Building a DIY greenhouse - where to start?

Question

I'd like to build an inexpensive, removable greenhouse over the back deck. The deck is approximately 20' x 8'. I'd like to build it for easy disassembly and storage in the summer.

I've never seen anyone attempt to cover their deck with a removable greenhouse. Are there some issues I should consider in planning a project such as this.

I'm looking for suggestions for the covering. Does it matter if the covering is clear or would it work better if it was green tinted or semitransparent? I'm also considering using Tuftex clear corrugated plastic roof panels. Is a viable option?

Any recommended resources pertaining to this would be greatly appreciated.

Additional info: Although, topically overcast and rainy we currently have about 9 hours of daylight. I live in "USDA Hardiness Zone 8a-8b" where the average low is 20°F through 10°F. Snow is unlikely and minimal at 0-3 inches in a year. High temperatures for the winter and spring would be in the low 40°F through 70°F.

As for orientation of the deck, the long axis runs east-west.

Here in the Pacific North Wet (as I call it, instead 'West') an abundance of rain seems to harm some of the plants, especially the tomatoes. Just the act of covering the tomatoes in the fall season seems to greatly extend their lifespan. In fact, I've got six tomato plants covered on the deck that are still producing and tomorrow starts the first of December. It is getting cold enough though that I have to pick the tomatoes green or yellow and let them ripen inside. I'm guessing that if I were to cover them in a greenhouse, it would produce an even better outcome.

Purpose for building a greenhouse over the deck: - Cover six large plants of tomatoes. - Keep the herbs through the cold, wet winter seasons. (Example: rosemary, thyme, basil, mint, lemon balm, lavender, green onions, etc.) - Growth of a late fall produce (Example: lettuce, spinach, etc.) - Shelter flowers - Grow starts in the early spring - Ability go out on the deck without getting cold & wet.

Answer 1

It would still be useful to add a couple of details to your question:

• What are you going to grow?
• What orientation does your deck have? (Which way is the long axis running -- north-south, east-west, in between?)

Based on what you've said so far, I can provide some general things to consider:

• I'm assuming you'll be using it to start plants from seed -- vegetables, flowers, etc. prior to transplanting them in the ground when the soil warms up.
• 20x8' is pretty big. (It's huge!)
  • For comparison: I start hundreds of plants (basil, tomatoes, squash, cole crops, celery, annual flowers, etc) every spring on two shelving racks in my basement -- about 6' wide x 6' deep x 7' high. That's enough to fill a 4000 sq ft veg garden plus smaller flower gardens in a third of the space you're proposing. (Well, sometimes there's overflow to a third rack upstairs, but that's only when I'm hardening off one batch of plants and starting a new one.)
  • If you are going to use it for sheltering dwarf fruit trees or something like that, then maybe you'll need that extra space.
  • (It just occurred to me that maybe you're going to use it as a warm place to hang out during cold but sunny winter days? Is this part of your plan?)
• Depending on how cold it gets at night, since you're not going to heat it, you may not be able to use it to start heat-loving plants like tomatoes, peppers, etc. I think it will get too cold at night. (You may be able to prove me wrong on this point by adding thermal mass to the greenhouse so that it stays warm -- 65-70°F at night.)
• You should research greenhouses that are built out of metal conduit. A friend built a small (6x6' or so) "gothic roof" style greenhouse, covered with poly sheeting, that worked out really well for her to start plants as described above. A structure this small could be firmly attached to your deck (to prevent blowing over) while in use, and then detached and carried away to be stored during the off-season.
  • I've seen plans for houses built out of plastic conduit too, you can check these out while you're doing your research.
• Consider very carefully how you are going to ventilate it. Roll-up sides might be handy. Self-ventilating roof panels might work on smaller structures. On a larger structure you may need to invest in some large fans that will need to be built-in to the endwalls and operated on a thermostat.
  • Ventilation is critical. A sunny 70°F day in February will create a very hot greenhouse, and you will kill all of your plants! Even a sunny 40°F day is dangerous.
  • You may want to consider using shade cloth to keep the temp down on warm days. (But this works against providing light, see below.)
  • On the flip side, you'll want to retain heat at night (see above), so you'll need a way to make sure that your ventilation shuts off / closes when the greenhouse starts to cool at the end of the day.
• Depending on how short the days are during your winter, you may need to provide supplemental lighting. In my basement "nursery", I put my seedlings under fluorescent lights on a timer for 12-14 hours of light per day.
• Regarding the corrugated roof panels you're considering: I've seen a wood-framed greenhouse built with something similar (roughly 8x8'). It lasted several years and worked very well but eventually collapsed under a heavy snow load. If you decide to use these, consider carefully how much wear and tear you're going to create in your setup and teardown. (I doubt poly sheeting will be very reusable, but it's much cheaper than the panels.)
  • Yes, it matters that the covering ("glazing") is clear. Plants need light to grow. The lower the light transmission of your glazing, the poorer your plants will do. I know of a greenhouse built using "smoked" glass (it was free) where (a) it doesn't warm up on sunny spring days like you might expect a greenhouse to do, (b) the plants don't thrive because they aren't getting full strength sunlight, (c) seedlings get "leggy" because they aren't getting full strength sunlight. I would go for the most transparent glazing you can get.
• If you're going to use it for starting plants as described above, what are you going to use for shelving? Are you going to stack vertically? If you're depending on overhead sunlight (versus sunlight entering at an angle), consider that top shelves will shade lower shelves. How high will the sidewalls be?
  • At 8' wide you can have multiple rows. Consider carefully which way your long axis runs, and whether stacked shelving on the outside rows is going to shade the inside rows.
• If you're going to build the large structure, and you plan to have a lot of plants out there, you're going to have a very heavy load -- all that soil and water, shelving, and the framing and glazing of the structure itself. (And possibly thermal mass -- e.g. barrels of water, bricks?) You may need to evaluate (by a pro) your deck to see if it has sufficient structural capacity to carry this large load.
• Greenhouses are warm, damp places. Warm and damp is generally good for fungus and bad for wood. Consider how this may impact your deck. (You're going to disassemble it for the summer, so maybe this won't be such a big impact. It will depend somewhat on your climate.)

