The floor of a greenhouse is doing more work than it looks like it is doing. It drains the water you apply and the condensation that runs down the glazing. It stores or fails to store heat from the day. It is the surface you damp down to raise humidity and drop temperature on the hottest afternoons. It is where weeds and slugs live if you let them. And it is what you stand on for hours at a time, in the wet, carrying things.
No single material does all of that well, which is why the right answer is almost always a mixed floor rather than one surface throughout.
The four options, honestly compared
| Floor | Drainage | Thermal mass | Cost | Best used for |
|---|---|---|---|---|
| Bare soil | Excellent, if the site drains | High, and it buffers humidity as well | Nothing | Under beds and growing areas, in any house where you intend to plant in the ground |
| Weed fabric with mulch | Excellent | Moderate | Very low | Under benches and around beds, where you want the soil to breathe but not to grow weeds |
| Gravel over fabric | Excellent | Moderate to good | Low | Paths and standing areas, the default hobby answer for the working line |
| Pavers or slabs on sand | Good, water passes through the joints | Good | Moderate | Paths and potting areas where a firm level surface matters, and where you might rearrange later |
| Poured concrete slab | None without a designed fall and a drain | Highest of all | High | Production houses, wheelchair access, anywhere hygiene is the priority |
Why a full slab is usually the wrong answer
A concrete slab is what people picture when they think about doing the job properly, and for a commercial propagation house it is right. For a hobby greenhouse it removes three things that are genuinely useful.
It removes drainage. Every drop of irrigation water, every litre of condensation, has to go somewhere, and on a sealed slab without a designed fall and a drain it goes into a puddle. It removes soil growing, which is the one thing a greenhouse floor can do that a bench cannot. And it removes the humidity buffer: a soil or gravel floor damped down on a hot morning evaporates through the day, absorbing heat as it goes, which is a genuine cooling mechanism worth several degrees.
A slab also costs more than everything else in this article combined, and it is the one decision that cannot be reversed without a breaker.
The mixed floor that suits most houses
The arrangement that works in the great majority of hobby greenhouses is simple. A firm path down the working line, bare soil or fabric under the growing areas, and nothing else.
- Level the whole footprint before the structure goes up, with a slight fall away from the door. This is far easier than doing it around a standing greenhouse.
- Lay landscape fabric across the entire floor. A heavy woven fabric such as DeWitt Pro-5 Weed Barrier Fabric, 3 x 250 ft ($86.99) stops perennial weeds coming up through gravel, which is the single failure that makes people rip a gravel floor out again. Cut openings where beds will go rather than covering the growing soil.
- Gravel the path 3 to 4 inches deep over the fabric. Angular crushed stone in the 10 to 20 mm range locks together and stays put underfoot; rounded pea gravel rolls and is unpleasant to walk on and worse to wheel a barrow over.
- Set pavers where you stand still. The potting area and the doorway benefit from a firm, flat, sweepable surface. A handful of slabs bedded into the gravel gives that without committing to a slab floor.
- Leave the growing areas open. Where a bed will sit, the soil underneath should stay connected to the ground below it, because that connection is what lets a deep bed drain and what gives roots somewhere to go.
Path width is the dimension people underestimate. Twenty-four inches is the practical minimum for walking through with a tray in both hands, and it is tight for a wheelbarrow. If the house is wide enough to allow 30 inches, take it. The whole layout arithmetic, including what path width does to the growing area you end up with, is in the greenhouse size calculator.
Damping down, and the floor as a cooling system
On a hot day, wetting a gravel or soil floor is one of the most effective cooling actions available in a hobby greenhouse, and it is free. Evaporating water absorbs a large amount of heat, roughly 970 BTU per pound of water evaporated, so a few gallons spread across a floor removes a genuinely useful quantity of heat from the air as it dries.
It also raises humidity at exactly the time of day when a hot greenhouse is at its driest, which reduces the vapour pressure deficit and takes stress off the crop. Do it in the morning so the floor is dry again by evening, because a wet floor overnight raises humidity into the range where disease starts. The mechanism and the numbers behind it are in cooling a greenhouse in summer.
This is a specific argument against a sealed slab in a hot climate, and a specific argument for gravel. A sealed floor cannot be damped down usefully because the water has nowhere to go and simply sits.
Slugs, weeds and the hygiene question
A greenhouse is a warm, damp, predator-free environment, which is excellent for plants and equally excellent for slugs. They live under anything that stays damp: pots on the floor, trays left flat, the underside of a paver, the gap under a bench. The floor design that reduces them is the one with the fewest permanent damp hiding places, which favours gravel over slabs laid flat on soil.
Weeds under cover are worse than weeds outside because nothing kills them off in winter. A weed in a greenhouse grows twelve months a year and hosts aphids and whitefly while it does so. Fabric under the whole floor except the beds is the answer, and a lighter fabric is adequate under benches where nothing walks.
At the end of the season, a floor that can be raked, swept and washed is worth having, because plant debris on the floor is where botrytis and leaf mould overwinter. That end-of-season reset is covered in greenhouse cleaning and the season reset.
Floor and path materials
Floor and layout are the same decision made twice. Once the path line is fixed, the bench and bed arrangement follows, and that is worked through in raised beds inside a greenhouse.