A clear cover is bought, kept, used and then replaced, and the stage a buyer has least control over is the one in which the film is still a roll. This page is about the film as stock. It sets out where a roll should be kept between delivery and fitting, how rolls are stacked, dated and rotated, what the sheet itself shows when it is close to the end of its working life, and how a replacement is planned so that the changeover does not land in the middle of a crop. The cover written about here is a clear polyethylene greenhouse film, UV protected, listed as standard clear greenhouse film at 6 mil with a covering material thickness of 0.15 mm or 0.12 mm, run as a single layer or as two, and held with aluminum clips. It is not about the chemistry of ultraviolet ageing, not about how much film a house takes, and not about the thickness grades themselves, each of which has a page of its own.

This is clear polyethylene film for a film greenhouse, supplied as the covering a house is listed with rather than as a general purpose sheet. It is UV protected, and that protection is what keeps the sheet both clear and workable over the seasons instead of yellowing and splitting early, which is the property most of its value rests on. The model lists it as standard clear greenhouse film at 6 mil, with a covering material thickness of 0.15 mm or 0.12 mm, and it is run either as a single layer or as two. Nothing else covers this frame: the model may only be covered by polyethylene held with aluminum clips, so the film and the clips are bought and kept as a pair, and a spare that does not include matching clips is only half a spare.

The crops it is bought for are vegetables, fruit, flowers and seed nursery stock, and the same sheet serves both a working commercial house and the smaller structures growers run at home or on a small holding. Its job on the house is to hold warmth in and keep cold air out, and because the cover stays closed and clear it keeps the light level and the temperature inside steady without extra energy being spent on either. That is the reason the sheet is treated as equipment rather than as consumable packaging: what it is doing to the crop depends on the condition of the film, and the condition of the film at the start of a season depends heavily on how it was kept before that season began.

A covering lives four lives, and they are not given the same attention. It is ordered, it is stored, it is run on the house, and it is replaced. Nearly all the discussion about greenhouse film is about the third stage, and almost none is about the second, even though the second is the stage in which the buyer is most often making an unforced mistake.

A roll that arrives at the yard is not finished material waiting to be used. It is a sheet already carrying its full reserve of stabiliser, and that reserve is spending itself slowly wherever the roll happens to be lying. Sunlight spends it, heat spends it, moisture and chemical fumes spend it, and none of the three is visible on the outside wrapping of a roll at the back of a shed. A roll kept for one season in the shade of a dry store and a roll kept for the same season against a south facing metal wall are the same product from the same pallet, and they will not behave the same way on the house. This is the whole reason the subject is worth a page: storage is the one part of a film's life that a grower fully controls, and it is the part most easily got right.

The cycle also has a natural rhythm that a purchasing plan can follow. Film is consumed in whole covers, not in metres, so the sensible unit is one cover plus a spare, bought once at the point in the year when the house is between crops rather than the day a panel starts slapping in wind. Treating the cover as stock rather than as a repair item turns the replacement into a scheduled job, and a scheduled job can be planned around the crop instead of interrupting it.

The three things a stored roll has to be kept away from are the three things a covered house is built to manage, which is not a coincidence. Light is first. A film is UV protected against the sunlight it will meet on the house, but the protection is a reserve rather than a shield, and every hour of direct sun on a stored roll draws on the same reserve the sheet will need when it is in service. Rolls should therefore sit in shade for their whole time in store, and a store with a translucent roof is not a shaded store.

Heat is second, and it arrives mostly through metal. A roll leaning against a dark steel wall, a hot sheet metal roof space or a boiler flue is being warmed from the inside out, and a warm roll is a roll whose layers begin to cling to one another, so that the sheet is pulled apart in patches when it is finally unwound. Water is third. A roll standing on a floor that stays damp, or on the ground beside a building, draws moisture inward between the layers, and a roll that has taken water clouds, smells of mildew and sticks when it is opened, none of which any amount of care at installation can undo.

Chemical fumes are the fourth item on the same short list, and it is the one a farm store most often gets wrong. Fertiliser, fuel, solvents and concentrated crop protection products all release vapour, and polyethylene standing in that vapour for months is being asked to absorb something it was never made to meet. A dry, shaded, ventilated building that holds none of those materials is the whole specification, and it is worth saying plainly that no film standard sets a storage life for a roll: what the standards fix is the performance a new film has to reach. Since the calendar cannot be relied on, rotation is what takes its place. The wider field this belongs to, protected cultivation and the handling of agricultural covering materials, is documented among much else by the FAO.

