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 the winter's high winds (60 to 70 MPH) and shelter the garden plants in the meantime!

Here is a photo of the almost completed building.  It is now in use; spinach, radishes, and other hardy cold tolerant plants have been planted.  This greenhouse will be used 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.  Eliot also has reviewed structures built in other countries like France, for instance, where there are some very innovative low cost concepts applied.

The high wind situation was concerning and shortly after completing and covering the greenhouse structure, weather with wind gusts to 70 MPH arrived.  Watching the building shudder in the wind was entertaining!  But it stayed there and is still sound.  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 Gardeners' Supply: 

Sorry my particular kit is no longer available.

Maybe it will be back in the Spring?

Start off by cheating

Start off by cheating, purchase a kit.  Remember the typical low cost kit does not 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 is a bit pricey but you can spend a lot of time buying all the little hardware pieces you will need.  A good source of UV plastic is Farmtek

A gothic arch style kit was selected which turned out to be a very good choice considering last winter.  There were extremely high winds and plenty of snow.  The building tolerated the winds, and the gothic arch truly helps the dissipation of the snow.  Snow would have accrued substantially on a semicircular design building.

The Foundation

Here is a photo of the foundation site from the front.  The previous year, using a front tine garden tiller,  a level pad was prepared, initially for a 10 x 20 foot building.  Since then my wife's eyes got bigger and the size was enhanced to 12 x 24 foot.  The tiller is a great way to do this leveling job at low cost, it is a lot of work though.  Please see the level stone (and dirt) article.

Preparation continued on 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 intentions were 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 a four inch perforated pipe was installed  around half the foundation, obviously on the uphill side.  It was ultimately decided to put the holes of the drain pipe down, which is probably unusual to do, but the requirement was maximum drainage and the pipes are still accessible for a possible cleanout even when the finished building is in place.  It is doubtful there will ever be a need.  Both drain pipes are on a slight down hill grade and are in a trench slightly below the topsoil pad.  Also note landscape cloth has been distributed 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 there was no need.

The 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 the growth of certain vegetables year round!  The choice of crushed stone was 1/2" minus.  This should pack down well, but still drain well.  Time will tell!  Between 3 and 4 tons of stone were ordered for the foundation.  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 (quarry) only had 22 ton trucks and did not want to be bothered with small loads, and I do not want a 22 ton truck on my grass!  Fortunately, 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 the stone was required.  The stone was leveled in a 12 x 13 foot pad on top of the landscaping cloth.  The topsoil pad did slope so the stone thickness varied from 4 to 12 inches.

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,  this time it was too wet near the greenhouse foundation so a wheelbarrow had to be used to transport all the soil in, ugh, and groan!  What is funny is  a compact tractor, with a loader, is now available; 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 there is a raised well drained 13 x 25 foot foundation for the homemade greenhouse.  The topsoil was carefully packed where the building perimeter frame would rest.  How?  By walking the perimeter repeatedly.

Here is a shot of the 12 x 24 foot 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 the ladder had to be propped in several positions.  There was some leftover oriented strand board and   the topsoil was covered so the ladder would not sink in, and also, so the soil would not become compacted.  Treated lumber was used for the entire frame. It costs a little more but it does not require paint.  Where the treated lumber contacted the soil it was wrapped with 4 mil black plastic and then the plastic was extended down the sloping topsoil foundation.  This was covered 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 inch aluminum flashing sliding it underneath the 2 x 4s on the ground and just curving it down the slope before it is mulched.  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 inch by 50 foot roll is $18 dollars at the local hardware store, Nicholson Lumber.  For some reason Home Depot and Lowes want $25 for the same thing!

Sorry Gardener's no longer offers this particular kit.

One surprise with the 24 foot greenhouse kit is that there is a third support wall in the center of the building.  It is sort of one and a half, twelve foot buildings.  There is no hint of this on the Gardeners' Supply website.  This central wall could be an advantage.  It is hoped to keep one end of the building warmer than the other so this wall will facilitate this.

Anchoring the building

Here is a neat trick for anchoring the building.  Drill half inch holes in the perimeter 2 x 4 and then drive a 3 foot piece of rebar through it.  In this case two foot rebar will suffice.  Also it was not driven in flush as the instructions say, a small tip was left and it was angled for a little better hold on the wood.  There is rocky soil and it was surprising how easily this went.  The rebar and the sledge hammer just shatter the rock.  The instructions say to space these every 3 feet around the perimeter of the building.  Be careful you do not place one in a spot where another structural member must go (like in this building!).  Home Depot and Lowes 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 to size at the store. 

Attaching the glazing plastic alone

Unfortunately two ends of the plastic were not squarely cut.  But what was tried was to staple the sheet at one end of the building pulling it tight down the arch.  Then unfurling the neatly folded plastic down to the other end of the building.  Get a good stapler!  This one was not 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, hope it lasts longer!   An air compressor and stapler are the best choice.

The instructions strongly suggest two people be present during the attachment of the large 24 x 26 foot sheet of polypropylene used to cover the buildings arch, but it was done alone, with one caveat.  I believe the 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 foot length along the 24 foot axis of the building and the 26 foot ends of the sheet were not cut perfectly straight.  Pulling and re-stapling one end of the greenhouse was required to get a good taught covering.

There were some minor mistakes in the plans that were specific to the 24 foot building.  While it was not used, Gardeners' Supply has a support phone number; they encourage you to call.

What is missing?

Screens!  Screens will be retrofitted to all five vent openings.   Also the door was completely trimmed out and door sealing foam strips were installed, with the hope of using this all winter long.  Wasps would love this place and just take over!  The wasp sprays may damage the plastic and you do not want it near your food!

A nice farm style door latch was added, that will latch the door while you are inside the building as well as outside.  Also, a nice touch, adding a door closer just like you would have on a screen door.

Some recommendations for this greenhouse kit

Try installing the windows before you cut the plastic.   Installing the door before glazing the end of the building may be helpful.  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, skip cutting 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, the cutting of the plastic was skipped 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.  For the door and window frames, which are just pine, one dutifully applies two coats of paint.

Had the initial plans required a airtight seal at the start, corner brackets would have been mounted 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 was a lot of custom fitting had to be completed which could have avoided. 

Was this an easy to build greenhouse?  I would observe that 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.  Some of the designs found on the Web would not have made it through our winters here in the North.   I hope this helps you with your homemade greenhouse project.

Thanks,

Bob