Let's start by getting the semantics out of the way; we built a 'hoop-house' that is strictly for starting seedlings, so technically we built a 'prop-house' (propagation house). It's a fairly simple PVC and wood structure that gives us the ability to grow a wider range of crops and start them earlier in the season than outdoor temperatures would otherwise allow. If you're considering building a small scale, low budget greenhouse, this general style is highly recommended. It is sturdy and most all the materials are readily available.
Before building our prop-house we located several greenhouse resources to help create the plans for a structure that fit our needs and budget. Here are a couple of online PDFs that we found helpful:
Basic steps for building a PVC greenhouse
1) Plan your structure — Considerations:
2) Build a foundation — We chose to use pressure treated 2' x 6' lumber as our foundation. Typically, we try to avoid pressure treated wood, but rot-resistant natural lumber (e.g., cedar, redwood, juniper) was out of our price range and we needed something that could withstand years of damp conditions.
We suggest leveling the high side of the foundation first, then using stakes to bring the lower side up to the same level.
3) Set sleeves — We used 24" lengths of 2" PVC. These were set into the ground at 3' intervals on the inside of the wood foundation. Pound into the ground with a mini sledge until sleeves are flush with the top of the foundation. Don't strike the PVC directly or it will chip or crack. Put a piece of wood on top of the sleve (it helps to have someone hold it in place for you) and tap on that with the sledge hammer.
4) Insert PVC bows — Our bows were 1.25" PVC cut to 23' lengths (which meant we needed to join two pieces, as PVC is sold in 20' sections). They were inserted into the sleeves and then attached with two 5" carriage bolts that went through both the wood foundation and the sleeve.
5) Add purlins — Purlins run perpendicular to the bows and provide sheer strength. We put the center purlin above the bows to create a small 'gothic' effect to help shed snow. The side purlins are mounted on the underside of the bows. All three purlins were attached with 4" carriage bolts.
Important note: we cracked several bows by drilling through them when they were under pressure. Use a graduated drill bit to open holes for the carriage bolts without damaging set bows.
6) Add hip boards — this is where the plastic actually attaches to the greenhouse. We chose to buy channel locks which hold the 'wiggle wire' and secure the plastic without putting holes in it. The channel locks are screwed onto the hip board.
Side note: You can also see in the above picture that we added weed fabric at this point.
7) Build end walls — This can be the most difficult part, since framing out rounded edges can be tricky. We used the plans in the Mother Earth article. One great trick that they recommend is to use a hole saw to cut the ends of the 2" x 4" where they contact the PVC bow. It creates a better connection and doesn't contort the bows. We connected the end bows to the end walls with metal strapping.
8) Duct tape points of contact — apparently PVC and poly plastic interact when in contact, degrading the life of the plastic. We created a barrier between these two by adding a layer of duct tape everywhere the poly would be in contact. It's a bit time consuming, but well worth it if you're trying to maximize the life of your structure.
Important side note: notice the wooden bracing behind Katie which connects the foundation to the end wall. This diagonal brace—when put on each side of both end walls—added significant strength to our prop-house and is highly recommended.
9) Add plastic — We used a six-mil poly plastic. It is pretty much the industry standard. Make sure to stretch your plastic on a warm, calm day. Have at least four people there to hold down corners as the plastic is secured. Even more hands make it an easier job with better results.
10) Compression strength — If you are ever expecting any snow loads, it is important to add support to the bows. We built three braces, each supporting three bows (remember, the outer bows are supported by the end walls). They were secured using metal strapping. Again, a hole saw was used to make a groove in which the bow rested.
11) Roll up sides and batten strapping — Since our plastic is attached at the hip board and not the ground, we were able to utilize roll-up sides. This allows us to increase the air flow when temperatures are getting too hot for our young plants.
We also added black webbing (batten strapping) that covers the plastic at each bow and helps hold it in place during strong gusts. It also helps hold the PVC roll up bars in place when they are down.
12) Build tables — Customize your prop-house to fit your growing needs. For us, that meant lots of table space for flats of plants. We chose to build higher tables to reduce the need for bending over when sowing seeds. Make sure to leave enough aisle space to maneuver. Also consider how you will water.
We are very pleased with how the prop-house turned out. It has created a nice protected area where our young plants can get a jump-start on the season without having to deal with cold temperatures or pests.
Eventually, it is our hope to cover the ground with a couple inches of gravel. We also intend to put in a mist system to reduce watering time. We'll be sure to share pictures of these features when they are added.