Greenhouse site assessment helps avoid future problems

Determining a location’s Exposure Category can make obtaining a building permit a lot simpler.

By Amy Rigazio

A greenhouse, like any building, is affected by the local environment. Rain is collected in its gutters and then runs to an outlet. Snow accumulates on the roof, slides down and settles into the gutters, and drifts up against the side walls. Wind blows against the greenhouse pushing down on one part of the roof while pulling up on another. In some parts of the country, floods and earthquakes can be real concerns to greenhouse operators.

A greenhouse needs to be designed to withstand all of these environmental factors. For any given area, the amount of rainfall, snowfall and maximum wind speed can be found on charts in almost any building code book. Different parts of the country use different codes, but the corresponding charts will be in the book.
One thing that cannot be found on any chart, but can drastically affect a building’s design, is Exposure Category.

Exposure Category refers to the characteristics of the ground surface specific to a building’s physical location. These characteristics include natural topography, vegetation, adjacent structures and proximity to large bodies of water. A wind gust of 110 mph affects a greenhouse 500 feet from the Atlantic Ocean much more severely than a 110 mph gust blowing in an urban area that is broken up by three-story buildings.
Before a greenhouse is purchased, a site assessment must be conducted. This is usually done by a manufacturer’s representative, but should also be reviewed by the landowner. This evaluation needs to have these observations: urban vs. suburban vs. open setting, size and location of adjacent buildings, size and location of wooded areas and proximity to large bodies of water.

Location, location, location
American Society of Civil Engineers identifies an urban area as a large city center with at least 50 percent of the buildings having a height in excess of 70 feet (21.3 meters). Although the majority of greenhouses will not fall within this category, some urban jurisdictions may require this type of evaluation to be done to safeguard against possible wind channeling between buildings. Construction sites meeting these criteria are considered to be located in Exposure Category A.

Smaller urban and suburban areas (Exposure Category B), according to ASCE, have “numerous closely spaced obstructions having the size of single-family dwellings or larger.” These obstructions, which can include a densely wooded area as opposed to a manufactured structure, must be within 1,500 feet of the proposed greenhouse site in the upwind direction. If obstructions are present on the predominately leeward side of a building, they cannot be taken into account for wind load calculations.
Exposure Category C is described by ASCE as “Open terrain with scattered obstructions having heights generally less than 30 feet (9.1 meters).” This category often becomes the default description of site, as it presumes no significant obstructions in the windward direction. If a site visit cannot be done, or if the information gathered is questionable, an Exposure Category C classification is a safe assumption.

Open water sites
Most greenhouse facilities will fall under one of the previous three Exposure Categories. Some sites though, may be located within 15,000 feet of an open body of water and be directly exposed to winds flowing over that water. Areas in this category (Exposure Category D) are located near the Great Lakes, an inland waterway or on the coast of California, Washington, Oregon and Alaska. The Atlantic coastline, because of to its susceptibility to hurricanes, is included in Exposure Category C.

Local building officials
A local building official can override any site evaluation done by a greenhouse manufacturer representative. Very often, towns have a set of guidelines that a building must meet, regularly including a recommended Exposure Category along with the required applied and environmental loads.
It is always advisable to contact the local building department before beginning any greenhouse project. If the required design parameters are known up front and provided to the manufacturer, a suitable structure can be designed and possible permitting problems can be avoided.

Design differences
All greenhouse manufacturers design their “standard” buildings to an individual set of criteria. When quotations are received from several manufacturers for a single project, the designs can have very significant differences depending on the information provided to the manufacturers by the prospective client.
If a site visit was not done by each manufacturer’s representative, they may not all be designing the greenhouse for the same Exposure Category. Some companies use Exposure Category B as a default, while others use Exposure Category C. This one discrepancy can lead to a significant cost variation between the structures as well as whether or not a construction permit is granted or denied.

When requesting building quotations from manufacturers, try to provide all of the pertinent information up front. This information will not only include the type and size of the building, but zoning and building information received from the town as well. When these specifications are given to the manufacturers, it will result in more comparable quotes, so that the best choice can be made between equivalent structures.
If there is a question as to the design criteria used for a given quotation, request a hard copy of all design parameters and assumptions made. This request can prevent future problems when a town asks for documentation.