Do some thinking about these aspects, a bit of research on what you think you need, and come back here and ask specific questions about your project.

(Disclaimer: I don't (yet) have my own greenhouse. Budget and siting have prevented me from getting one set up -- wind (50 mph+) and snow load (4') are major considerations for me, so I'll need a very sturdy structure. I've done a lot of research, and watched various hobbyists operate their own greenhouses, which is where the info above comes from.)

Answer 2

The coverage should be transparent or semitransparent. Both have advantages and disadvantages, but I would prefer semitransparent glass (or plastic), because the solar radiation will be scattered. Maybe a plastic-foil could also be a possibility. Plastic materials must be UV-resistant. Don't use colored materials (green or whatever), as plants react on many spectral ranges of sunlight.

Answer 3

Think of a greenhouse as a storage for plants and vegetation and you might be able to get a better idea of how to build one. You see, many people have a preconceived notion of a greenhouse, i.e. one that has glass or one that has high tech climate control. Really, as long as your plants are in this storage unit where they are protected from strong winds, have enough (but not too much) sunlight and good ventilation, you have a good greenhouse going on.

Start small first and build from there. You can start with one shelf of a few varieties of plants that you feel you can handle and go from there as you gain more experience. For cheap but good shelter, ordinary acrylic will do but do note that it yellows in sunlight so you might have to change it every 2 years or so. I suppose you could see that as a sort of maintenance.

Answer 4

Have you considered making your deck into a permanent 'outdoor' room? Something that will vastly increase the value of your home as well as saleability, be able to grow vegetables or exotic plants in beautiful pots and raised planters year 'round, add a few pieces of comfortable furniture so you can entertain or go hide from the world in an elegant, glass outdoor room that could be showcased in 'Sunset Magazine'?

I don't think you are going to want to do anything temporary. Putting a greenhouse up and taking it down (8'x20') every year will be a big pain. Wood and water don't mix. As a Landscape Architect/Project Manager for 30 years in the Pacific North Wet, grin, I always talked my clients OUT of wood decks attached to their home. If they HAD to have a deck, we made it using 'Treks'...I think that is how it was spelled...which is recycled plastic bottles made into dimensional lumber. A bit more costly to purchase and you have to pre-drill for all your screws...but! No Maintenance. Lower home insurance costs as wood gets slippery, dangerous. It comes in a lot of colors but PLEASE, please stay with dove gray! Any other color will look like you have a moldy or dirty deck. Dove gray is the best color to use for hardscape in Northwest gardens. Same with pavers, concrete, CMU (concrete modular units) for garden walls, retaining walls...even wood arbors, screens, fences, walkway edging...gravel is wonderful especially in the PNW!!

You want the color in your landscape to be the colors in the forest, not Disney Land, grin!

How high off the ground IS your deck? Is it the same elevation as the main floor or do you have to step down from your home onto the deck? If you are only a few feet off the ground and/or the deck surface is not the same elevation as your home...I'd talk you into considering building an outdoor room, with garden walls, planters built right into the walls that are for sitting, potted plants and areas for raised beds big enough for trees or vegetable gardens. Glass french doors opening from the home that can STAY open allowing the heat from your outdoor room, green house to heat your HOME. Lower your heating bills drastically. The flooring should be 7X9X2 inch dove gray 'Roman Cobble Pavers' or similar. No mortar allowed. Pavers are framed with dimensional lumber, PT The walls would be 18" in height from the pavers to the top of the wall (which would include a 2" thick concrete cap stone, about 2' in width). The walls would be either REAL stone (in gray) and pretty expensive and not so DIY friendly...or CMU's. Concrete lego blocks, big grin! Every year this product gets better and more natural looking. I used an 'ashlar' pattern with 3 or 4 different sizes of blocks. It takes a bit more IQ but worth it! You should get bids from Landscape Design Build Companies to see what they'd charge. Let me put it this way. It isn't something for DIY...this is your HOME, your investment. Unless you've done this kind of thing before, now is NOT the time to learn. Landscape companies are COMPETITIVE. You'll get more for every buck you spend with a good landscape company rather than using an Architect, Engineer, Electrician. A good landscape company DOES IT ALL. Yes, even low voltage lighting (essential in the PNW...). I could recommend a few depending on where you live. Not sure if that is 'kosher' on this site but I could at least tell you what to look for in a landscape company.

An outdoor room that provides passive solar will vastly increase the value of your home. Done in concrete, pavers, 6X6 posts, proper glazing, french doors or the big, big bi-fold doors that fold out of the way allowing the outdoor room to come inside. It should be designed so that the glazing is oriented to the sun, allows light and heat in, insulation so you don't lose heat, ventilation to release heat when necessary, a wood stove or open fireplace for sure...this would be more for human ambiance and comfort. A room-sized cadet heater for sure.

I think you need to think bigger. Break it all into manageable chunks so you can afford it. A wood deck in the PNW does nothing for your home. It has been shown to be a liability and actually reduces the value of your home. Concrete and stone is the way to go in my experience with lots of glass and BIG, THICK POSTS. No 4X4's. (4X4's are in proportion maybe for a kid's play house but I won't use them if people can see them in connection with the home or landscape). I'll try to send a few pictures if you are interested.

Oh I just realized you just might be renting and that is why you want a portable 'greenhouse'...smack my forehead and grin. Goodnight!