Greenhouse Foundation

Old 08-04-21, 04:39 AM
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Greenhouse Foundation

I have a question about a foundation for a small (4'X6') greenhouse. I want to use treated 4" X 6" timbers for the foundation with crusher run under the timbers. Then fill inside the 4X6's with 1" crushed stone. The crushed stone is partly for drainage and partly for a bit of heat sink. I'm confident with the crushed stone, but will the crusher run work? Thanks in advance.
Old 08-04-21, 06:49 AM
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Without a foundation starting below the frost line, the greenhouse will be shifted as the ground freezes/thaws below it. If the greenhouse is built sturdy enough so it doesn't twist during the shifting preventing windows/doors from functioning, I see nothing wrong with your approach.
Old 08-04-21, 08:56 AM
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Yes, crusher run might work but a clean crushed stone will be better and easier to install. Crusher run will require careful installation and compaction in layers where a clean stone can just be dumped in and is easier to compact. I like #75 which is a "clean" (no dust or other smaller particles) crushed stone about 3/4" in size. As a side benefit it is easier to walk on than 1" stones, is easier to spread and shovel and still drains well.

I would excavate a trench for your footers a foot deep and a foot wide at the minimum. Deeper is better and best would be to go down below the frost line. Then place your 4x6 in the center of this stone. You can pin it in place with long rebar rods driven through the 4x6.

The crushed stone does two jobs. It provides a base for your building. More importantly it provides drainage. Since clean, crushed stone drains readily it will not hold water. So, when it freezes there is no water in it to freeze and expand. To work best it should extend down below the frost line but I've seen it work well without going to full frost depth. If you do ever get frost heaving it is more minor than when built on soil and it's easy to reset/re-level the building if needed.

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