example: What A Hoop Of A Given Length Gives
For a semicircular arch, arc length L = π × r, so r = L ÷ π, span = 2r = 2L ÷ π = 0.637 L and height = r = 0.318 L. For a cattle panel 4.9 m (16 ft, a commonly stocked length; measure yours) long: span = 2 × 4.9 ÷ 3.1416 = 3.12 m, call it 3.1 m, and height = 4.9 ÷ 3.1416 = 1.56 m. A flatter arch of the same length is wider and lower but sheds snow worse and is weaker; a narrower, taller arch is the opposite. For a high tunnel of the carrier’s 20 ft (6.1 m) width, a semicircular arch needs L = π × 3.05 = 9.6 m of material.
What You Need
- Arch material: conduit, cattle panels, stout wire, poles, bent green timber.
- Ground anchors: posts, rebar, driven pipe, buried timber.
- Purlins and bracing: timber or pipe along the length and across the ends.
- Cover and battens or clips to hold it.
- Doors and vents.
Method
Step 1: Level and mark the footprint with the long axis east-west.
Step 2: Set the hoops in the ground, each side anchored to a post or driven pipe, at spacing small enough for the cover and snow (closer where snow is common). If using cattle panels, join them end to end with ties.
Step 3: Tie the hoops together with a ridge pole and side rails so the structure resists racking.
Step 4: Frame the ends, with doors at both ends so air can pass through.
Step 5: Add vents: doors, roll-up sides or a ridge opening. More opening area is better; check on a hot day with a thermometer.
Step 6: Cover in calm weather, with several people. Pull the film taut, secure it with battens and bury or weight the edges.
Step 7: Brace and check after the first storm; tighten any slack film.
Diagnosing Failures
- Film flapping and tearing — loose cover; tighten and add battens.
- Hoops leaning along the line — no ridge pole or longitudinal bracing.
- Frame collapse under snow — arch too flat or too lightly built, or snow was not cleared.
- Structure lifted by wind — anchors too shallow.
Common Mistakes
- Building in a frost hollow or the shade of trees.
- Anchoring only with stakes pushed into loose ground.
- Forgetting the doors and vents — the tunnel overheats.
Safety
Wind, snow and fuel heaters
Do not put a cover on in strong wind; film can drag a person off a ladder or off the ground. Do not enter or work inside a tunnel that is sagging under snow, which can collapse without warning; clear snow from outside or support the frame. Do not burn charcoal, wood or fuel inside a greenhouse without a proper flue and ventilation; combustion makes carbon monoxide, which is colourless and can kill. Use ladders on firm ground.
What’s Next
- Learn next: Thermal Mass Heating — what to put inside
- Combine with: Ventilation and Temperature Management — running it day to day