Lean-To Greenhouses
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Traditional Shaped Greenhouse
One key advantage of the traditional shaped greenhouse is that there are far more sizes available than other types, they are generally cheaper to buy and they are available from a large number of suppliers. So, if this greenhouse shape matches your needs, buy it rather than the more expensive octagonal or lean-to greenhouses.

Even with the traditional garden greenhouse shape, there are subtle variations and they generally arise in order to maximise light transmission in the winter and spring months. At these times of year, you may well be growing plants in your greenhouse, and maximum light transmission is important because the sun is so low in the sky and days are generally more cloudy. Light transmission is not so important in the summer months because there is more than enough light to go round.

When thinking about light transmission, the obvious needs to be mentioned! There is only one sun and it shines on your greenhouse from roughly the same direction each winter and spring day. The point in maximising light transmission is to provide light to all parts of the greenhouse and not just the side facing the sun. 

There are four main variations on the traditional shaped greenhouse, each is a compromise as shown below.

	The standard greenhouse shape. With the sun low in the sky during winter and early spring, much of the light bounces off the roof because it is not at the ideal angle of 90 degrees. However, the angle of the sun on the sides of the greenhouse is roughly an 80 degree angle which means a lot of light is absorbed and reflected round the greenhouse.
	This is the basic greenhouse shape but the side walls are leaning in. This means that the angle of the sun against the side walls is very close to a perfect 90 degrees.  The roof is very similar to the greenhouse above which causes much of the light to bounce off.  Overall, a very good shape.
	An unusual variant on the standard greenhouse, but some companies do sell this type of greenhouse. The greenhouse must be positioned so that the short steeper side of the roof faces the sun. The sun-facing side of the roof presents an angle nearer to 90 degrees to the sun than all the other greenhouses.  The walls are as per the standard greenhouse shape.
	Probably the best compromise, this greenhouse has a very small roof area which reflects light out of the greenhouse. The long middle side wall is at an ideal 90 degrees to the sun.  The shorter base wall is as per the traditional greenhouse. Unfortunately this type of greenhouse is expensive to manufacture.

The positioning of a greenhouse is also a key factor for maximum light transmission. All traditional type greenhouses should be positioned such that their longest side runs east to west to allow maximum light transmission in darker months.

Octagonal Shaped Greenhouse
These do tend to be the most expensive greenhouses, but their shape makes them an attractive feature in many gardens. In addition, their shape ensures there is always part of the greenhouse at any time which is at the ideal angle of 90 degrees to the sun. 

Another advantage (although at considerable cost) is that the circular benching which can be bought for them gives a large and convenient area for potting up plants and growing them on.

One possible disadvantage of an octagonal greenhouse is that ventilation during the hot summer months is difficult. There are always pockets of heat which develop because of the round shape. Lots of opening ventilation windows are required to ensure an even temperature during the summer.

 

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