Construction Of A Polytunnel

Construction Of A Polytunnel
In this Krosagro guide on foil tunnels, we will talk about elements, parts and accessories needed in the construction of a polytunnel. What does a standard design consist of, what makes it durable, what are the guarantees with a hot-dip galvanized frame, how to maintain it?

1. Construction of a polytunnel: what is it ?

Every polytunnel manufacturer offers his own construction solution, so there are quite a few differences. The main difference, of the utmost importance, is the material from which the garden tunnel was made. This has repercussions on its durability, reliability, and especially its price. Of course, the better the quality of the material, the higher the price , while the cheaper the construction the more fragile and unreliable. The greenhouses themselves, although they may come in different shapes, consist of such elements :
  • Tunnel frame – the base of the structure onto which further foil segments will be built. In a square or a rectangle shape, the two sides are usually longer than the width. The shorter sides are the front and back, where doors are mounted.
  • Tunnel posts – their number depends on the length of the structure, they are fixed on each corner and at the base of every arch.
  • Tunnel arches – these are the elements giving the polytunnel its dome shape. They consist of two parts, or three for hobby tunnels (single and multi vegetation) .

Steel elements –  foil tunnel

  • Tunnel door – in this case, one can choose between one or two doors, fitted on both sides of the greenhouse. Opening them will contribute to ventilate the greenhouse, allowing users to get rid of excess heat and humidity under cover.
  • Lateral ventilation – optional equipment on both sides of the greenhouse. In addition to this opening system, an insect net is provided, preventing pests from entering the tunnel when ventilating. The foil is folded and lowered by means of a crank. This creates a beneficial air circulation for crops and allows the gardener to modify the micro climate inside.
  • Small steel elements – the aforementioned parts are the main structure skeleton. The whole construction includes many other accessories. They have a significant impact on the greenhouse, stabilizing and strengthening it. It is very important to assemble each part in accordance with the instructions.

2. What are garden tunnels made of?

Everything depends on the producer. Some companies focus on PVC constructions, aluminum… others choose tunnels made of more durable materials such as steel. The difference in strength and quality is noticeable. Which one is better or worse is a subjective matter. Of course, as always, you can find advantages and disadvantages in every type. Many people start with cheap structures, and then turn to steel, which is the best compromise in many respects.
An investment in a greenhouse should last for years. It is not worth it if you have to buy spare parts every season, or fix fragile and defective elements. The price of a garden tunnel, which previously looked like a bargain, will increase several times with each repair. Additional expenses will not encourage growing under cover. Unsurprisingly, cultivation results will be lesser than those announced in marketing slogans. The solution is simple, just choose a strong construction with good parameters, offering a certified foil and an optimal design.

Cheap plastic tunnel or solid foil tunnel?

PVC, aluminum, although available on the market, are not the best material for a garden tunnel. The construction itself should resist strong winds and storms without additional reinforcements. In the case of PVC and aluminum, deformation and bending often occurs following atmospheric hazards. During a storm, tunnels can literally be swept off the ground. It is also not recommended to hang pots off the frame. Even with a small weight, this may bend, and over time break the arches. Steel is a much more solid solution which copes with strong gusts of wind and tough weather conditions. Pots can be hung without unpleasant consequences .
The advantage of a Krosagro construction is that engineers and designers apply professional polytunnels technology to hobby greenhouses. Using 25 mm pipes and galvanized construction provides a structure fit for years of cultivation on a plot or garden.

Anti corrosion layer

A galvanized steel foil tunnel is a structure made of parts treated with a hot dip galvanizing process. This means that elements have been coated with an anti-corrosive layer. There are several types of zinc used in this method, and the advantages of galvanizing are numerous: aesthetic appearance, resistance to rust, higher resistance to mechanical damage, lack of porosity, etc.
During immersion, zinc reacts with steel, metal and other alloys. The galvanizing steps may differ in terms of method and duration, but the goal remains the same: a protective layer with a thickness of 70 to 220 micrometers (μm) covering pipes and profiles.

Maintenance of a galvanized steel frame

There are three methods to maintain and repair zinc coating, which comply with the recommendations of the PN-EN ISO 1461 standard.
  • thermal spray with zinc
  • treating elements with zinc-rich paint
  • using zinc based soldering alloys
All maintenance and repair begins with thorough cleaning of the elements. Remove any dirt from the surface. You can get rid of them using organic acids or special means available on the market. During the whole procedure, it is very important that these substances do not dry out on the elements.
The most popular way to repair cavities in a galvanized frame is the second method. After cleaning, several layers are applied onto the pipes and tunnel profiles until a similar thickness to the previous one is obtained.

Modular structure

The tunnel structure has been designed with the possibility to extend it freely or build on to it for extra bays . You can add new segments of arches to the hobby constructions, and even combine both structures to avail of more cultivation space. Tunnel fittings are compatible with any given model, so the gardener can modified his growing unit. This is a big advantage for users who can create custom length facilities.


A foil tunnel, specifically its frame, is made of steel. After shaping the structural elements in the factory , they head for the galvanizing treatment, where they are subjected to a zinc bath. Dipping metal parts in zinc creates an anti-corrosive layer on their surface. The complex construction of a polytunnel ensures long term service for users, despite intensive use.

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