Last reviewed 1 Oct 2026 · 7 min read
Site selection
| Factor | Requirement |
|---|---|
| Land and topography | Level or gently sloping (< 1–2 %), well-drained, not in a hollow (cold-air pockets and water-logging); soil with good bearing capacity |
| Drainage | Good natural drainage; no flooding; high water table avoided |
| Water | Assured supply of good-quality water (low EC, free of excessive iron, bicarbonate and pathogens); tank and filtration space |
| Power | Reliable three-phase supply (for fans, pumps and controllers) with a generator back-up for high-tech units |
| Access | All-weather road, close to the market, airport/transport (for flowers), labour and inputs |
| Sunlight | Free of shade from trees and buildings (keep clear by a distance of at least about 2–3 times the obstacle height) |
| Wind | A windbreak on the windward side, but not so close as to restrict ventilation |
| Air quality | Away from dust, smoke, industrial pollution and brick-kilns |
| Climate | Check temperature, humidity, rainfall, wind speed and solar radiation records; choose the structure type accordingly |
| Land tenure, cost, expansion room, and pest/disease history of the soil | — |
Orientation and layout
- Gutter-connected multi-span houses are generally laid with the gutters running North–South, so shading by the structure moves evenly across the crop; single-span houses in latitudes where winter light is vital are often oriented East–West (the long side facing south) to maximise winter light. The choice depends on latitude and the prevailing wind direction.
- Ventilators should be placed to take the prevailing wind (windward side vents + leeward ridge vents create cross-ventilation).
- Spacing between houses ≥ 1.5–2 times the height to avoid mutual shading and permit ventilation; leave service roads (3–4 m).
Geometry (typical for Indian naturally ventilated polyhouses)
| Parameter | Typical value |
|---|---|
| Span (width of one bay) | 8 m (6–9.6 m) |
| Bay length (column spacing) | 4 m |
| Gutter height (side height) | 4–5 m for NVPH (2.5–3 m for small low-cost houses) |
| Ridge height | 5.5–7 m (NVPH); about 4.5 m low-cost |
| Roof slope | About 22–30° (film roofs: 25° commonly; glass 30° minimum) |
| Number of spans | Multi-span for larger plots; length of one bay row ≤ 60–100 m for natural ventilation |
| Vent area | Roof + side vents ≥ 15–25 % of floor area |
(Standards and subsidy norms prescribe exact limits; use the version applicable to the scheme.)
Structural design
Loads
| Load | Notes |
|---|---|
| Dead load (DL) | Self-weight of the frame, cover, gutters, fittings — small for film structures (about 0.1–0.3 kN/m²; heavier for glass) |
| Live (crop) load | Hanging crops (tomato, cucumber, roses) and trellis wires: about 0.15–0.3 kN/m² (hedged — use the code value); maintenance live load on the roof |
| Wind load | The governing load for lightweight houses; includes suction/uplift on the roof and sides |
| Snow load | For hill sites |
| Earthquake | Minor, but included for large structures |
| Thermal and installation loads | Specialised |
FormulaDesign wind pressure (IS 875 Part 3 form)
with the design wind speed (basic wind speed × risk coefficient × terrain, height and size factor × topography factor); the pressure on a surface is using external and internal pressure coefficients (for a house with large openings, internal pressure coefficients are significant).
Worked ExampleExample — design wind pressure
If the design wind speed at the gutter level of a greenhouse is m/s: (0.96 kPa). With net pressure coefficients of about 0.8 (suction on the roof), the design uplift on the roof is N/m², which for a 3 m × 8 m roof panel is kN, which the columns and footings must resist in tension.
Design checks
- Members (columns, rafters/arches, purlins, bracing, gutters) checked for combined bending and axial load under the governing combination (DL + LL, DL + wind) as per IS 800 (steel) and IS 875 (loads) — use the version prescribed in your syllabus.
- Foundations designed for uplift (pull-out), bearing and overturning; concrete pedestals about 0.3–0.45 m square, 0.5–0.9 m deep, with anchor bolts or embedded column (typical, depending on soil and wind).
- Deflection limits; bracing on end walls and cross-bracing for stability.
- Pipe sizes for columns and trusses follow the design (typically GI pipes of 40–100 mm diameter in medium-sized NVPH).
Ventilation design
Natural ventilation
- Driven by wind pressure and thermal buoyancy (stack effect); air-change rate (changes per hour).
- Rule of thumb: vent area (roof + sides) of 15–25 % of the floor area; roof vents should be bigger on the leeward side and the vertical gap between the lowest inlet and the highest outlet raises the stack effect.
- Insect nets (mesh) cut the air flow by 20–50 %, so vent area must increase accordingly.
Heat load and fan ventilation
Sensible heat to be removed (kW) , i.e., the air volumetric flow needed for an allowed air temperature rise :