How a roll is laid down matters as much as where. The first rule is to leave the original wrapping on until the day the film is going on the house. The wrap is the only barrier the roll has against dust, splash and light, and a roll unwrapped early so that it looks ready is a roll that has been exposed for exactly as long as it has been unwrapped. The second rule is to keep rolls off the ground on pallets or bearers, and to keep them off wet concrete in particular, because concrete stays cold and damp long after the air has dried.

The third rule is about weight. Rolls store well standing on their own cores or lying flat in a low stack, and they store badly under anything heavy. A roll that has carried the weight of a pallet above it holds a flattened profile, and a crease set into polyethylene in store does not simply iron out on the house; it is a thin line in the sheet from the first day of its service. The same caution applies to the ends of the roll. A crushed or deformed core makes the film unwrap unevenly, and a roll that has been dragged across a rough concrete yard picks up the score marks that later open into tears.

So the handling list is short. Keep the wrap on, keep rolls off the floor, keep nothing stacked on top of them, keep sharp edges and loose metal away from the stack, and carry a roll rather than drag it. One further habit is worth adding, because it costs nothing: when a roll comes out of store after a long wait, unroll a metre or two and look at the colour and feel of the sheet before it goes up on the house. A roll that has taken damage in store shows it in that first metre, where it can still be set aside, rather than halfway along a bay where it cannot.

Because the film is spending its reserve in store, the order in which rolls are used is as important as the conditions they are kept in. The practice is the one every parts store uses: first in, first out. It takes two habits, and neither of them is complicated. The first is to mark the delivery date on the wrapping of every roll the day it arrives, in marker, where it can be read without opening the stack. The second is to take from the oldest roll in the store rather than the nearest one to the door, which is the opposite of what a hurried morning will do by itself.

Alongside those two habits it is worth keeping a one page register, held wherever the tools are kept, with three columns: how many rolls are in store, what thickness they are, and when each arrived. That register answers the only question that matters at the start of a covering season, which is whether the film the house needs is already on site or still has to be ordered, and it answers it in seconds rather than after a walk round the buildings. It also makes the spare roll visible, which is the subject of the section after next.

There is one more consequence of treating rotation seriously, and it concerns ordering rather than storing. Film is made and packed to order, and a roll bought in a calm month arrives in the calm month. A roll bought the week a cover fails arrives after the event it was bought for. Placing the order at the far end of the previous covering season, so that the replacement is in store before the window for fitting opens, is what makes the scheduled changeover possible rather than merely hoped for. That is a purchasing decision, and it is the one that a stock register makes easy.

A replacement cover is a like for like order, and the details that have to match are few enough to list. The film is standard clear greenhouse film at 6 mil, or a covering material thickness of 0.15 mm or 0.12 mm, run as a single layer or as a double layer. The fixing is aluminum clips into the frame channel, so the clips are ordered with the film and to the same channel. Thicker or thinner grades are a separate decision with its own page, and they are not needed to re-cover a house that was working well.

The frame the cover returns to is hot dip galvanized steel pipe, which is what keeps the structure free of rust across its working life, and it is available with spans of 6.4, 8, 9.6 and 10 metres and a total structure height of 4.3 metres to 6.5 metres, under an arch roof, with ventilation that is manually or electrically operated. The covering is intended to prevent heat loss and cold air intrusion in any climate, and the structure is drawn for a design wind load of 0.35 kN/m2, which is a pressure on the exposed surface rather than a wind speed. Any of these figures can be held or changed with the supplier, and the arrangement is quoted to the customer's own drawing: a house of 64 metres by 40 metres, 2,560 square metres, is one arrangement built regularly and not a fixed size that the covering only comes in.

Systems are ordered with the house rather than with the film, and keeping the two lists apart is part of keeping a stock room useful. Cooling and heating, irrigation, ventilation, fertilisation, shading and benches are all added to the structure on request, and none of them is a covering spare. What belongs on the film side of the store is the sheet and the clips, plus the offcuts described further down. The systems themselves are shown with the rest of the polyethylene covered range on our film greenhouse range page.

A covering does not fail politely. The tear that ends a season usually arrives with wind behind it, on a day when nothing can be ordered and nothing can be fitted, and the crop is standing underneath while the decision is being made. That situation is the argument for a spare roll, and it is the only argument that is needed. A roll in store turns a storm loss from a crop emergency into a repair job, because the sheet is already on site, already acclimatised to the local conditions, and already matched to the channel and the clips.

