Part Two: The Foundation

The Harbor Freight 10 x 12 greenhouse kit supplies a steel base to be placed directly in the ground. It's a rectangular C-shaped piece, the idea being that the hollow inside of the base gets filled with the earth and gravel of your greenhouse floor, and that anchors the greenhouse. The instructions don't mention using metal stakes, concrete piers, or anything driven into the ground for extra anchoring. Most folks believe this greenhouse needs much better anchoring, and would rather not bury a steel foundation in wet ground.

The popular solution is to build a wooden foundation, anchor it into the ground somehow, and mount the steel base on top. Everyone finds their own way to do this, but most use at least 4 x 4 sized timbers for the base.

Tip: Lay out the steel base pieces on the ground and measure them before you assemble your wooden base. The measurements listed in the manual are for a trench in the ground, not the actual size of the steel base.

We live in a warm desert climate, and our area was flat, so we kept our foundation simple. We used pressure-treated 4 x 6 timbers on edge, set into a shallow trench, levelled and squared. Temporaried with long screws, then drilled and bolted at corners. General Omar Bradley (large hairy dog in center) holds down the dirt.






24" long rebar stakes, 5/8" diameter, are driven into ground inside the foundation. Two on each side, and three on the west side (where we get the most wind.)





After driving into ground, stakes were anchored to timbers with strapping and bolts. Not very fancy, but we think the aluminum frame of the greenhouse will fail before this foundation goes anywhere.




Pressure treated 4 x 4 timbers are cut to fit diagonally inside the corners of the 4 x 6 foundation. This should help keep the foundation frame square (especially as the wood dries.) We set these a bit lower than the 4 x 6 timbers, and they should be covered up by the gravel floor later.








Four corner braces in place. Bradley holds down the interior dirt.




The steel base that comes with the Harbor Freight 10x12 kit looks to be powder-coated, in a silver color. It's not as flimsy as we thought it would be, but the shape of it does allow flex, which we'll deal with later.

Before we bolted the base pieces together, we drilled holes about 18" apart in the bottom of each piece (closer in the corners) with a cordless drill. Then we bolted them together as per the kit, sitting the base on top of our wooden 4 x 6 foundation.

As a barrier between the steel base and the pressure treated wood, we used polyethylene foam sill insulation (Lowes, 50' roll, 3 1/2" wide, for under $5.00.) I cut it in half since I didn't need the full width, and slid it under the metal base. We checked for squareness and screwed the base to the wood. Later, I trimmed off the extra insulation with a box knife, inside and outside.

You can check squareness by simply measuring diagonally from corner to corner, and hopefully the two measurements will be the same. If not, slide the pieces around until they are, and then screw them down. Squareness is important throughout this kit!

Update: I had hopes the sill insulation would compress with the weight of the structure and form a seal to keep out water, but no luck. I had to go back later and caulk the seam between the wood and the steel base, inside and out, with clear silicone caulk. That stopped rain water from seeping in under the steel base.

Now it's time to start using the manual. We'll assemble the greenhouse as per the manual, and then, before we attach any of the polycarbonate panels, we'll add some important bracing to modify the frame for strength.

Click here to go to Part Three: Assembling the Greenhouse Walls.