An order for greenhouse film is measured in metres, and it is measured wrong more often than any other part of a film house, because the figure most people reach for is the one number they already know: the floor area. This page stays with the arithmetic that turns a drawing into a film order on a commercial film house of 64 m by 40 m, 2,560 square metres, covered with standard clear greenhouse film at 6 mil, about 0.15 mm, in a single or double layer on a hot dip galvanized steel pipe frame with an arch roof. Nothing here is about light, and nothing here is about how long the sheet lasts. It is about how much film a house of this size actually takes, what widths it arrives in, how laps and ends are allowed for, and what is left on the floor after the longest panel has been cut.

Two thousand five hundred and sixty square metres is the floor of this house. The film never lies on that floor. It starts at the ground on one side, climbs the sidewall to the eaves, follows the curve of the arch over the top, comes down the far sidewall and finishes at the ground on the other side; it also closes both ends. What is bought is therefore a surface, and a surface is always larger than the plan of the same house, because the arch is not a flat roof and the sides are not part of the floor plan at all.

The gap between the two numbers is not a rounding difference. On a wide house, the two end walls alone are a large area of film: the house is 64 m across, the structure stands between 4.3 m and 6.5 m, and closing one end takes a gable-shaped piece roughly the width of the house and most of its height. There are two of them, one at each end, and they are cut from the same material as the roof. A buyer who orders from the floor figure arrives on site with the roof covered and nothing to close the ends.

There is a second reason the plan figure misleads. Film is quoted and delivered by area, but it is ordered by width and length, and the two are not interchangeable. A roll 8 m wide and 100 m long carries 800 square metres of film, and so does a roll 4 m wide and 200 m long, but only one of them suits the bay it is going onto. The quantity question and the width question are the same conversation, which is why this page works along the profile rather than across the floor.

On a flat roof, the width of a panel is the width of the bay. On an arch it is not, and that difference is the single most common source of a short sheet. The distance the film travels from one fixing line to the other, over the top of the arch, is longer than the straight distance between those two lines. Geometry guarantees it: a curve between two points is longer than the chord that joins them, and the rise on this model is whatever the span and the 4.3 m to 6.5 m structure height make it.

So the figure to work from is a developed width, and it has three parts. The first is the arc of the roof, measured over the crown of the arch rather than across it. The second is the drop on each side, from the eaves down to the fixing line at the bottom of the sidewall; a house standing 4.3 m tall carries a different drop from one standing 6.5 m, and the model lists both ends of that range, with the span choice of 6.4 m, 8 m, 9.6 m or 10 m setting the roof half of it. The third is a small turn-in at each edge, the piece that has to go past the fixing line to be held by the clip rather than pulled out of it.

None of those three numbers can be measured across the floor, and none of them can be guessed from the span. The practical answer is a tape laid along the profile: start at the fixing line on one side, run the tape over the arch at the crown line, finish at the fixing line on the other side, and add the turn-in at both ends. That measurement, multiplied by the number of runs and by the length of each run, is the film area the house needs. It is worth doing twice on different arches of the same house, because it is the figure everything else is built on and it is the figure a supplier will ask for first.

Film is supplied on rolls, and no roll that can be handled on a farm is 64 m wide. That fact, not a preference, is why a house of this size is covered in a series of runs rather than in one piece, and it is also why the span figure on the model matters to the film order as much as it matters to the frame. Arithmetic on the two dimensions the model gives makes the pattern clear: across 64 m, a 6.4 m span gives ten bays, and an 8 m span gives eight. A 9.6 m span and a 10 m span do not divide 64 m exactly, so on those two the last bay is whatever the setting-out drawing makes it, and the panel widths follow that drawing rather than a round number.

Each run goes on over the length of the house, so its length is the 40 m dimension plus whatever the ends need, and its width is the developed arch width of one bay. The joins between neighbouring runs are not floating somewhere in the middle of a bay: they land on an arch line, where there is a frame member under the film and a fixing line along it. That is what sets the target for the roll: the nominal width quoted is chosen so the run reaches a fixing line on each of its edges rather than so the total area comes out neatly.

A second layer is the simplest change to the order and the largest. Double layer means two sheets of the same area, one above the other, and on a film house they are normally run as a pair and held apart by air rather than laid flat on each other; the blower units that do that are part of the systems that can be added to this model, and the panel in the artwork below shows them. The area arithmetic does not change at all when the second layer is ordered; it is simply run twice, so the film quantity for a double layer is the quantity for a single layer multiplied by two, and the fixing, the laps and the end allowances are then counted twice as well.

