ROOF VENTS - NOT AN OPEN AND SHUT CASE...

Roof vents typically are the first form of natural ventilation chosen, followed by sidewall and endwall vents. Generally, the hardier the crop, the more likely that natural ventilation will be used.

Vent openings are created by suspending a shaft or drive line from the greenhouse frame, along the length of the vent run. Then, a pinion on the shaft is attached to a rack secured to the vent. A drive unit (either manual or electric) operates the gear box. This spins the shaft that rotates the pinion that pushes the rack, opening the vent the desired distance. Roof vent location varies from either side of the peak or ridge to just above the gutter line if the building is gutter-connected.

The amount and rate of air exchange at the vent varies according to the temperature differential between the inside and outside of the structure; wind level; size of the opening; structure profile and glazing; altitude; light intensity; and whether it is used alone or in conjunction with sidewall vents or shade screening. Vent openings can vary in size from fractions of an inch to as much as 12 feet. Glazing ranges from high-end double insulated glass to low-end single polyethylene film or shade screen.

When incorporating roof vents into an existing greenhouse, structural integrity must be maintained at all costs. As roof vents create additional lift off the ridge, any frame onto which they are installed must be able to handle increased wind pressure, as well as shed rain, snow or ice as climatic conditions demand. In most cases, the wider the structure, the greater the increase in wind pressure and lift at the peak. As with structural design, the specific needs of the grower and crop dictate how much detail and strength must be incorporated into the roof vent. Technological advances in covering materials and the integration of sophisticated computer controls, using the delicate sensoring and weather station data, are enabling dramatically different types of roof vents to evolve. Some new structures do not have peaks, others lack rafters or a roof line made of rigid material. In some cases, the roof line is the roof vent. Environmental screening is used as the roof line, and opens and closes automatically, according to parameters established by the owner / operator.

Roof vents are not for everyone. They do have limitations and are not cost-effective for all operations. Materials and installation costs need to be considered up front. But if used with environmental screening under an existing covering, or as environmental screening itself, a roof vent is an invaluable tool for most growers.