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Community and Q&A

base for greenhouse

this_page_left_blank | Posted in General Questions on

How thick a layer of gravel (chips& dust) do I need for patio stones that a glass greenhouse will rest on? Intuitively it seems like 2″ would be plenty, but the supplier suggests 4″. One yard will get me to about 2.5″ with enough left over to fill a couple of low spots in the driveway. If I want 4″, that means getting 2 yards and having a large amount left over that I don’t really want to deal with.

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  1. Expert Member
    Michael Maines | | #1

    Trevor, it really depends on the condition of the soil and other site specifics. If the site is well-drained and the soil has good bearing capacity (i.e., it's gravel and not silt or clay) then my guess is you would be ok with less than 4".

  2. STEPHEN SHEEHY | | #2

    Gravel is a trivial part of your total greenhouse cost. Use up the two yards, even if it means you end up with 5" . Or just buy a yard and a half. Most of the cost of small amounts of gravel, less than a dump truck load, is trucking.

  3. this_page_left_blank | | #3

    The reason I don't want 4", and want 5" even less is that the greenhouse door already has a pretty sizeable step-up to get over. If I add the 1.5" patio stones plus 4-5" of base, then I'm almost going to need a step ladder to get in and out. Yes, I could have excavated down further, but I didn't and I'm not going to now in this weather. Another option is to have the 2x6 PT foundation directly on the ground, and the gravel and stones just on the interior of the greenhouse. The supplier of the greenhouse said that was acceptable. What concerns me is the lifespans of the wood in direct ground contact, but maybe that's the lesser of two evils. I guess I could also do a hybrid, in which I lay a couple of inches of gravel, put the wood foundation down, then raise up the interior another couple of inches plus the stones. Having to cut the stones to an exact fit inside the greenhouse is the only real pain in either of the latter two options.

    The place will sell 1.5 yards, but they do it by delivering 2 yards in something he called "super bags", then I have to return the leftover. It's certainly not worth my time to try to transfer a half yard of gravel into a bag in the bed of a pick-up truck and drive it back.

  4. Expert Member
    BILL WICHERS | | #4

    You could stack some pavers to make a sort of low staircase in and out of the greenhouse if you have too much gravel.

    I would prefer a wood foundation here. You'd have a better mounting surface for the greenhouse frame and you NEED some kind of anchoring to deal with the wind. The easiest way is to use PT timbers (usually 4x6), and anchor the corners with some screw-in augers. Make sure to put some protection between the PT wood and the presumably aluminum greenhouse frame. This "protection" doesn't need to be much: strips of poly sheet, stainless steel strips or washers (washers leave an air gap though), etc. Then use stainless fasteners to secure the greenhouse to the timbers. You don't want the aluminum in direct contact with the PT lumber due to corrosion concerns.


      1. Expert Member
        BILL WICHERS | | #6

        It's hard to tell how big those are, but if the picture is to scale, then I'm guessing they're fairly small by comparing them to the bolts. I'd use something a little beefier like this:
        Just attach them with a big bolt and a fender washer.

        I'm pretty sure the Canadian version of Tractor Supply has them too. I think I remember seeing them last time I was visiting family outside Hamilton.


        1. this_page_left_blank | | #7

          Not exactly, but similar:

          Only trick will be to get them screwed in to the exact right spot, since with bolts and washers there won't be much wiggle room.

          1. Expert Member
            BILL WICHERS | | #8

            Those anchors look pretty much the same, but galvanized instead of painted. Galvanized is actually probably better.

            Screw them in with a piece of pipe as a handle. It helps to dig a small hole with a trowel to get them started. so that they can bite in as you screw them in with the pipe. There is also a hook-like attachment for power post hold diggers to screw these in but BE CAREFUL if you try that. My crews use those attachments for installing utility guy anchors on poles and it's easy for them to get away from you.

            You can use a turnbuckle to make anchoring easier. Put a WELDED eyebolt into the foundation timber (not one of the ones that is formed and open, you want one where the loop is welded back to the main shaft, they are MUCH stronger), then use shackles to connect the turnbuckle to the eyebolt and anchor. If you don't have enough room, sometimes the eyebolt can act as a turnbuckle with a shackle connecting it to the anchor.

            Try to keep the eyebolt in as straight a line with the expected force on the anchor as possible. You don't want the eyebolt pulled at a right angle (don't install the eyebolt and anchor such that they are tightly perpendicular to each other). If you have a tight angle like that, you're better off using a turnbuckle in between with the anchor a little ways out from the timber. You get a stronger connection the straighter that connection is.


          2. Expert Member
            MALCOLM TAYLOR | | #9


            When I do small outbuilding close to grade I use a variation on Bill's method.

            I make a base out of 6"x6" pt lumber fastened together with GRK RSS structural screws. On the top at about 4 ft oc I fasten pt 2"x4"s on the flat. The whole base gets flipped over into the 4" or so deep excavation, so the top of the 6"x6" is about 3" above the surrounding grade. The aggregate, pavers and garden beds provide ballast to endure the base can't move or lift. The only downside of doing it that way is having to cut some pavers to fit.

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