The spare is not only the roll. Matching aluminum clips belong with it, since a cover cannot be held without them, and so do the offcuts taken from the last fitting, which are the patch material for the small holes that are not worth a panel. Those offcuts should be rolled, labelled with their thickness, and kept by the same rules as the main stock, in the shade and off the floor, because the metre of film that saves an afternoon is worthless if it has been lying in the light for a year.

How much to hold is a judgement about the house and the site rather than a number this page can supply. A short house in a sheltered place and a long house on an exposed coast are not the same risk, and the figure that suits one will be wasteful for the other. What can be said is that the spare is stock like any other stock, so it is dated, rotated and used in its turn, and the roll it replaces becomes the new spare. Held that way it never becomes the forgotten roll at the back of the store that this whole page exists to warn against.

Deciding that a cover is finished is a judgement about the material, and the material gives four signs in a fairly consistent order. The first is colour. A film that was clear begins to look washed or milky in low sun, and the bright look that a well covered house has is the first thing a grower notices leaving it. The second is feel. The sheet that was soft and pliable stiffens and stops behaving like cloth, and a fold that is pressed into it no longer flattens out but shows a pale line where the material has begun to give.

The third sign is light. The house begins to look duller from inside, and a film that is spending its final season is passing less light than the crop is being managed for, which is a loss that costs yield before it costs anything visible. The fourth sign is tear behaviour, and it is the one that forces the decision. A small nick stops being a small nick and starts to run, and the runs characteristically travel away from a clip or from where the sheet turns into the channel. When the fourth sign appears the choice has already gone, because a split that is travelling will not stop at a convenient point.

The purpose of reading the first two signs is to buy the time to act on the fourth. A grower who sees the colour and the feel going has a season in which to order a roll and choose a window, and a grower who waits for the tear has none. What the film is doing when it ages, and why ultraviolet light is the principal cause rather than heat or handling, is set out in full on our how ultraviolet ages the film page; this page keeps to what the sheet shows and what to do about it.

A replacement costs least when it is planned, and the planning has three parts that can all be done well before the day. The first is the order. Film is produced and packed to order, so the date the roll is needed has to be worked backwards into a date the order is placed, and the safe habit is to order at the end of the previous covering season so that the roll is in store, dated and rotated into position, well before the window opens.

The second part is the window itself. A covering change is an empty house job if it is to be done at all comfortably, and the empty house is found between crops, not in the middle of one. That is the reason the reading of the film described above matters: the grower who reads the film early is the grower who can pick a window rather than take one. The third part is the crew and the day, and it is deliberately not covered here. How a cover is run out, seated in the channel and tensioned is a job with its own order of operations and its own common mistakes, and it is written up on our how a cover is fixed and tensioned page.

What comes off the house on the day is worth planning too. The old sheet, the clips that came out of the channel and the offcuts are three separate things leaving the job, and each has a use or a destination. The old film is the finished product and goes for disposal; the clips are checked and returned to the store with the spare; the offcuts are labelled by thickness and kept as patch material. Sorting the three at the foot of the house takes minutes, and sorting them a week later out of a pile does not happen at all.

From the buying side, the trade reporting that commercial growers read tends to treat a covering as a running cost renewed on a cycle rather than as a one off purchase, which is the same conclusion this page reaches from the other end of the transaction. That reporting is published by Greenhouse Grower.

It is worth being clear about which part of a film house is stock and which part is not, because the two are bought together and then live on completely different clocks. The cover is a consumable renewed on a cycle of a few seasons. The frame is not. This model is built on hot dip galvanized steel pipe, chosen precisely so that the structure does not need the attention the film needs, and the frame's own maintenance expectation is set by the corrosivity of the air where it stands rather than by anything the grower does.

The classes come from ISO 9223, and applied to a standard galvanized coating they read as follows.

Those four figures are what make two identical houses in two places different maintenance propositions, and naming the class is what turns them into one number for one site. The pipe we supply is specified at 70 to 85 micrometres of zinc under BS EN ISO 1461, assessed to BS EN ISO 14713-1, and cut and drilled points are treated so that the coating stays continuous. The frame on this model has therefore been manufactured for above 35 years, and it is the one part of the house that a cover replacement does not touch.

The house travels in three groups: metal structures, covering system and install accessories. For a buyer planning storage, that grouping is not just a packing list, because it divides the delivery into the part that will stand for decades and the parts that will be renewed. The covering group is the one that has to be kept indoors and kept shaded, and the install accessories group is where the aluminum clips arrive, which is why a second order for film alone is worth matching to the clips already in the store. The packing is also the practical reason to date a roll: a roll is packed with the covering group and its delivery date is known at the moment it is unloaded.

