Easy to build greenhouse, 12 x 24, greenhouse foundation and assembly

The goal, build a homemade greenhouse, including a greenhouse foundation on a slope in a wet area, that will survive our winter's high winds (60 to 70 MPH) and shelter our plants in the meantime!

Here's a photo of the almost completed building.  It's now in use, we've planted spinach, radishes, and other hardy cold tolerant plants.  We plan to use this greenhouse in the winter as well as the rest of the season.  You can harvest plants like lettuce all winter long even living in Vermont as Eliot Coleman's book Four Season Harvest will attest.  If you are looking for plans or even just ideas for a build it from scratch do it yourself greenhouse, Eliot's book has quite a few unique suggestions.  He also has reviewed structures built in other countries like France for instance, where there are some very innovative low cost concepts applied.

Click Greenhouses at the Garden Center at homedepot.com for some kits at decent prices.

I was actually very worried about our high wind situation and shortly after completing and covering the greenhouse structure we had weather with wind gusts to 70 MPH, I just watched the building for a while!  But it stayed there and is still sound.  We thought the hurricane the next week would be the real test but the wind peaks were only 60 MPH; nothing!

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A link to the kit at Gardener's Supply: 12' x 24' Super Grow House

Don't forget you can usually save yourself sales tax perhaps offsetting the shipping charges.

Start off by cheating

Start off by cheating, purchase a kit.  Remember the typical low cost kit doesn't provide the foundation, nor the structural wood.  The kit contains instructions, custom brackets for joining lumber, assorted hardware, PVC pipe and PVC brackets for the arches, plastic UV resistant glazing, batten tape, adequate window and door hinges, some nice frictional window supports or closers, some cheap handles, and the deluxe model does provide window and door framing wood.  The kit's a bit pricey but you can spend a lot of time buying all the little hardware pieces you'll need.  A good source of UV plastic is Farmtek

We chose a gothic arch style kit which I think turned out to be a very good choice after last winter.  We had extremely high winds and plenty of snow.  The building tolerated the winds and the gothic arch truly helps the dissipation of the snow.  I believe snow would have built up quite a bit on a semicircular design building.

The Foundation

Here's a photo of the foundation site from the front.  Actually the year before, using my front tine garden tiller I had begun to cut a level pad initially for a 10' x 20' building, but since then my wife's eyes got bigger and I'm glad we did go with 12' x 24'.  The tiller is a great way to do this leveling job at low cost, it is a lot of work though.  Please see my level stone (and dirt) article.

I further prepared the site with the tiller until it was large enough for a 12 x 24 greenhouse.  Again this is on a slope and there is a moisture problem so I intended to also bring in more material to raise the foundation above this prepared pad of topsoil. The photo is from the rear of the building site looking toward the front.  So at this point I went ahead and installed 4" perforated pipe around half the foundation, obviously on the uphill side.  I ultimately decided to put the holes of the drain pipe down, which is probably unusual to do, but I wanted maximum drainage and the pipes are actually accessible for a possible cleanout even when the finished building is in place.  I doubt there will ever be a need though.  Of course both drain pipes are on a slight down hill grade and are in a trench slightly below the topsoil pad.  Also note I've put down landscape cloth around the perimeter of the site and on only half the topsoil that will be under the foundation.  The pipe is on top of the landscape cloth hopefully preventing topsoil from migrating into the pipe.  You can buy "socks" for the pipe to prevent this but in this case I saw no need.  

Our plan was to have half the foundation of the building as stone and the other half top soil.  One end will have benches and the other 12 foot section will provide for direct planting in the ground allowing us to grow certain vegetables year round!  For the crushed stone I chose 1/2" minus.  I hoped this would pack down well, but still drain well, I'll have to wait and see.  I ordered between 3 and 4 tons of stone.  This stone is about $8 a ton and $35 for delivery.  Call your local crushed stone supplier, they can usually give you the name of one or two people with small trucks capable of hauling three or four tons.  The stone supplier only had 22 ton trucks and didn't want to be bothered with small loads, and I sure don't want a 22 ton truck on my grass!  Fortunately for me it had been dry enough and having prepared the topsoil pad the year before, my delivery helper was able to back his truck onto the topsoil pad exactly where I wanted the stone.  I leveled the stone in a 12' x 13' pad on top of the landscaping cloth.  My topsoil pad actually did slope so the stone thickness varied from 4" to 12".

Next another truckload of topsoil between 3 and 4 yards.  Many of the mulch and topsoil dealers now offer prepared mixes of soil.  In one case topsoil with peat moss already blended.  In another case topsoil with mushroom soil blended in.  Unfortunately for me this time it was too wet near the greenhouse foundation so I had to wheelbarrow all the soil in, ugh, and grown!  What's funny is I've now purchased a compact tractor with a loader, that job would have been easy with the tractor!  This soil was used to fill in the 12 x 12 area toward the rear of the building.  It is graded from about 12" to 18" deep.  Now I have a raised well drained 13' x 25' or foundation.  I carefully packed the topsoil where the building perimeter frame would actually rest.  How?  I used my feet and kept walking the perimeter!