Put the parts together and the order falls out of four numbers, only one of which is a measurement on site. The first is the developed width of one arch, taken along the profile with a tape. The second is the run length, which on this house is the 40 m dimension plus the ends. The third is how many runs there are, which comes from the 64 m dimension and the span. The fourth is the number of layers, one or two. Multiply the first two together for the area of one run, multiply by the number of runs for the whole roof and sides, and multiply by the layers. Everything after that is allowance, which is the next section.

Worked on the numbers this model gives, the shape of the answer is easy to see. Ten runs of 40 m with a developed arch width of a few metres each is a roof and side area in the low thousands of square metres; the two ends add a few hundred more; and a second layer doubles the whole of it. The floor is 2,560 square metres and the film order is not, and on the double layer version it is not by a wide margin. That is the whole reason a quotation is asked for as a drawing and not as a floor area.

Area tells you how many square metres of covering a house needs. Mass tells you what that area weighs, and on film it is a calculation rather than a catalogue figure, because the material is thin and its density is a property of polyethylene rather than of a particular roll. The thickness this model lists is 0.15 mm or 0.12 mm, and 6 mil, the cover called out on the model, is about 0.15 mm. Polyethylene used for agricultural film sits in the region of 0.92 to 0.95 grams per cubic centimetre, and multiplying thickness by that density gives the mass of a square metre.

The table is a conversion, not a measurement, and it is worth saying plainly what it is for. It lets a buyer turn an area into a weight, and a weight into a number of rolls, before a quotation is asked for; it lets a delivered roll be checked in the yard, because 100 kg of film that measures far less than 700 square metres is not the thickness that was ordered; and it matters for the transport side of a global order, where the film is one line of a shipment and its mass sits alongside the mass of the steel. The range in each row comes from the density range of the material and not from a tolerance on the film, and the declared figures on the supplier's own roll are what govern the order in the end.

Thickness also changes the order in a way that has nothing to do with weight, and it is the reason the two figures on this model are a quarter apart rather than a rounding apart. Moving from 0.12 mm to 0.15 mm raises the mass per square metre by about a quarter, so the same house takes about a quarter more material by weight at the same area. If the choice between the two is being made on the thickness itself rather than on the quantity, the covering for the 8 mil heavy duty version of this house carries that comparison on our heavy duty cover page, and the reinforced construction that changes how a sheet behaves once it is up is set out on our reinforced cover page.

The area from the calculation is the covering the house needs. It is not the film that gets ordered, and the difference is the part of a film order that is most often under-estimated. Three allowances sit between the two numbers, and each of them has a home on the drawing rather than a percentage attached to it.

The first is the lap at a join. Where two runs meet, they overlap rather than being butt-jointed, because a butt joint in a sheet held by clips has nothing to grip and nothing to seal. The width of that overlap is set by the fixing detail: the join is taken over an arch line and the overlapping edges are carried down into the fixing with the rest of the sheet. On this model the covering is held with aluminum clips, so the join and the fixing are the same operation, and the lap is as wide as the fixing line needs it to be rather than as wide as someone in the yard decides.

The second is the turn-in at each fixing line, the short distance the film travels past the point where it is gripped. A sheet cut flush to the fixing line has nothing to hold; every edge has to pass the line, and the same is true at the ground and at every intermediate fixing the film meets on its way over the arch. It looks like a small allowance on one panel and it is not small when it is counted on every edge of every run across a 64 m house.

The third is the end allowance at each end of a run, and it is where the arithmetic usually goes wrong in the other direction, because rolls do not come in the exact length of a house. A roll is cut to a nominal length, and the standard for agricultural covering films puts a tolerance on that declared length of 0 to plus 4 percent; in plain terms a roll is at least as long as it is declared and may be up to a few percent longer, and it is never cut short to suit one customer's 40 m run. So a run that needs slightly more than a whole number of rolls is a run that needs one more roll, and the surplus is offcut rather than waste.

Offcut is worth planning for rather than tolerating. On a house covered in runs, the longest panel is cut first because it is the one that cannot be pieced; the ends, the door surrounds and the smaller pieces are then taken from what is left. On this model that order matters more than usual, because the two end walls are large areas of film in their own right and can be closed from offcut if the runs have been planned around them. And a house this size is normally left with one bay of film kept back, because the day a panel is damaged over a standing crop is not the day to be ordering a roll.

A reasonable question at this point is what widths the film is available in, and the honest answer is that there is no single number. The standard that covers thermoplastic covering films for agriculture and horticulture applies to polyethylene and ethylene copolymer films from 20 micrometres to more than 250 micrometres, and it handles dimensions by defining terms rather than by fixing values: nominal width is the width declared by the manufacturer or supplier, and nominal length is the length declared by the manufacturer or supplier for a roll or a sheet. The declared figure is the one that binds an order, and it is the one that should be written into a quotation.

