Greenhouse Foundation

seamommy(7bTX)February 26, 2009

I am having a greenhouse built but having a little bit of confusion regarding the foundation. Concrete is ugly, but seems like the most practical material. Brick might be another option and I have some on hand that matches the house. Any other suggestions out there? Cheryl

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Our is all concrete with a concrete slab. That made it much easier to put in floor drains & much easier to keep clean.

    Bookmark   February 26, 2009 at 1:13PM
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kudzu9(Zone 8a - PNW)

I have a 10' X 15' aluminum greenhouse. I built a concrete block perimeter foundation for it (down to the frost line) and was going to put in a slab, but then I decided to put in a brick floor laid in a sand bed. I like the looks of it, spills go immediately into the cracks in the floor, so it isn't slippery, and I can get into the floor if I ever want to modify the water supply line or get at the electrical conduit for modifications. If you go with a slab floor, be sure to put a drain in the center and maybe even slope the floor slightly toward the drain. Here's what mine looks like...

    Bookmark   February 26, 2009 at 5:46PM
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Kudzu9, I also put in a block foundation on a concrete footing - about 2' high. Very neat, and easy to build upon and keep things tidy.
I also see that you have one of the oil filled radiant heaters - I have one of those around here somewhere. How does that work for you? I like the idea as they have built in thermostats. I am in zone 9 and don't have extensive need to heat the gh - only a few nights a year where problems may be presented, but am wondering how that heater works out for you.

    Bookmark   February 27, 2009 at 11:39AM
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Well, I didn't want to lay a whole concrete slab, just build a perimeter foundation either of concrete (poured) or brick or maybe cinderblock. I planned to lay a brick paver deck on a sand bed (or maybe pea gravel) for the floor. Did you use any fabric under the sand to stop weeds? We have some horrible briers here in Texas that sprout from runners about 18-24" deep and I wasn't sure if that weed-stop fabric would keep them from coming up in the gh. Roundup doesn't kill them, and other brick paths that I have put in haven't stopped them either. Any suggestions? Cheryl

    Bookmark   February 27, 2009 at 1:49PM
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kudzu9(Zone 8a - PNW)

I've been quite satisfied with the oil-filled heater for my mild climate (Zone 8), and yours should be even better. It has two settings (750 W and 1500 W) and I have it on the 750 W setting with the thermostat at about 40F. Even though I've only got single pane glazing, the greenhouse is tight and the heater is able to maintain that temperature even on the low setting. As a safety precaution, I have a wireless thermometer that transmits to the house so I can check that the temperature is where I want it without having to go out with a flashlight at midnight! I also have an inexpensive, unvented, backup propane heater in case of an extended power outage (though I've never needed it). One other thought: a standard 15 amp circuit will trip at about 1800 W, so, if you run an electric heater at 1500 W and have other electrical stuff on that circuit, you could trip a breaker and lose power to the heater without possibly realizing it.

To lay my pavers, I actually excavated about 8" below finished floor height, then put in 4" of gravel and compacted it by hand, 2" of sand and compacted it by hand, and then laid in the 2" thick pavers and swept sand between the joints to lock them in. My perimeter foundation is about 2' deep (so that it's just below our frost line), and it's about a foot above grade. The advantage of a raised perimeter is that it effectively gives you more headroom. The process for doing the perimeter involved excavating down, and then putting in a 16" wide by 4" thick base (with a little rebar thrown in). Then I laid my courses of cinder block on top of the concrete base. When that was done, I dropped in some short pieces of rebar in the top holes and filled the voids with a soupy concrete mix to lock it all together. I then mortared on a decorative top course of colored concrete pavers and set my frame on that. The frame is set on caulking to keep out drafts, and is also bolted to the foundation. I also attached 1" thick styrofoam to the outside of the foundation to cut down on heat loss through the foundation. Here is what it looked like when I was almost done:

I didn't put weed barrier in and haven't had any problems. Because I excavated for the perimeter foundation and excavated to prepare the floor, there wasn't any noticeable vegetation left under the footprint of the greenhouse, and nothing, including my blackberry vines, has returned in the 3 years I've had the greenhouse. The only slight problem I've had was last year when some ants started excavating sand out of the floor to make tunnels and I had to zap them with the little bait cans. If you are concerned about something growing back under a paver floor, barrier fabric would likely be a good insurance policy. If you went with an entirely poured perimeter foundation, that would work fine, too. I just didn't want to have to build forms and pay the minimum for the cement truck and pumper to come in. I managed fine with my little cement mixer for the concrete I did need and the mortar is easy to mix in a bucket. Good luck.

    Bookmark   February 27, 2009 at 2:41PM
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kudzu9, thanks. I think that will work out well for me. I already have a wireless thermostat in the gh which transmits temps to the house which I can download to the computer. . .it was a "weather station" (Honeywell) I picked up at Costco (the local membership discount store). So that is very convenient and I can look back and see what the temps were in the gh and outside and compare hour by hour if necessary.
I ran two 20 amp circuits to the gh - though I have not powered up yet as I am still installing things. . .so now using a heavy duty (#10 wire) extension cord.
I have wired my motorized shutters and exhaust fan to a dual stage thermostat and a variable speed fan control, and have put in several GFI outlets. I have 2x6 walls all around with only the south side open for light. Thanks again.

    Bookmark   February 27, 2009 at 5:53PM
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kudzu9(Zone 8a - PNW)

Sounds like you've really thought this through and are going first class. Enjoy...

    Bookmark   March 1, 2009 at 3:30AM
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