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Foundation Checklist

  • Foundation Start by creating a solid shelf of gravel around the perimeter, at least 6" deep and about 2-3' wide. You won't want gravel under your planting bed, ultimately, but you do want a solid shelf that you can work on. You'll be compacting it, mechanically or by hand, you'll build forms (and they can't be too strong), you'll place some rebar, and pour concrete. We'll go through it step by step. Of course, a concrete foundation is probably not essential but we did one because.
    • With Iowa's high winds we wanted to be dang sure that the greenhouse would be solidly anchored and not blow away.
    • We wanted to make it hard for rodents, like chipmunks to get in and dig everything up inside
    • We wanted to have a good solid foundation to fasten the pvc pipe/rebar ribs of the greenhouse
    • Place about 6" of stone x about 2' wide around the perimeter. This is going to make a base for your ultra simple foundation. You want to compact it. You can rent a gas compactor, or do what we did - just keep tamping it with a heavy tamper - a rod with a ~10" square heavy piece of iron on the bottom. Get it compacted good and firm.
    • Build your forms You can't make them too strong. You need to use 2x material, minimum - use at least 2x10s - we used 2x12s.
    • The outside of the form will be the aluminum and foam insulation. It needs to be strongly supported. We ran a 2x4 the length of the foam ("rim 2x4"), at the top, and fastened it in place with braced 2x4's (stakes) driven in a minimum of 1'. The stakes were driven the ground tightly against the rim 2x4, and screwed into it with 4" star bit screws (easy to remove w/out pounding, prying.) The stacks were braced with additional 2x4s, about 2' long, pounded into the ground and screwed into the stakes. You do not want these suckers to move! We put them in about every 4 feet, with one 1 foot from each end of each side.
    • The inside of the form was 2x12s. We fastened the lengths together with 6x 12" pieces of plywood, 4-5 screws on either side of each joint. We staked the 2x12s with 2x2 stakes and scrap rebar every 2 feet. We started out with them every four feet and hastily added more. The corners were secured with 12" lengths of 2x2 - screwed into each piece of 2x12.

  • Place rebar Concrete has plenty of compressive strength, not as much tensile strength.
    • Probably overkill, but we put in 2 pieces of rebar the entire length of each side, and 3 pieces on the north side, which would be under the water barrels.
    • We overlapped the rebar 1-2 feet, and tied each joint tightly with tie wire.
    • We used 2 sizes of (erg, what are they called) rebar 'platforms' - 3" high and 6" high - every 2 feet to hold one row of rebar 3' above the bottom, and the other 6" above the bottom.
    • Note on insulation If for some reason you have not buried the bottom of the 2" foam insulation in that drainage trench, plus the gravel base for the foundation, it is going to want to float up, and you are going to have to do something about that.

  • Pour the concrete We had it delivered.
    • Before we had the concrete delivered we carefully measured and placed 8 stacks which would hold a guideline for anchor bolts - to be placed after the concrete was poured. We measured so that the string went down the center of the outside 3 1/2" of the future concrete (below where the wall would be). Then we moved that string out of the way, but handy
    • We made a drawing of exactly where the future wall bottom plates would go, and measured and marked on the forms those locations with bright chalk. We planned for them to be about 4' apart, and 8" from the end of each bottom plate.
    • We made a crude funnel out of plywood - just ~4' x 2' pieces fastened together at right angles, to keep the concrete coming out of the truck more or less where we wanted it to go. It was fine without the help as long as he didn't go too fast, so we abandoned the funnel, and made him slow down.
    • The concrete trucks' extension wouldn't reach the full 24' and it would have been inconvenient for him to drive around to the other side (piles of stockpiled dirt), so he made the concrete a little more liquidy so we could move it easily.
    • We used hoes to pull the concrete down to the far end, it went fairly smoothly.
    • We used trowels to get the top of the concrete as smooth as possible.
    • We put the string lines back on the stakes which would mark where the anchor bolts were to go
    • We added 6" x 1/2" anchor bolts into the wet concrete, being careful (measuring) that exactly 1 1/2" was left showing above the concrete, and that they were in line with the string line. We kept checking their position as the concrete set up, since in wettish concrete, they move/sag a little. Even so, we ended up with a few that were too deep, too high, or too close to the edge of the future bottom plate.
    • Resist temptation to remove the forms before the concrete is fully set up. Wait at least 2 days.
    • Remove the forms and celebrate. That step, for us, felt like a huge step had been completed.

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