More hobby greenhouses are destroyed by wind than by anything else, and the destruction is rarely a frame failure. What happens is that the structure moves. The whole house slides, tips or lifts off its base, and the frame then fails as a consequence of moving rather than as a cause of it. Anchoring is therefore not a refinement, it is the thing standing between a greenhouse and the neighbour's fence.
It is also badly served by kit instructions, which typically show four small pegs into soil and call it done. Four pegs will hold a greenhouse on a still day. They are not what the loads require.
How much force is actually involved
Wind pressure rises with the square of speed, which is why the jump from a stiff breeze to a gale is so much bigger than it sounds. The standard approximation used in building work is that dynamic pressure in pounds per square foot is roughly 0.00256 times the square of wind speed in miles per hour.
| Wind speed | Pressure | On a 6 x 8 ft house side | What typically happens |
|---|---|---|---|
| 20 mph | About 1 lb per sq ft | Roughly 44 lb | Film flaps, loose panels rattle |
| 40 mph | About 4 lb per sq ft | Roughly 175 lb | Unclipped panels start leaving the frame |
| 60 mph | About 9 lb per sq ft | Roughly 400 lb | Unanchored houses slide or tip |
| 80 mph | About 16 lb per sq ft | Roughly 700 lb | Structural failure of light hobby frames |
| 100 mph | About 26 lb per sq ft | Roughly 1,100 lb | Total loss of most hobby structures |
Two things make this worse than the raw pressure suggests. Wind flowing over a curved or pitched roof generates lift in the same way a wing does, so a large part of the load is upward rather than sideways. And a greenhouse weighs very little, so there is almost no dead load resisting that lift. A 6 by 8 foot polycarbonate kit might weigh 120 pounds in total. The uplift on it in a strong gale is a multiple of that.
The two failure modes
The first is the house leaving its base. This is what happens to a structure that is either not anchored at all or anchored only at the corners into soft ground. It slides a few inches, the frame racks out of square, the glazing pops out of its channels, and then the whole thing goes. Corner-only anchoring is a specific trap because the long unsupported spans between corners are exactly where the base rail lifts first.
The second is internal pressurisation. A greenhouse with a door or vent open on the windward side fills with air that cannot get out fast enough. The internal pressure then adds to the external lift on the roof rather than opposing it, and the roof comes off from underneath. This is why the standing advice before any serious wind is to close and latch everything, and it is why a door that does not latch properly is a structural problem rather than a convenience problem. A Shed and Greenhouse Door Hardware Kit with T-Handle Lock ($39.99) that gives a positive latch is cheap insurance.
Anchoring into the ground
The principle is that the anchor has to resist being pulled straight up out of the soil, not just being pushed sideways. That means depth and surface area, and it means anchoring into undisturbed ground rather than into a few inches of topsoil or turf.
- Screw-in earth anchors. A helical anchor screwed 15 to 18 inches into firm ground, connected to the frame with a tensioned strap, resists uplift far better than any peg. A kit such as Ciyivak 18 in Screw-In Ground Anchor Kit with Straps ($32.99) is the standard fix for tunnels and light kit houses, and the straps matter as much as the anchors because a strap that can be re-tensioned stays tight as the ground settles.
- Concrete anchor points. A short post or a length of threaded rod set in a bucket-sized concrete pad at each corner and at intervals along the sides. Slower and permanent, but this is what actually holds in exposed positions, and it is the standard approach where the ground is loose or sandy.
- A perimeter base. A treated timber or concrete kerb the frame bolts to, itself anchored to the ground. This has the additional virtue of raising the frame out of standing water and giving glazing panels a straight, square channel to sit in, which is often the difference between a kit that seals and one that never quite does.
- Ballast. Where nothing can be driven into the ground, such as on a paved terrace, weight is the fallback. Allow 60 to 100 pounds per corner as a starting point on a small house and considerably more on an exposed site. Paving slabs on the base rail are the usual method, and they are the least reliable of the four because they do nothing about a frame that racks.
Spacing matters as much as anchor type. Corners plus intervals of no more than 4 to 6 feet along each side stops the base rail lifting between fixings. On a film tunnel, every hoop should be secured, not every other one.
Film tunnels are a different problem
A tunnel has no rigid glazing to hold its shape and no meaningful weight at all. It relies entirely on the cover being tight and the skirt being sealed to the ground. Wind that gets under the skirt inflates the tunnel from below and takes it into the air intact, which is exactly as spectacular as it sounds.
The standard method is a trench along each long side into which the film is folded and then backfilled, sealing the structure to the ground continuously rather than at points. Where the ground cannot be dug, landscape staples through a folded hem at close intervals, plus sandbags along the whole skirt, is the substitute. On a smaller structure such as an OOTONAT 8 ft Fiberglass Garden Hoop Tunnel Kit ($17.09) over a bed, staples through the cover into firm soil at every hoop are usually enough.
Cross-bracing is the other tunnel-specific measure. A row of unbraced hoops can fold over like a concertina under a side load. A single diagonal from the top of the first hoop to the base of the third, and the same at the far end, converts a row of independent hoops into a structure.
What to do before a storm
Most storm damage is preventable in ten minutes, and the checklist is short enough to actually do.
- Close and latch the door, and close every vent. An open house pressurises and lifts from the inside.
- Restrain automatic vent openers, which will otherwise hold a vent open on a mild windy day.
- Remove or lash down shade cloth. A sheet of cloth over a roof is a second sail, and it is fastened to the structure with clips designed for gentle loads.
- Take loose items off the benches and off the ground outside. Most broken panels are broken by something the wind picked up, not by the wind itself.
- Check the anchor straps are tight. Straps loosen as ground settles, and the time to find that out is not during the storm.
- Where the house is on a paved base, check the ballast is on the base rail rather than merely near it.
Anchoring and storm-proofing hardware
Anchoring interacts with the site: a sheltered position needs less of it, and shelter is chosen rather than bought. That is covered in siting and orientation. The related question of what the structure has to carry from above rather than from the side is in snow and wind load ratings.