An external shading system is a set that stands above the house and takes the sun before it reaches the cover: a steel frame held clear of the roof, the screen carried on that frame, the drive that rolls it in and out, the limit that stops the travel, and the control that decides when it runs. Because the set is an accessory in its own right, frame, screen and drive are chosen and run on their own terms rather than as part of the covering. This page stays on the system itself: the parts above the cover, the shade rate, the fabric, drive and limit, and the daily rule. The version laid out for a film house is on external shading on a film house.

The frame is a structure of its own. Posts go down both sides into their own footings, beams span between them, and diagonal bracing keeps the grid square against wind. On the grid run the guide wires the screen travels on, the shaft it rolls onto, and the drive that turns the shaft. The set is tall enough that the screen clears the ridge with room for air to move between screen and cover, and that height is a selection point: too little clearance and the warmed air has nowhere to go, while a taller frame carries more wind load and needs heavier posts.

Standing on its own frame lets the two be serviced apart: the cover can be repaired while the shading frame stays up, and a worn screen can be changed without touching the film or the glass. The screen may run over the whole span or only over the ridge, and it is drawn to one end where the drive and shaft sit, so the frame carries screen and motor in one place.

The shade rate is the share of the sun the screen takes out, and it is chosen for the crop rather than for the house. A crop that scorches in strong midday sun asks for a denser screen; a crop already working at the edge of the light it needs asks for a lighter one. A dense screen over a crop that wants light grows the crop slowly and colours it late; a light screen over a crop that burns still lets the leaves scald.

Two readings decide the figure: what the crop and its stage want, and what the site delivers in its brightest month. A tender, thin-leaved crop on a hot, high-radiation site asks for more shade than the same crop on a mild coast, and a young plant after transplanting often wants more than that crop at full canopy. The rate is set as a step when the screen is ordered, so the crop plan comes first and the fabric follows; the fabric itself is set out on shade net.

An external screen lives outdoors and takes sun, wind and hail directly, so it is woven from high-density polyethylene tape with a UV stabiliser in the tape. The weave, the tape width and the stabiliser set how it behaves: a screen that fades and turns brittle has used up its protection and begins to tear at the fold lines. Because it is spread and rolled hundreds of times a season, it runs on guide wires, is tensioned by springs, and is fastened along its edges.

The edges fail first. The hem and the eyelets carry the whole pull of the fabric, so a slack screen flaps in the wind until it tears from the edge inward. That is why finished edges are reinforced and tension is checked rather than guessed: a taut screen does the work its rate promises, while a slack one shades less evenly and wears its own edge out.

There are two ways to move the screen, and the choice follows the span. With a rack and pinion drive, a gear motor turns a shaft carrying pinions that run along racks fixed to the frame; the racks hold the screen where it is put, so a long screen stays put under wind. With a cable drive, a drum winds cables that draw the screen open, which suits a lighter frame and a shorter span. In both cases the screen travels on guide wires, so the travel is straight and even across the house.

Whatever the drive, the travel must be stopped at both ends. Adjustable limit switches cut the motor when the screen is fully spread and again when it is fully rolled; without them the screen is pulled past its end so the fabric tears or the drive stalls. Springs at the far end keep the screen flat so it does not sag onto the frame. Upkeep sits in three places: guide wires kept straight, racks and pinions kept clear, and shaft bearings greased, since a screen that runs heavily is usually a tension or alignment problem rather than a drive one.

The two screens are not substitutes, because they work on opposite sides of the cover. The external screen takes the sun before it reaches the cover, so much of that heat never enters the house. An internal screen works on light and heat already inside, and in winter the same screen can be closed at night to hold warmth over the crop. External shading is about load avoided, internal shading about load already in the house.

When the heat is inside anyway, evaporative cooling is the next line rather than more shade, and the wetted pad is on the cellulose wet curtain. On a summer day the screen is spread early rather than at noon, because a screen that comes out once the house is hot has lost the morning; it is retracted in the late afternoon when the light is wanted, and stays retracted through a dull day. In strong wind it is retracted, since a spread screen is a sail, and at night it is normally retracted unless used as a second layer over the house.

Independent references help when a screen and its rate are specified: FAO publishes material on protected cultivation, and Greenhouse Grower follows daily practice in the greenhouse industry.

To have a set quoted and laid out, send the span, length and ridge height of the house, the crop and its stage, how strong the summer sun and typical wind are at the site, and whether the frame must be added to a standing house. That is enough to work out frame, screen rate and drive as one specification. Our factory has built greenhouses and their equipment for above 35 years; the set sits in the range of greenhouse accessories, alongside the rest of the greenhouse equipment we make. Call +86 191 5068 3942 or write to sales@cngreenhouses.com.

Q: What does an external shading system consist of?
A frame of its own above the cover, the screen on it, the guide wires and shaft, a gear motor with a rack and pinion or cable drive, limit switches at both ends of the travel, and the control. The set stands clear of the roof rather than being part of the covering.

Q: How is the shade rate chosen for a crop?
From the crop and its stage first, the site second: a crop that scorches at midday asks for a denser screen, a light-limited crop for a lighter one, and a hot high-radiation site for more shade than the same crop on a mild coast.

Q: Where does an external screen fail first?
At its edges. The hem and the eyelets carry the whole pull of the fabric, so a slack screen flaps in the wind and tears from the edge inward - which is why the edges are reinforced and tension is checked rather than guessed.

Q: What stops the screen at the open and closed ends?
Adjustable limit switches on the drive. They cut the motor when the screen is fully spread and again when it is fully rolled; without them the screen is pulled past its end and the fabric tears or the drive stalls.

Q: Does an external screen do the same job as an internal one?
No. The external screen takes the sun before it reaches the cover, so the heat never enters the house; an internal screen works on light and heat already inside and can double as a night layer.

Q: When should the external screen be spread and retracted?
Spread early, before the sun is high. Retract in the late afternoon and on dull days when the light is wanted, retract in strong wind since a spread screen is a sail, and normally retract at night.