Part One: Greenhouse Shape

If you’ve decided you want to build a greenhouse, you probably already have a pretty good idea of what it will look like and where it will go. This article will examine a few advantages and disadvantages of specific greenhouse designs, as well as some of the things you’ll need to consider about greenhouse orientation and placement.

Let’s start with shape of the greenhouse (we’ll deal with structure and materials in a later article).

Greenhouses come in a variety of designs: semi-circular “Quonsets,”  “Gothic” arches, vertical  walls with round roofs, and “ridge and eave” structures. Which is best for you depends on multiple factors.

If you’re going to build it yourself out of wood, a design with straight edges on the roof line (i.e. not rounded) is going to be a lot easier to construct (you can make either laminated or steam-bent curved wooden roof trusses, but they’re a lot of work). If you go with a peaked roof, you’ll have problems cladding the greenhouse with a double layer of polyethylene film (“double poly”) – the plastic will tear on the corners, and you’ll need to use more expensive materials like polycarbonate. Double poly has the advantage of being the lightest and lowest capital cost. This means fewer support structures in the roof to shade your crop (typically a truss every 8 feet as opposed to a 2-foot truss separation for polycarbonate), but you will have to replace the poly every few years.

If you can get your hands on bent metal hoops (I had a metal shop bend some 1” square tubing for my greenhouse), double poly is a good choice if you don’t mind the look of it. Rigid materials like polycarbonate do look at lot “cleaner,” especially compared to poly film that is a few years old and has those taped over holes and tears that will inevitably develop (e.g. from a lawn mower throwing rocks at it). Be aware, however, that all plastic will age in sunlight, and become stained and /or yellow over time. If you canvisit a greenhouse of similar design and materials that’s been standing for a few years you can make sure that you can live with the look of it once it gets a bit “long in the tooth.”

If you live in an area where heavy, wet snow is part of your winter (most of our province), you’ll want a fairly steep roof to shed snow, especially if the greenhouse is not heated in winter. Remember too that heavy, wet snow can deform the cladding (especially double poly) and that nay structures at the edge of the roof can prevent snow from sliding off. Significant snow load can crush a greenhouse. A trick I discovered with my greenhouse (double poly with vertical walls) was to have the roof supports transition from curved to vertical about 8 inches above the eave – the built up snow forms a valley in the poly, down which the snow can slide and the bottom of this valley is always above the eave line.

Some greenhouse designs like Quonsets or Gothic arches can have sidewalls that slope inwards towards the center of the greenhouse. This shape sheds snow easily, but any snow that builds up at the base of the wall is likely in part to rest on the wall of the greenhouse. This snow can build up to the point where the lower sidewalls get damaged. Another consideration for sloping sidewalls is the loss of usable space in the greenhouse, particularly near the edges there will be less vertical height, making the perimeter useless for walkways or tall crops (e.g. tomatoes, cucumbers, peppers) if the slope is shallow.

The footprint of a greenhouse is usually square or rectangular, with the roof sloping away from the center line. Those gable ends are typically more structurally complex – equipped with doors, vents (especially in double poly structures) and more structural supports than the side walls (a few posts and eaves). This means that light transmission of the gable ends is usually less than that of the side walls and sloping roof.

Light entering the greenhouse at low angles will be mostly absorbed by the crop closest to where it enters, and plants farther away will be shaded by the ones in front. For these two reasons, it makes the most sense to orient the greenhouse so that the gable ends are facing east and west, so that the light penetrates the greenhouse through the south wall and roof, and light absorption by the crop is maximized.

This is most important when sun angles are low (during the winter and further north) but in clear weather a lot of light still comes from the north on cloudy days. You’ll get less reflection (more penetration) from steep roofs as well when winter sun angles are low. If you want to operate the greenhouse in winter with no heat or minimal heat, insulating the north wall (or building the greenhouse into a south-facing slope) and placing most of the equipment on that wall (fans, pumps etc.) makes the most sense. Moving the ridge line of the roof off-center (closer to the north wall) can also improve light absorption, as will making the interior surface of the insulated north wall reflective to maximize light. If you want to maximize solar heating, make that north wall highly absorptive (e.g. black or deep red.)  

This, of course, assumes that orienting the long dimension of the greenhouse east-west is actually an option in the space you have. Practical considerations do come into play. You probably don’t want your greenhouse at a 45 degree angle across your back yard – you’ll want to line it up with your house, driveway or some other aspect of your property, as well as keep it out of the shade of buildings, trees, and so on.  Remember that you’ll also want access to the greenhouse door year round, so you might want to put a gable end on the south side if that’s where having the door makes the most sense and is easiest to clear snow from. You’ll also need access to power for running pumps, fans and double poly inflation blowers (if that’s part of your design), and water for irrigation (a garden hose or permanent water line is best – my 256 sq ft greenhouse goes through about 40 L a day – a lot to carry in a pail.)

In short, the greenhouse you end up with will largely be determined by the site you have available and by the type of structure you can reasonably buy or build. This should be balanced by your convenience (how hard will it be to clear the snow?) and what you want it to look like.   I hope this article has presented some food for thought.  Next time, structure and materials.