Detailed Explanation of Greenhouse and Polytunnel Construction Methods and Steps

A greenhouse, also known as a hothouse, mainly includes glass greenhouses and plastic greenhouses. The greenhouse structure should be sealed and insulated, yet it should also allow for easy ventilation and cooling. Modern greenhouses are equipped with devices that control temperature, humidity, light and other conditions, using computers to automatically create the optimal environmental conditions needed by plants. Below is an introduction to greenhouse and polytunnel construction methods.

1. Leveling the land and setting out lines

According to the designed solar greenhouse plan, measure and determine the azimuth angle, identify the four corners of the greenhouse or polytunnel, drive stakes at the four corners of the greenhouse, and then determine the positions of the gable walls and the back wall.

2. Building the walls

The soil used for building earthen walls can be taken from the outer side of the greenhouse back wall, or from the soil below the cultivation layer at the front of the greenhouse. If using the soil from the front of the greenhouse, the cultivation layer (about 25 cm thick) can be dug out and set aside, water the subsoil below, and after one day, dig the subsoil to make the earthen wall.

First, set up clamping boards according to the thickness of the earthen wall, fill in the freshly dug moist soil, and compact it with a soil rammer or electric tamper, about 20 cm per layer. After compacting one layer, do the second layer until the required height is reached. The gable walls and the back wall should be built together, not in sections; only in this way will they be sturdy. If the soil viscosity is insufficient, wheat straw can be mixed in. In some areas the soil viscosity is very low, and the compaction method cannot be used to build walls. In this case, a certain amount of wheat straw can be mixed into the soil to make mud, formed into adobe bricks, and after the adobe bricks dry, an adobe brick wall is built.

When laying the wall, the gaps between adobe bricks should be sealed tightly with straw mud, and both the inner and outer surfaces of the wall should be coated with straw mud. When constructing a brick wall, the foundation must first be compacted before laying bricks. During construction, the mortar should be full, the brick joints neatly pointed, and the plaster surface well applied. Both the inner and outer surfaces of the wall should be plastered to prevent air leakage. The hollow space between brick wall layers should not be too large or too small. Generally, the width of the hollow space should be kept between 5-8 cm. The hollow space should not be left all the way to the top; every 3-4 m, bricks should be used to connect the layers to improve the wall's sturdiness. Hollow walls can be filled with slag, perlite, or wheat straw as filler, or nothing can be added, using only air for insulation. Hollow walls without filler must be made completely free of cracks. When capping the brick wall, it is best to use mud and chaff to cap 30 cm, so that the back wall and the rear roof surface connect tightly and improve insulation performance.

3. Burying posts and erecting roof trusses

According to the drawings, determine the position of each post and mark them with lime. Dig holes 30-40 cm deep, use stones as post footings to prevent the posts from sinking, then install the dug beams on the rear row of posts, with the beam heads resting on the posts and the beam tails resting on the back wall or on the rear auxiliary posts. Place 3-4 purlins on the beams. The ridge purlin connects at the beams in a straight line, and the other purlins are placed in a staggered manner. To prevent the purlins from sliding down, a small wooden block can be nailed onto the beam below the purlin to block the purlin. Some greenhouses use only posts to support the ridge purlin.

4. Covering the rear roof surface

Cover a layer of waste plastic film over the purlins or rafters, place bundles of corn stalks on the film, with the direction of placement perpendicular to the purlins or rafters. Then spread wheat straw or rice straw on the corn stalks, and then lay another layer of plastic film on top, and apply straw mud on it. The rear roof surface, composed of straw and wheat straw wrapped in two layers of plastic film, forms a covering like a quilt, and its insulation performance is greatly improved compared to an ordinary rear roof surface without plastic film. After the rear roof surface is covered, use straw mud to seal tightly the junction between the inner side of the rear roof surface and the greenhouse back wall.

5. Digging the cold-proof trench

Dig a cold-proof trench 20 cm wide and 40 cm deep at the front edge of the greenhouse or polytunnel.

6. Burying ground anchors and fixing the galvanized wire for the rear roof film-pressing line

Lay a length of No. 8 galvanized wire equal in length to the greenhouse flat at the bottom of the cold-proof trench, with ground anchors threaded on it. The ground anchors are No. 8 galvanized wire with iron rings made at both ends. According to the spacing of the arch rods to be buried, every 3 m, tie a brick or wooden stick to the wire and place it between these fixing objects.

On the outer side of the greenhouse back wall, dig a trench and bury ground anchors in the same way, except that the anchor spacing can be increased to 2-3 m. After burying, fill in soil and tamp it firm, with the upper ring of the iron anchor exposed above the ground surface. On the rear roof surface of the greenhouse, stretch a line of No. 8 galvanized wire, with both ends buried in the ground outside the greenhouse gable walls; when burying, tie a heavy object to the ends. Use galvanized wire or nylon rope to fix the wire, with one end tied to the wire and the other end tied to the iron anchor buried outside the back wall.

