- info@infunplastic.com
- +86-18530860523
- Zhoukou city, Henan province, China
A film ordered half a meter too short cannot be stretched to fit — and a whole roll can be written off in one morning of installation. This guide walks through how to measure width and length, what allowance to add for the channel and tensioning, when seaming is the right call, and the six things to send a factory before ordering a custom size.
Film is forgiving in thickness and forgiving in price — but not in length. A cover cut 0.5 m short of the anchor cannot be rejoined convincingly; the roll becomes salvage. It is the most common one-time loss in greenhouse covering, and it almost always traces back to one of three measurement mistakes: measuring the floor span instead of the arch, forgetting the channel depth, or ignoring how much the film must wrap to anchor. The good news: all three are prevented by a tape measure and ten minutes of arithmetic.
Start from the foundation on one side, follow the arch over the ridge, and end at the foundation on the other side — that is the arc the film must cover. To that arc length add:
| Addition | Typical allowance | Why |
|---|---|---|
| Channel locking, both sides | +20–30 cm per side | Film must insert into the lock channel with enough tail to grip under tension |
| Buried anchoring (trench method) | +30–50 cm per side | Film wraps into the trench and is held by soil instead of a channel |
| Tension stretch reserve | Included above — never subtract | A taut cover is a long-lived cover; slack film flaps and fails early |
For a multi-span gutter-connected house, each bay is covered separately or with a factory-joined sheet — the same arc principle applies per bay, plus the gutter connection detail. Note that a bowed arch has a longer surface than a Gothic or sawtooth profile of the same floor width; when in doubt, run a rope over the frame and measure the rope.
Length runs along the ridge, from anchor to anchor. Add the two end anchoring lengths — buried ends or end-wall channel — using the same allowances as the sides. On sloped ground, measure along the actual frame line, not the horizontal projection: a 3° slope adds real length over a 60 m house. If the film will also cover the gable ends, order separate end panels sized to the gable arc; do not try to fold the main cover around the corners.
Film can be trimmed; it cannot be grown. The rule our installation team repeats: add generously at the anchor, then verify the total against the order sheet before the factory slits the roll. Excess tail inside the channel costs nothing — it improves grip. A missing 20 cm at the windward corner costs the roll. When we quote a custom size, we return a size check drawing with the arc calculation so the two sides can be compared before production.
Ordering tip: write the allowances into your purchase note (e.g. “arc + 25 cm per side channel”). Factories size film every day; a stated allowance removes the guesswork on both ends.
Single-piece extrusion tops out around 16 m of developed width. Wider single-span arches, and multi-bay covers ordered as one sheet, need seams. Seaming is routine when it is planned: the decisions that matter are where the seam sits and how it is made.
A proper seam is a specified seam. Three numbers define it:
If seaming must happen in the field — repairs, retrofits — use the manufacturer’s tape or a handheld sealer on a clean, dry surface, and never overlap anti-drip layers face-to-face: the anti-drip side must stay inward.
Wind load concentrates at the windward face and the four corners, and that is precisely where most covers tear first. The standard countermeasures are simple: order extra width at the windward anchor, run an extra locking profile or strap line lower on the windward face, and pad every channel edge that film crosses at an angle. In exposed sites — coastal plains, open plateau — we recommend the higher end of the thickness range (100–200 mic class) together with the windward reinforcement, because thickness buys tear resistance exactly where the load lands.
With those six details we return a size check drawing, a thickness recommendation within the 100–200 mic range, and a quotation — usually within one working day. Custom widths are our normal business, not an exception: MOQ is 1,000 kg per specification, free samples are available for fit trials, and lead time is typically 30+ days.
How much wider than my greenhouse span should the film be?
Measure the arc over the frame, then add roughly 20–30 cm per side for channel locking (or 30–50 cm per side if burying the edges in trenches). The arc — not the floor width — is the base number.
Can greenhouse film wider than 16 m be made in one piece?
No — single-piece extrusion tops out around 16 m. Wider covers are produced as factory-seamed sheets, with the seam positioned on the leeward side or ridge and reinforced for wind load.
Where should a seam be positioned?
Away from the windward face and away from condensation drainage lines. A leeward ridge line is the standard position; the seam should also be backed by a locking profile or strap on exposed sites.
Does film stretch when installed?
Yes — film is tensioned to a drum-tight finish and elongates slightly. That is why the allowance is added at the anchors before tensioning, and why a slightly generous anchor tail is never a defect.
What happens if the film arrives too short?
Short film cannot be reliably joined back to spec — the roll typically becomes salvage. This is the most common one-time loss in covering, and it is why we return a size check drawing for confirmation before production.
Do you offer custom widths and sizes?
Yes — custom width up to 12 m single-piece, custom roll lengths, and factory seaming beyond that. MOQ is 1,000 kg per specification with free samples for fit trials; lead time is typically 30+ days.
Related reading: Greenhouse Film Installation & Maintenance Guide · PO Film vs PE Film · Choosing Film by Crop & Climate · Request a Quote
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