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Here's a shot of the 12' x 24' frame.  You can click on this for a larger view.  You can see the rear of the greenhouse foundation is raised quite a bit, about 2 1/2 feet.  This made working on the rear of the building very difficult.  The soil was soft, steeply sloped, and I had to prop up my ladder in several positions.  I had some leftover oriented strand board and I covered the topsoil end so I wouldn't sink in, and also, so the soil would not become compacted.  I used treated lumber for the entire frame, costs a little more but I don't have to paint it.  Where the treated lumber contacted the soil I wrapped it with 4 mil black plastic and then extended the plastic down the sloping topsoil foundation.  This I will cover over with mulch.  You will be lucky and could just use the new, non-arsenic, ACQ lumber at Home Depot!  I may also use 14" aluminum flashing sliding it underneath the 2 x 4's on the ground and just curving it down the slope before I mulch.  Why, a good way to prevent pests from tunneling into the building.  Pests tend to walk up to a barrier and then dig, what will they hit, solid aluminum!  A 14" x 50' roll is $18 dollars at my local hardware store, Nicholson Lumber.  For some reason Home Depot and Lowe's want $25 for the same thing!

One surprise with the 24' greenhouse kit is that there is a third support wall in the center of the building, it's sort of 1 1/2  12' buildings.  There's no hint of this on the Gardener's Supply website.  For us this could be an advantage.  We hope to keep one end of the building warmer than the other so this wall will facilitate this.

Anchoring the building

Here's the trick for anchoring the building.  Drill half inch holes in the perimeter 2 x 4 and then drive a 3' piece of rebar through it.  I used two foot rebar.  I also didn't drive it flush as the instructions say, I left a small tip angled it for a little better hold on the wood.  I have rocky soil and I was surprised how easily this went.  I guess the rebar and my sledge hammer just shatter the rock.  The instructions say to space these every 3 feet around the perimeter of the building.  Be careful you don't place one in a spot where another structural member must go (like I did!).  Home Depot and Lowe's have precut rebar pieces, a 2 foot piece was $0.99.  They had one foot, four foot, and ten foot pieces as well.  Get out your hack saw if you want 3 foot, or, you can probably have them cut it at the store. 

Attaching the glazing plastic myself

Unfortunately two ends of the plastic were not squarely cut.  But what I tried was to staple the sheet at one end of the building pulling it tight down the arch.  Then I unfolded my neatly folded plastic down to the other end of the building.  Get a good stapler!  Mine wasn't the greatest.  There is a lot of stapling to do with this design!  And you do have to replace the plastic perhaps every 3 years, boy, I hope it lasts longer!  

The instructions strongly suggest two people be present during the attachment of the large 24' x 26' sheet of polypropylene used to cover the buildings arch, but I did it myself with one caveat.  I believe my method of installation would have worked very well had the ends of the polypropylene sheet been cut perfectly straight.  The manufactured edge of 26 x 24 sheet provided was straight, but you must use the 26' length along the 24' axis of the building and the 26' ends of the sheet were not cut perfectly straight.  Oh well, I had to pull and re-staple one end of the greenhouse to get a good taught covering.

There were some minor mistakes in the plans that were specific to the 24' building.  While I didn't use it, Gardener's Supply has a support phone number they encourage your to call.

What's missing

Screens!  I'll be retrofitting screens to all five vent openings.  I also completely trimmed out the inside of the door and put in door sealing foam strips, since we hope to use this all winter long.  I think wasps would love this place and just take over!  The wasp sprays may damage the plastic and you sure don't want it near your food!

I've added a nice farm style door latch that will latch the door while you are inside the building as well as outside.  I've also added a door closer just like you would have on a screen door.

Some recommendations for this greenhouse kit

I would recommend installing the windows before you cut the plastic.  I'd even suggest installing the door before glazing the end of the building.  In this way the glazing will already be taught, you then staple it to the doors and windows, as well as the frame and then cut it!  Also to make the vent windows more air tight don't cut the plastic at the joint of the window and frame, cut it as far out as you can adjacent to the batten tape.  This will give you an overlapping plastic flap when the vents are closed.  And also for the windows I actually didn't cut the plastic at the top of each window where the hinges are located.  This lets rain run down the outside of the building and prevents it from running in at the top of the window frame.  Of course for the door and window frames, which are just pine, I dutifully applied two coats of paint.

Had I planned on screens and a airtight seal at the start I would have mounted the corner brackets that hold the doors and windows together in a position which would allow placing a trim sealing strip around the perimeter of the door and window frames.  As it is I had to do a lot of custom fitting which I could have avoided. 

Was this an easy to build greenhouse?  I'd say it was a medium difficulty project.  The foundation and the structure were both quite a bit of work, a lot of stapling.  But the kit's design is fairly strong and has survived very high winds, and heavy snow falls.  I don't believe some of the designs I've seen on the Web would have made it through some of our winters here in the Endless Mountains of eastern Pennsylvania.   I hope this helps you with your homemade greenhouse project.

Thanks, Bob