Greenhouse Foundation Alternatives

greenhouse kit foundations


DIY Greenhouse Foundations

Cultivating plants and vegetables in a greenhouse is a rewarding and worthwhile endeavor. If one decides to build one, it is always best to start out with a clear plan and a proper foundation.

A sound foundation is the basis for any successful greenhouse growing environment. A greenhouse can provide a year round supply of greens without having to be overly concerned about the adverse effects of the outdoor environment.

Greenhouse Location


Once, the greenhouse location is selected with abundant sun and shade. As well as proximity to essential services such as water and power. Your greenhouse, whether it is a home attached greenhouse addition or a free standing greenhouse structure, always start by mapping out a layout.

Once the area is mapped out with a string line or the ground is outlined with a biodegradable paint marker, there are a few choices to choose from so let’s start with simple alternatives.

Greenhouse Foundation details

Depending on the greenhouse chosen and whether it is a seasonal or an all year around growing project. These two factors should be the basis for the type of foundation to build.

If it is seasonal, then a topical perimeter base made out of landscape timbers or a 6-8″ wide concrete base wall also called a grade beam, is sufficient, making sure it is properly anchored into the ground. This is assuming that the interior floor of the greenhouse will remain dirt.

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greenhouse kit foundation, foundationAnchoring can be accomplished by digging concrete corner piers and by inserting 10″-12″ galvanized anchor bolts embedded into the center of the concrete pier so as to secure a six inch wide landscape tie to the concrete pier, making sure to expose the threaded part of the anchor bolt minimum 6″-7″ above the level of the concrete.

If using landscape ties the concrete pier should be flush with the dirt, the landscape tie is then placed on top of the concrete flush with the dirt. Anchoring the landscape tie with properly placed holes drilled in the center of the tie, minimum 6″ from the end corner of the tie.

The holes are for the anchor bolts to thread through the landscape tie for anchoring down with galvanized washer and nut.

If going with a concrete perimeter grade beam, install anchor bolts 6″ in from one corner and 12″ in from the adjacent corner. As well as placing an anchor bolt every 3-4 feet along the entire perimeter of the concrete grade beam, excluding the cut out area for wherever the doorway is located. Anchor bolts near the doorway should be placed 6″ in on both sides away from the doorway edge.

If going with concrete, another option is to dig out a shallow trench, 8″-10″wide /4-6″ deep and form it with 2×8 or 2×10 timbers to create a level, square base perimeter concrete grade beam. This will allow the concrete grade beam to rise out of the ground minimum 6″ above the grade (dirt) or higher. Preferably 12-18″ is ideal to prevent snow and ice build up during the winter months. A higher grade beam or base wall will also allow for adding new rich top soil on top of the existing greenhouse interior ground.

Regardless of the foundation chosen these three building rules must always be true …square – level – and plum!


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Top plates for greenhouse frames should be standard green treated Douglas fir. Always install and secure a 2×6 or wider, green treated top plate on top of the concrete grade beam.

Greenhouse foundation flashing

Although it is perfectly OK to anchor the greenhouse frame on top of the wood sill plate, as a rule its best to install sill plate flashing. Covering the entire width of the sill plate and bent over the outermost edge 2″ down with an outer drip edge. This creates a smooth even surface for sealing the gaps with quality silicone caulk between the greenhouse base frame and the sill plate. This also prevents moisture from leaking and soaking onto the wood sill plate. This flashing feature provides a clean looking greenhouse base appearance protecting the wood sill plate and provides a much longer life span.

Greenhouse concrete slabs

Building a greenhouse on top of a concrete slab is not considered the best alternative. This option will only allow for seasonal use.

Greenhouses built on concrete slabs should have a centrally located waste water drain. They must also use plant benches with planting beds filled with soil to grow in.

This type of application is highly labor intensive and preserving the quality of the soil can be majorly exasperating. You will find yourself having to remove the majority of last season’s dirt and replacing it with new dirt every year. Yikes!

Plants love the earth, which is why, utilizing the ground inside a greenhouse will always provide a more prosperous and bountiful harvest. Building planter boxes on top of the soil makes for easy watering and maintaining soil nutrients. Raised plant boxes on top of plant benches, will dry and deplete the soil from many of its nutrients very quickly, inside of a greenhouse with a concrete slab.

Ever notice!.. The same can be said about the dirt from a dried out flower pot. Although this alternative is a doable choice, the labor needed to maintain robust growth inside a greenhouse is always the gardener’s choice.