Most disappointing greenhouse projects are not built badly - they are specified badly. The structure stands, the film is intact, and yet the house runs too hot in summer, too humid in winter, and too expensive to ventilate. Almost every one of those problems traces back to a decision made on a specification sheet before the first steel was cut.
This guide covers the six decisions that define a multi span greenhouse for commercial vegetable production, in the order they affect performance.
1. Start With the Crop, Not the Structure
Tomatoes, cucumbers, peppers, leafy greens and strawberries impose different demands on the same footprint:
- Tall fruiting crops need high gutters, crop wire systems and strong roof ventilation
- Leafy greens need airflow close to the canopy and reliable shading
- Seedlings and transplants need precise humidity and temperature control, which usually points to polycarbonate rather than film
Write down the crop, target season, expected yield per square meter and harvest window before you discuss span width. Suppliers who quote before asking these questions are quoting a building, not a growing system.
2. Span Width and Bay Length
Span width drives both cost and climate uniformity:
- 6 m spans - more columns and gutters per square meter, but simpler trusses and lower unit risk
- 8 m spans - the most common commercial choice for vegetables, good balance of cost and open working space
- 10-12 m spans - fewer internal columns, better for mechanization, requires stronger trusses and careful roof vent design
Bay length is usually set by standard 4 m to 8 m frames. Longer bays reduce column count but increase the load carried by each truss. For mechanized operations, plan bay length around your equipment turning radius, not just around steel economy.
3. Gutter Height Is Not a Detail
Gutter height determines air volume, crop headroom and summer temperature. For tall vegetable crops, a gutter height of 3.5 m to 4.5 m is common; low-cost houses at 2.5 m to 3 m look cheaper and then underperform:
- Less air volume means faster temperature spikes under midday sun
- Limited headroom caps crop height and constrains internal working conditions
- Shorter sidewalls reduce side ventilation effectiveness
The extra steel for 0.5 m of additional gutter height is small compared with the cost of cooling a low house in summer.
4. Film Specification Decides Longevity
Not all polyethylene film is equivalent. Ask for four numbers:
| Specification | What to check | Why it matters |
|---|
| Thickness | 150-200 micron typical for commercial houses | Thinner film fails faster in wind and hail |
| UV additive | Service life stated in years (3, 4 or 5 seasons) | Determines replacement frequency and cost |
| Light transmission | 90% or higher for clear film | Directly affects photosynthesis and yield |
| Thermal / anti-drip | IR additive and anti-condensation layer | Reduces heating loss and leaf disease from dripping |
A 4-season or 5-season film costs more per roll and less per year of service. Cheap film that is re-covered every two seasons is the more expensive option over a ten-year horizon.
5. Ventilation and Climate Control
Ventilation is the highest-leverage performance decision in a vegetable house:
- Roof vents release heat at the highest point and are the primary cooling mechanism in most climates
- Side vents provide horizontal airflow through the canopy and are essential in humid conditions
- Automation keeps temperature and humidity inside the crop's tolerance band without manual intervention - this is usually worth the cost on any house above 1,000 m2
- Shading protects summer crops in high-radiation regions; internal or external shading should be decided at design stage, not added later
- Cooling pads, fogging and heating are climate-specific and should be sized from local design conditions, not copied from another project
A useful rule: ventilation capacity should be sized for the hottest month, not the annual average. Under-ventilated houses cannot be fixed cheaply later, because retrofitting vents means cutting the structure.
6. Structural Loads and Local Conditions
Confirm the design snow load, wind load and any local building requirements in writing. Greenhouses are light structures and are particularly sensitive to:
- Wind exposure on open sites and coastal locations
- Snow accumulation patterns between spans
- Foundation design for local soil conditions
Ask for the design loads stated in the technical offer and confirm that they match your site. A quotation without stated loads is not a design.
Specification Checklist for Suppliers
- Crop, target season and expected yield per square meter
- Total area, span width, bay length and gutter height
- Steel section sizes and galvanizing standard
- Covering material, thickness, UV life and light transmission
- Roof vent and side vent area, manual or motorized
- Shading, cooling, heating and control system scope
- Irrigation type: drip, sprinkler or hydroponic gutters
- Design wind and snow loads
- Installation scope, supervision and spare parts list
Frequently Asked Questions
What span width is best for tomatoes?
8 m to 10 m spans with 4 m to 4.5 m gutter height are the usual commercial configuration for tall fruiting vegetables. They provide working space for crop work and enough air volume to moderate midday temperatures.
Can a multi span film greenhouse be used year-round?
In mild and warm climates, yes, with adequate ventilation and shading. In cold climates, year-round vegetable production usually requires a thermal screen or a polycarbonate house to control heat loss.
How much area do I need to be commercially viable?
Viability depends on market price and labor cost rather than area alone, but most commercial vegetable operations start at 1,000 m2 and scale in blocks of several thousand square meters.
Manual or automatic control?
Manual vents are acceptable for very small houses with constant attention. Above 1,000 m2 automatic control pays for itself quickly in labor savings and in avoided crop stress from late venting.
Talk to an Engineer, Not a Catalogue
Send us your crop plan, site location and target area, and we will return a specification with span layout, covering recommendation, ventilation sizing and a cost breakdown you can compare line by line. Browse our multi span greenhouse range, read more technical guides, or contact our team for a project-specific proposal.
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