How much covering a house of a given size actually takes, once the arch profile, the laps at the ends and the roll widths are counted, is a calculation in its own right and is worked through on our how much film a house takes page. The short version for stock purposes is that a cover is bought in whole rolls for one house, and that a spare is bought as a whole roll rather than as a length.

The company behind this cover is a manufacturer rather than a trading house, and the greenhouse items it sells are produced in its own works, including the plastic film houses and the standard agricultural structures they belong to. Producing in house is what allows both quality control and competitive pricing to be offered on the same order, and it is also why a re-order of covering matches the cover it replaces rather than being matched to it from a catalogue.

Covering for this model is available in custom sizes on request, and a quotation is put together from two pieces of information: the quantity of houses required and the crops that will be grown in them. Choosing the wider system is a third conversation, and it runs better when the buyer can give the local temperature range and average wind speeds, since those are the figures that decide which configuration is durable in a particular place rather than merely adequate. Samples of the greenhouse products, including covering film components, are supplied free of charge, with the freight on sample deliveries borne by the customer.

After the order, an after sales engineer provides layout drawings and the standard installation manual, and remains available for questions through the assembly. Where on site supervision is wanted, engineers can travel to the customer's country to oversee the installation. The company has been manufacturing greenhouses for above 35 years, works from its own frame factory, and ships to customers all over the world every year, with many of them visiting the factory for business cooperation. Choosing a poly tunnel cover in the first place, and the selection questions that go with it, is set out on our choosing a poly tunnel cover page.

If you need to ask a question and get some help to find the greenhouse that fits, call us on +86 191 5068 3942 or send an e-mail to sales@cngreenhouses.com. Tell us what the cover has done and how long it has been on, and the storage and replacement questions on this page can be settled against your own house rather than in the abstract.

Q: How should unused greenhouse film be stored before it is fitted?
In shade, in a cool dry building, off the floor and away from fumes. The three things to keep a roll away from are direct sunlight, heat arriving through metal, and standing moisture. Keep the original wrapping on until the day the film goes on the house, store rolls on pallets or bearers rather than on concrete, put nothing heavy on top of them, and keep fertiliser, fuel and solvents out of the same building. Since no covering film standard fixes a storage life, the protection that would otherwise come from a calendar date has to come from rotation instead.

Q: Why does a damp floor damage a roll of film?
Because the moisture does not stay outside the roll. Water drawn up through a damp floor or in from the air passes between the layers, and a roll that has taken water clouds, develops a smell of mildew and clings to itself when it is unwound. The inner layers are the ones affected and they are also the ones nobody sees, so the fault is usually discovered when the sheet is already half way along a bay. Pallets or bearers under every roll, and a floor that drains, is the whole remedy.

Q: What signs in the film itself show that a cover is past use?
Four, in a fairly consistent order: the colour goes washed or milky, especially in low sun; the sheet stiffens and a pressed fold leaves a pale line instead of flattening; the house looks duller from inside as less light gets through; and a small nick starts to run, characteristically travelling away from a clip or from where the film enters the channel. The first two signs are the time to order, and the fourth is the point at which the choice has gone. The cover on this model is clear film at 6 mil, about 0.15 mm, or 0.12 mm, single or double layer.

Q: How much spare covering should be kept on site?
That is a judgement about the house and the site rather than a figure this page can give, because a short sheltered house and a long exposed one carry different risk. What can be fixed is the form the spare takes: at least one whole roll rather than a length, matching aluminum clips with it, and the labelled offcuts from the last fitting kept as patch material. The spare is dated and rotated like the rest of the stock, so that the roll it replaces becomes the next spare instead of becoming the forgotten roll at the back of the store.

Q: What should be ordered to re-cover this house?
The same covering the house is listed with: standard clear greenhouse film at 6 mil, or a covering material thickness of 0.15 mm or 0.12 mm, run as a single layer or as a double layer, with aluminum clips to suit the frame channel. The frame it returns to is hot dip galvanized steel pipe with spans of 6.4, 8, 9.6 or 10 metres and a total height of 4.3 to 6.5 metres under an arch roof, designed for a wind load of 0.35 kN/m2. Sizes and systems are quoted to the customer's own drawing, so a re-order is confirmed against the drawing rather than assumed from the previous invoice.

Q: Are samples free, and who pays the freight?
Samples of the greenhouse products, including components of the plastic film houses, are supplied free of charge. The freight cost of a sample delivery is borne by the customer. For a covering order rather than a sample, the quotation is built from the quantity of houses required and the crops to be grown in them, and where the wider system is being decided as well it helps to know the local temperature range and average wind speeds at the site.