What the standard does fix is how far a delivered roll may sit from that declared figure, and those tolerances are worth knowing before a roll is unloaded. On width, flat film is allowed a tolerance of 0 to plus 4 percent of nominal width, and tubular film, which is supplied as a sleeve, is held to a tighter 0 to plus 2.4 percent. On length, a roll is allowed 0 to plus 4 percent of nominal length. On thickness, the average across the roll may sit within 5 percent either side of nominal, while a single point measurement may run 15 percent under or 25 percent over. Nothing in that list runs in the direction of less material, which is the practical point for a buyer: a roll is a whole roll, and it is a whole roll that is at or above its declared size.

Two consequences follow for a film order on a house this size. The first is that the thickness figure on the model is a nominal one, so 0.15 mm describes the sheet that is ordered and a delivered covering may weigh a few percent more or less per square metre than the conversion table above suggests; the band is small enough not to change the number of rolls, and it is large enough to explain a weighbridge reading that does not match a calculation exactly. The second is that where a house needs a run of a particular width, that width belongs in the enquiry as a declared nominal width, together with a note saying whether the film quoted is flat or tubular, because the two have different tolerances and are not interchangeable in the way they are handled on site.

Film ordered on assumptions is film that arrives in the wrong shape, and the list of things worth settling is short enough to work through in one sitting. Everything on it is a figure that exists before the order and cannot be changed afterwards without a cutting table and a wasted roll.

One item on that list is a specification choice rather than a measurement, and it is the only one where a longer look at the covering as a whole is worth taking first. How the film is selected and changed over the life of a house, and what that means for what is ordered at the outset, is the subject of our tunnel cover selection page.

The artwork supplied with this model carries a statement that matters more to a quantity question than to any other: it offers OEM and ODM customisation, and it states that custom sizes can be made upon request. On a house covered in runs rather than in one sheet, that is where manufacture and measurement meet. A standard nominal width is a starting point, not a requirement, and film can be produced to the width and the run length the setting-out drawing gives, so that each run arrives at the bay it was measured for.

The gain is arithmetic. Every panel that has to be trimmed on site produces an offcut, and every trimming line is a cut edge in a material that will be under load for years. Ordering to the drawn width removes the trim, which reduces the offcut and reduces the number of fresh cut edges that have to be handled carefully when the film goes on. It does not remove the lap or the turn-in, which are part of the fixing rather than part of the film, and it does not change the area the house needs; it changes how much of that area arrives already the right shape.

A custom size is a specification, and a specification is only useful if it is complete. Width, length, thickness and the number of layers are the four figures that have to be given, and the layer count matters here for a reason that is easy to miss: a double layer house takes two sheets that may be ordered in different thicknesses or in the same one, and only the buyer knows which. Film produced to size still ships in the same three packing groups as the rest of the house, and it is worth treating the film line of the order with the same care as the frame line.

The bay width that decides the panel width is a frame dimension, so the frame belongs in a film order rather than beside it. This model is built on hot dip galvanized steel pipe, used, as the sheet puts it, to avoid rust, with an arch roof and a covering material thickness of 0.15 mm or 0.12 mm. The listed advantages are a structure suitable for all climate conditions and adaptable to any type of landscape, effective prevention of heat loss and cold air intrusion, good insulation properties and low energy consumption, and the frame runs to a span of 6.4 m, 8 m, 9.6 m or 10 m at a total structure height of 4.3 m to 6.5 m.

The coating on that pipe is judged against the air where the house is built rather than against anything the film does, and the classes are the ones set by ISO 9223. In heavy industrial air, class C4, a standard galvanized coating reaches first maintenance at about 13 years. In urban conditions, class C3, the figure is about 30 years, and in coastal conditions, class C5, about 50 years. In clean suburban air, class C2, 50 years or more is reached without repair. The frame specified for this model carries 70 to 85 micrometres of zinc under BS EN ISO 1461, assessed against BS EN ISO 14713-1, and cut and drilled points made during the build are treated afterwards so the coating is not left broken. Naming the class for the site is what turns those four figures into one number, and it is the number that says how many times the film will be renewed over the frame beneath it.

This model can be supplied with systems added at the customer request, and a few of them add covering, which means they add to the film and fabric side of the order rather than only to the equipment side. A shading system puts a second membrane above the roof, on its own fixings and its own area. A ventilation system brings openings with roll up curtains and mesh panels, and every one of those is an area of material measured the same way as the roof. Cooling and heating systems bring penetrations and, in the case of a wet pad, a surface that has to be fed and drained. Irrigation and fertilization systems, and benches, are inside the house and do not add covering, but they do decide how much room there is to move when the film goes on.

