A quarter inch out of square in the base is enough to pop the glazing loose.

Two diagonals, matching within a quarter inch

The one thing to take from this page. Measure from the front left corner to the back right, then from the front right corner to the back left, and adjust the base until the two measurements are the same, within a quarter inch. Every source describes the same check in the same words. And the reason for the tight figure is the consequence chain: a foundation even a quarter inch out of alignment puts the frame out of square, which stresses panel corners, pops glazing sheets loose and causes sliding doors to stick. Do it at set-out, again after the base has settled, and again once the perimeter frame is fastened, because each of those steps can move it and each check is free.

A sticking door has two causes, and timing tells them apart

Worth knowing because the same symptom appears in two different places in this category. An out-of-square base produces sliding doors that stick from the moment the greenhouse is assembled. Structural deflection under snow load also produces difficulty operating doors and windows, but it arrives later, under load, alongside sagging and creaking. So the question that separates them is when it started. A door that has never closed properly is a geometry problem in the foundation. A door that closed fine for two winters and now binds in February is telling you the frame is deflecting. Same complaint, opposite responses, and the distinction costs nothing but a moment of memory.

You almost certainly do not need a slab

The money-saving item, and it runs against the instinct that permanent means concrete. Asked directly whether a full concrete slab is necessary, one guide answers no, describing compacted gravel pads with a perimeter lumber frame, or concrete blocks on a gravel base, as highly effective and cost-effective alternatives. A gravel supplier puts the saving at sixty to seventy percent less than concrete for comparable performance in most applications, which is worth reading with their interest in mind, though they are also candid about the exceptions: very large or heavy glass greenhouses may need the solid support only concrete provides, and extreme frost heave or expansive clay soils may call for something else. Gravel also drains, which a slab does not.

If you do pour a slab, the drain goes in at pour time or never

Two slab details that cannot be added afterward. A drain should be placed in the center of the greenhouse slab, draining into a gravel pit or a pipe leading to a drainage area outside the perimeter. A greenhouse floor gets water on it constantly, from watering and from condensation running down the glazing, and a sealed slab with no drain holds all of it. The dimensions matter too: make the foundation one inch longer and wider than the greenhouse outside dimensions, with a three inch floor adequate for home greenhouses, thicker outside edges to resist cracking from frost, over at least four inches of compacted gravel or stone. None of that is retrofittable once the concrete has gone off.

Frost depth is local, and the published figures disagree wildly

The number that decides whether the base moves seasonally, and the one place to refuse a general answer. One source gives footings extending below the local frost line as ranging from twelve to eighteen inches in moderate climates to forty-eight to sixty inches in cold northern regions, a spread of four to five times. Another describes digging below the frost line as typically six to twelve inches, which cannot be right for a northern zone. That disagreement is itself the lesson: get the frost depth for your own location rather than a figure off a page. Where it matters, the reason is specific, since perimeter footings below the frost line prevent frost heave from warping the frame and cracking the panels.

Kit anchoring legs usually do not reach frost depth

Worth knowing before deciding the base is optional. A manufacturer notes that while the anchoring legs supplied with a greenhouse help hold the structure, they usually do not reach frost depth, especially in colder regions, which is why many growers build a base rather than relying on them. So the legs are a wind and stability measure rather than a frost measure. Where anchors do go into a base, the spacing is consistent across sources: bolts within one foot of each corner, then roughly four feet apart, or rebar driven at the same one-foot-from-corner and four-foot-on-center pattern.

The sill is a thermal break and the thing the greenhouse bolts to

Easy to treat as an optional extra and it is doing two jobs at once. Guidance recommends fastening a two by four sill on top of the foundation whether you use a slab or a wall, because the sill acts as an insulated buffer between the concrete and the greenhouse frame, reducing heat loss, and it is also what the structure gets screwed to. Materials matter given what it sits in: naturally resistant woods such as cedar, redwood or cypress, plastic composite lumber, or treated wood with a barrier. And one small step with a real payoff, a thick bead of exterior-grade silicone caulk under the sill plate to block cold air drafts, which is trivial at assembly and impossible afterward.

The gravel is not finished when you stop working on it

A step nobody would think to include and it protects the quarter inch you worked for. Guidance advises allowing twenty-four to forty-eight hours for gravel to settle before assembling the greenhouse, particularly if water was used during compaction, then walking the surface and checking for low spots so a thin top-off layer can be added and tamped level before the kit goes up. Depth varies with conditions: six inches rather than four in cold climates, and on clay-heavy soils excavate eight inches and lay a four inch base of coarse gravel under the compacted surface layer, which cuts frost heave risk by improving drainage below the frost zone. Put woven geotextile under the gravel for weed control, and grade the surrounding site to fall away at a quarter inch per foot, since standing water softens the sub-base and causes the uneven settling this whole page is about.