Greenhouse Footings on Slate Bedrock


Old 08-24-15, 04:59 PM
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Greenhouse Footings on Slate Bedrock

I am building a 12 x 16 greenhouse from reclaimed windows. My township in Pennsylvania said I only need a zoning permit for under 1000 sq feet, which has been approved. I am having difficulty finding specific code requirements for footings for my area, and there is no way I can dig below the frost line.

I am researching footing options, and from what I have read, most people are digging a trench, pouring a solid concrete footing, and laying two coarses of masonary block, then bolting on a 4x6 sil plate.

The issue I have ran into is that my yard is all slate. My grass sits on about two inches of top soil, and the proposed GH location is flat, but once the grass is removed, I only have about two inches of loose slate before I hit solid layers of slate. That doesn't seem to be enough depth to run a full trench, and it would take a lot of chipping to dig the slate to an eight inch depth for a solid concrete footing. I believe that I do not have to dig below the frost line if I am building over rock.

Here is what I am proposing:

I have a hammer drill, and a 1 inch x 16 inch masonary bit. I drill 16 inches into the solid slate, insert a masonary anchor and threaded rod into the hole, place eight inch masonry tube over the threaded rod, and pour Quickrete 5000 psi into the drilled hole and tube, to form pier footings. I need added support due to occasional heavy winds. Then set a 4x6 galvanized post anchor into the footing. Then bolt 4x6x16 pressure treated horizontal beams on the long side, and cut 4x6x 11'4" PT beams for the short sides, and frame the GH on top of the beams.

Does that satisfy International Building Code?

Also, I want the GH floor at 24" above grade due to heavy snow in my area. Should I build it up on vertical 6x6 pier posts and attach the beams like building a deck?

Or would I be able to pour the tube pier footings to 24" above grade if I drill more holes in the slate and add rebar for support, then place the 4x6 beams on post anchors?

Suggestions are appreciated.

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Old 08-25-15, 09:32 AM
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Talk to your Building Inpsections Dept. They can tell you what is acceptable for your area.

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