How to Anchor a Greenhouse to the Ground

Anchor a greenhouse by fixing its base rail to something heavy and fixed: a pressure-treated timber base frame pinned into the ground with rebar or ground screws, a concrete perimeter footing, or a slab with masonry anchors. On soil, the timber-frame-plus-ground-anchor approach is the usual answer and the most forgiving. Whatever the method, the load you are resisting is uplift, not sideways push -- wind over the roof lifts the whole structure, and a greenhouse that is merely sitting on a level patch will leave the yard in one piece in the first real storm.

A kit greenhouse is aluminum and plastic. An 8 by 12 structure might weigh a couple of hundred pounds, which is nothing against the force of wind moving across a curved or pitched roof. The wind does not mainly push the greenhouse over; it reduces pressure above the roof and lifts it, exactly as it does over a wing.

That is why the anchoring has to connect the frame itself to the ground, continuously around the base, rather than just stopping the structure from sliding. Weighting the inside shelves with pots does almost nothing, because the weight is not attached to the frame.

1. 1

   Set the footprint and check it is square

   Lay out the exact external dimensions from the manual and measure both diagonals. Equal diagonals means square. A base that is out by an inch makes the panels fight you for the whole build.

2. 2

   Level the ground, and level it again

   Remove turf, rake flat, and check across both axes with a long level. A greenhouse on an out-of-level base twists, and doors on a twisted frame never close properly.

3. 3

   Build the frame from pressure-treated timber

   Typically 4 x 4 or two courses of 2 x 6, cut to the manufacturer's base dimensions, screwed at the corners with exterior structural screws.

4. 4

   Pin the frame to the ground

   Drive rebar through holes drilled in the timber at each corner and at intervals along each side, at least 18 inches into undisturbed soil. Ground screws or earth anchors do the same job with more holding power.

5. 5

   Assemble the greenhouse on the frame and bolt the base rail down

   Use the fixing points the manufacturer provides. Every one of them, not the corners only -- an unfixed span in the middle of a long side is where a frame starts to peel.

6. 6

   Add diagonal bracing if the kit supplies it

   Corner braces and roof bracing are not optional extras. They are what stops the whole frame racking into a parallelogram in a gust.

7. 7

   Re-check every bolt after the first month

   Everything settles, and aluminum frames loosen. One pass with a wrench after a few weeks of weather is worth more than any upgrade.

• Rebar pins -- cheap and effective in soil. Use 1/2 inch stock, 24 to 36 inches long, driven through the timber frame and well into undisturbed ground.
• Ground screws and earth anchors -- screw-in spirals or augers, far better holding power in uplift than a straight pin, and removable.
• Masonry anchors -- for slabs. Match the anchor to the slab thickness; a thin patio slab may not hold a heavy expansion bolt.
• Auger-style dog anchors with guy ropes -- the right answer for portable cover greenhouses, used in addition to the supplied stakes, not instead of them.
• Concrete deadmen -- buried blocks with a strap up to the frame, used where you cannot pour a footing but need real holding power.

Depth matters more than the anchor type. A pin that goes 12 inches into topsoil will pull out; the same pin at 30 inches into undisturbed subsoil will not. Where the ground freezes deeply, think about whether frost heave will lift your anchors over a winter, and consider a footing rather than pins.

The best anchoring strategy starts with not putting the greenhouse in the windiest corner of the yard. A structure sheltered on its windward side by a hedge, a fence or the house experiences a fraction of the load, and permeable shelter such as a hedge is better than a solid fence, which creates turbulence on the lee side.

Orientation helps too: the shorter end into the prevailing wind presents less area. Where to put a greenhouse in your yard works through sun, shelter and access together, and the best greenhouse for high winds covers the kits with published wind ratings for sites where shelter is not available.