The crop does not change how much film a house takes, and it is worth being clear about that, because the crops on this model are listed as vegetables, fruit, flowers and a seed nursery and they are the reason a house is built rather than a reason the order changes shape. What the crop changes is which systems are added and how the house is arranged inside, and only those systems that bring their own membrane or their own openings change the covering line of the order. A nursery house and a fruiting vegetable house of the same size with the same systems take the same film, in the same widths, with the same laps.

Trade reporting on how commercial protected cropping businesses buy, store and work with covering film is carried by Hortidaily, and technical material on greenhouse structures, covering materials and protected cultivation is published by the USDA. Both are worth reading before a large film order is placed, because the figures a supplier quotes have a physical basis and it helps to know which of them are specifications and which are conventions.

The house ships in three groups: metal structures, covering system and install accessories. For a film order that grouping is useful, because the film travels with the clips and the channel that hold it rather than arriving separately, and the roll mass worked out earlier is what decides how the covering group is packed and how it is lifted at the other end. The company behind the house has been manufacturing greenhouses for above 35 years and ships to customers all over the world every year. There is a professional engineer department, and where a customer requests it engineers can be sent to foreign countries to guide the installation, which on a run covered film house is where a measured order and an unmeasured one stop looking the same. Many customers visit the factory each year for business cooperation, and the question most of them bring about film is the one this page is written around: how much of it the house takes, and in what widths it should come.

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. Send the developed width of one arch, the run length and the number of runs, and the answer comes back as a film area, a mass, a nominal width and a roll count rather than as a guess. The rest of the range this house belongs to is on our film greenhouse range page.

Q: How much film does a 64 m by 40 m house actually take?
More than the floor area, and the floor area is not the number to start from. The house is 2,560 square metres on the plan, and the film has to cover an arch roof, both sidewalls and both ends, so the quantity is a developed surface. Work it out as the developed width of one arch measured along the profile, multiplied by the run length, multiplied by the number of runs, multiplied by one or two layers, and then add the laps, the turn-ins and the end allowances. Ten runs of 40 m with a few metres of developed arch width each is a film area in the low thousands of square metres before the ends and before a second layer.

Q: Does a second layer simply double the film order?
Yes, and it doubles the allowances with it. Single or double layer is a specification choice on this model, and the film area is run twice: the same number of runs, the same lengths, the same joins, the same laps. On a film house the two layers are normally held apart by air rather than laid against each other, so the pair is run as one assembly with its edges joined, and the blower units that keep it apart are one of the systems that can be added to the order. What does not double is the frame.

Q: What width does the film come in?
There is no single industry width, and any supplier who quotes one without asking about the house is quoting a stock size rather than a fit. The standard for agricultural covering films defines nominal width as the width declared by the manufacturer or supplier, and it is that declared figure, not a fixed value in the standard, that an order is placed against. The tolerance applied to it is 0 to plus 4 percent for flat film and 0 to plus 2.4 percent for tubular film. The artwork with this model states that custom sizes can be made upon request, so the width can be set to the bay rather than the bay being made to fit a stock roll.

Q: How much should be added for laps and ends?
The lap comes from the fixing detail, not from a rule of thumb, because on this model the covering is held with aluminum clips and the join between two runs is taken over an arch line and carried into the same fixing. The end allowance is a different matter, and it is where whole rolls come in: a roll length has a declared value with a tolerance of 0 to plus 4 percent, so a roll is never cut short to suit one run, and a run that needs slightly more than a whole roll needs one more roll. The surplus is offcut, and offcut closes ends and cutouts if the longest panel is cut first.

Q: How can a delivered roll be checked against the thickness that was ordered?
By mass, before it is unrolled. A square metre of polyethylene film weighs its thickness multiplied by the density of the material, and at 0.92 to 0.95 grams per cubic centimetre that is about 110 to 114 g/m2 at 0.12 mm and about 138 to 143 g/m2 at 0.15 mm, which is roughly 877 to 906 square metres per 100 kg and 702 to 725 square metres per 100 kg respectively. Weigh a roll, divide by the mass per square metre, and compare the answer with the declared nominal dimensions. Allow for the standard tolerance of 5 percent either side on average thickness before calling a roll wrong.

Q: Why is the house not covered in one piece?
Because the house is 64 m across and a roll that wide cannot be made, handled or lifted. The covering goes on as a series of runs, each one measured over the arch and run along the length of the house, and the joins between them land on an arch line where there is a frame member and a fixing line underneath. That is what decides the panel width and therefore the nominal width to ask for: the target is a run that reaches a fixing line on both of its edges, with the number of runs coming from the 64 m dimension divided by the span listed on the model.