How To Build Your Own Greenhouse from Carport – Part 2

Note: This post continued from Part 1, so go read it if you haven’t yet. I’ll wait.

Now on with the show!

Step 3: Get greenhouse film.

Note: I put this step here, but I waited until I was ready to need the plastic film before I ordered it. This meant I could not use the greenhouse during the winter, which I really regret. Learn from my mistake!

But first, measure the carport, then add some to allow for overlap and “oops”.

Because it’s better that it be a foot too long than an inch too short!

Piecing the over-the-top section and end panels together in different ways to find the most compact size (soooo glad I spent all those hours as a kid playing Tetris!), I ended up with 17′ x 37′ total size.

The next larger standard size was 20 ft. wide x 38 ft. long, and I bought my plastic from agriculturesolutions.com (no affiliation).

Their plastic is the lowest per-foot cost I found, for the size I needed. Oh and just FYI, their website maybe isn’t the most modern, but I had no problems ordering or receiving. They also sell the wiggle wire and base for better greenhouse film connection.

Design decisions

I covered the whole carport in greenhouse plastic, and that let me “dry in” the greenhouse, as the cheapest/fastest option. It may not have been the best option, but for me it works.

You could cover it with polycarbonate panels, or build the north wall (and partially the ends with plywood or siding), or any combination of those.

Note: The following comments assume northern hemisphere, at significant (>20 deg. N) latitude.

But why you would want to make a solid north wall? Why wouldn’t you want all the sun you could get?

  • The hours of solar exposure (and heat, natch) are most needed in the winter, and are most scarce
  • In the winter, it’s very important to hold onto the heat you’ve gathered during the day, into the night
  • In the winter, the sun rises south of west, and sets south of east
  • Greenhouses are usually oriented with the long axis east/west, to maximize solar gain. This is tweak-able if you understand the effects
  • You will (normally?) never get sun coming in the north wall in the winter

So, it’s better to build a well-insulated solid north wall, and maybe the north part of each end wall.

Of course, this is not what I did! That would make sense, and we wouldn’t want that, now, would we? Ha!

I call it an “efficient use of time/material resources“, because a not-fully-perfect completed project is better than a never-finished “perfect” project. That’s gotta be tweet-able…

And I really like to cross projects off the list.

Step 4: Assemble & orient carport frame

This process will vary depending on the carport you get, ‘course.

The one I got said specifically it was not rated for snow.

This concerned me a little, because we can and do get some significant snow.

It also didn’t have an easy way to securely connect the slip-fit pieces, and I really didn’t want them to un-slip.

So I used self-tapping screws between the joints to make sure they didn’t come apart during a windstorm or heavy snow load.

Carport "cross" connectors

Carport “cross” connectors, together with self-tapping screws. See the bad welds? Ugh.

I leveled the site and put long 2×6″ wood runners under the frame for a foundation. I thought about using cinder/concrete blocks but that would be a lot more work and I wasn’t 100% sure of the location.

So I built it on 2×6 runners that could be easily moved. Ask the wind how that works.

I screwed the vertical supports to these runners. I’m not sure if the runners need to be anchored to the ground or not, but it’s not a bad idea, especially in windy areas. I should do mine, but maybe later…

Me from the future: NO! NOW! DO IT NOW! 60 mph winds will shove it over it and kill plants! Anchor it down!!!

Orientation

Now, it’s very important to locate and orient the carport properly. I can’t stress enough how important this is.

If I were to build it and put it in the wrong place, it becomes a liability instead of an asset!

With the number of trees (blocking the greenhouse’s sunlight) and the fact that it’s a north slope (even longer tree shadows), good sun-exposure locations were limited.

But I found a place (seemed like the only one close!) nestled between trees that cuts off hot summer sun yet allows the limited winter sun.

I also put it so the long side is east/west (for max solar gain). I knew I had a good location when where I picked turn out to be where the snow first melts off in the spring. Yay for confirmation of good design choices!

Side note: One option is to face it slightly more to the east so the hot afternoon summer sun doesn’t make the greenhouse too hot. Contrary to what one might assume, two of the biggest management issues with a greenhouse is getting too hot and humid. Weird, right? They actually need a significant amount of ventilation. Now back to the show!

 

Greenhouse frame base and greens

2×4 vertical on 2×6 base. Never mind the snow, just keep growing.

Step 5: Attach 2x4s to frame.

I thought about how to attach the film to the wood. I wanted to set it so that the film doesn’t rub on any sharp corners or edges.

That wasn’t going to work for both directions (end panel and side panel), so  I set them a little proud of the outside of the frame. The film stretches over a cushioned, rounded edge, then attaches to the 2×4.

But how to attach them? The classy way, ‘course. Plumbers tape & screws, baby!

Other people use stainless clamps and strips of metal to secure the tubing to the wood, or drill through the tubing and carriage bolt the tubing to the wood. Either works. My way was cheaper, but their way is maybe better. Nah, can’t be!

Connecting wood frame to carport frame.

Connecting wood frame to carport frame.

Step 6: Frame out ends.

More wood and screws. Wish I had a lighter drill…

Note here that I didn’t plan on having a door at the far (west, prevailing wind side) end, so I framed it internally differently than the door (east) end.

I used the wood attached to the carport frame as connection points for the end frames. These points are both toe-nailed and screwed together with plumbers tape.

We have high winds many times of the year, and I don’t want this greenhouse falling apart!

Step 7: Plumb & level the carport assembly.

The carport tubing was slightly bent improperly (and probably not put together perfectly straight, oops), so I compromised between making the door end plumb, versus the far end.

Down the rabbit hole sidenote: Plumbing/straightening and leveling the frame is made much easier by having a level foundation, but I choose to live on the wild side. It helps to increase my tolerance for non-perfection. The Japanese have made an art form out of finding beauty and celebrating imperfection, called wabi-sabi (though it usually is applied to more natural objects). Wow, I’m a zen artist and didn’t even know it!

Step 8: Install corner braces.

Corner braces

Corner braces. Never mind the wrinkles.

The corner braces are necessary to keep the greenhouse from shifting or racking while under wind or rain/snow stress, like perhaps, oh I don’t know, maybe falling over without permission.

That would be bad, right?

Excessive movement can also damage the film and it’s expensive to replace. As you can see I braced the end walls to the bottom frame runners. It was much stiffer after I installed these and didn’t move when shaken like it did before I install the braces.

 

This post series continues in Part 3.

 

If you have any questions about the greenhouse or gardening in it, please comment down below and let’s talk!

 

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Thanks!

The Crew at TPL