7. Building the front roof surface

After the post positions are buried, adjust them front and back so that the rows and columns of posts are aligned. Bind 4 m long bamboo strips together in pairs, with the length appropriate, insert one end into the cold-proof trench, and press the lower part tightly against the south side of the cold-proof trench with bricks. The angle should be such that when the arch rods are erected, they are vertical to the ground or slightly inclined toward the south. Bind a crossbeam on the posts supporting the front roof surface, with the crossbeam 20-30 cm from the top of each row of posts. On the crossbeam, erect small hanging posts. Both the upper and lower ends of the small hanging posts should be drilled with holes, and No. 8 galvanized wire is passed through the holes respectively to bend the arch rods. One end of the small hanging post is tightly bound to the arch rod, and the other end is supported on the crossbeam and bound tightly. The upper end of the arch rod can be inserted onto the ridge purlin. Then, continuously adjust the small hanging posts so that the height at the same position on the front roof surface is consistent.

8. Covering with film

The greenhouse film consists of two or three sheets. When using two sheets, their widths are 3 m and 5 m respectively; when using three sheets, their widths are 2 m, 4 m, and 2 m respectively. First, roll back one edge of the 3 m or 2 m wide film, bond it with adhesive or iron it with an electric iron into a 5-6 cm wide tube, insert a nylon rope, fix the 3 m wide one at 2.5 m above the ground, and fix the 2 m wide one at 1.5 m above the ground.

The film is first rolled into rolls, and while covering, it is pulled tight while soil is filled into the cold-proof trench. The nylon rope should be pulled tight and, together with the film, buried in the ground at the greenhouse gable walls. The upper sheet or sheets of film are likewise first rolled into rolls, with one end buried in the ground near the gable wall, and then spread out toward the other end, and after reaching the end, buried in the ground near the gable wall. There are two ways to fix the end of the film near the rear roof surface. One is to directly fix it to the ridge purlin with bamboo strips and iron nails; the other method is to fix it to the ridge purlin with bamboo strips and iron nails, then fold it back and hook it onto the rear roof surface. The width that can be hooked onto the rear roof surface is about 0.5-1 m, the more the better, and it is compacted with straw mud on top. For a rear roof surface without added waste film, this method has a relatively good effect of improving insulation performance.

9. Fixing the film-pressing lines

After the film is covered, film-pressing lines should be used to press it tight and fix it. The film-pressing lines can be commercially available polypropylene greenhouse-specific film-pressing lines, or nylon rope or iron wire can be used as substitutes, but it is best not to use them. Special film-pressing lines are best. First, tie one end of the film-pressing line to the No. 8 galvanized wire on the greenhouse rear roof surface, throw it down from the greenhouse, press it onto the film between two arch rods, and tie it tightly to the ground anchor ring at the lower end.

The order of fixing the film-pressing lines is first sparse then dense: first fix a few film-pressing lines at large spacing, then gradually fix one film-pressing line between each arch rod. Both the film-pressing lines and the plastic film have a certain elasticity, so on the second and third days after fixing the film-pressing lines, tighten them 2-3 more times to ensure they are truly pressed tight. When pressed tight, the front roof surface film appears wavy.

10. Installing straw mats and paper quilts

The paper quilt is made of 4-6 layers of kraft paper. Straw mats are made of rice straw or cattail grass. Rice straw mats are 1.2-1.3 m wide, and cattail grass mats are 1.5-1.6 m wide. The length should be sufficient to cover the front roof surface of the greenhouse. In the absence of a paper quilt, two layers of straw mats can be covered, or the overlap between straw mats can be increased. Each straw mat uses two nylon ropes, each respectively 2 times the length of the straw mat or slightly longer, for pulling and laying. Both ends of each rope are respectively fixed at a point near the edge of one end of the straw mat, forming two loops that fit over the straw mat.

By pulling the two ropes on the surface of the straw mat, the straw mat can be rolled up or spread out on the front roof surface of the greenhouse. The rolled-up straw mats are placed on the rear roof surface in a staggered manner or one in front of another. To prevent the straw mats from sliding down, a stone or two or three bricks can be placed behind each rolled straw mat as a block.

11. Handling the entrance and exit

A solar greenhouse can have a door left at the east gable wall of the greenhouse. The door should be as small as possible, with an insulation room built outside the door, and door curtains hung on both the inner and outer sides of the door. Generally, no door is left at the west gable wall or the back wall of the greenhouse.

The above is Tutuliu.com's introduction to greenhouse and polytunnel construction methods. If the budget is sufficient, a glass greenhouse can be built; if the budget is smaller, a plastic greenhouse is also a good choice.