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This article throws light upon the four steps for the construction of green house. The steps are: 1. Foundation 2. Frame Work 3. Pitch of Proof 4. Determination of the Pitch.
Step # 1. Foundation:
It provides support to the structure. Small green houses may have shallow foundation continuously around perimeter of the building. A typical foundation consists of concrete footings located below the frost line. The spacing between footings is 2-3 m. The posts are embedded in concrete piers which rest on footing. This system will provide the main support for the green house. The curtain wall of desirable height is made up of bricks.
Step # 2. Frame Work:
The frame of green house is to provide support for covering material and to other accessories like water pipes, electrical fittings ventilators and other. Thus the frame work of green house should be light, strong but casts less shade, and has long life. Earlier wood was used for framing the green house.
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It has several advantages like bad conductor of heat, cheap and easy to use but frame work of wood will cast lot of shade and its moving shadow affect the performance of crop. On the contrary, cold roll steel or aluminium is used now-a-days. It has all desirable characteristics like it is light, strong, casts less shade and has long life, but being bad conductor of heat, it became hot or cool quickly.
Step # 3. Pitch of Roof:
The angle of divergence from the horizontal will decide the degree of slant and is called as pitch of roof. The glass or other transparent material of green house will allow light to penetrate through it but some light will be reflected back in the atmosphere. This will depend upon the angle of incidence which makes the light over the transparent material.
The amount of light lost by reflection is inversely proportional to the angle of incidence. Thus, if the sunrays fall upon the roof at right angle, little or no light is reflected back. Similarly, if light falls at low angle, loss of light by reflection will be more.
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Thus, it is vivid that maximum amount of sunrays may be conserved by a roof when it makes a 90° angle by the falling light. During winter it becomes much more important. To make a such roof, it would need a pitch of 65 degrees.
The roof with such a high pitch of angle will have certain disadvantages like:
(i) It will be too high for house having width of 3 or 3.5 m.
(ii) It well be very expensive to build and maintain.
(iii) It will expose too large surface area for radiation of the space covered.
To overcome above disadvantages if pitch is reduced to 35 degree, the sun rays will strike the roof at an angle of 55 degree then loss of light through reflection will be only 2-3%. Such roof has not many disadvantages.
Roof having a pitch of 26° or less, they are not satisfactory on accounts:
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(i) Loss of light through reflection is more,
(ii) Snow does not clear off efficiently, and
(iii) Condensed drops of water will fall on the plant like bullet which will impair the quality of product.
Step # 4. Determination of the Pitch:
The pitch of the house can be calculated trigonometrically when the width of the house and the height of the ridge is known. Half width and height will make a triangle so the pitch can be determined with the formula
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A = b/a
where A is pitch, a is half width and b is height. However, it is more desirable to determine the length of rafter to make a roof of a house on given pitch. This may also be trigonometrically solved be applying a formula
A = Half width (b)/cosine of pitch angle (c)
whereas A is the length of rafter, (b) is half width and (c) is cosine of pitch angle.
