How Are Wind Uplift Ratings Determined?
The cost of a roof system is affected by the wind design, but how are wind uplift ratings determined? A balance should be achieved so that the cost to prevent blow-off isn’t unnecessarily high while the wind-resistance capacity is sufficient for a roof to stay intact according to the appropriate wind-related classification. If actual wind load exceeds the wind-resistance capacity of a roof system, the roof may not be considered wind-resistant. The following provides insights for the layperson as to how wind uplift ratings are determined.
Wind Uplift Defined
The suction that wind create as air moves parallel to the surface of a roof is called wind uplift. A pressure differential is created when a gust of wind meets the side of a building and part of the air is directed upward and then across the roof. The air pressure on top of the roof is less than the air pressure below. As the differential tries to equalize, a suction is created that pulls at the panels of the roof. The uplift is more forceful in faster wind. Wind uplift can pull shingles and panels off of a structure in extreme wind events.
Before a wind uplift rating can be determined, certain building calculations must be gathered. All of the following criteria below is factored in.
- Terrain surrounding the building
Wind speeds up if there are abrupt topography changes nearby, such as ridges, hills, and escarpments. If a structure is near a hill, for example, it will be subjected to higher wind loads than a building surrounded by flat terrain.
- Height of the structure
Wind speed is higher with above-ground height. A taller building is subjected to higher wind loads.
- Positive or Negative Building Pressure
When wind strikes a building, it can either:
- Increase the pressure inside the building, which is positive pressure, or
- Decrease the pressure, causing negative pressure.
Porosity of the building envelope causes internal pressure changes. Factors that contribute to porosity include:
- A window or door left open,
- Openings around window frames and doors, and
- Infiltration of air through walls that are not completely airtight.
Internal pressure in a depressurized building pulls the roof down, reducing the amount of uplift exerted on the roof. The decrease in internal pressure pulls inward on the windward wall, which increases that wall’s wind load.
In a fully pressurized building–due to breakage of even a small window, for example–there is a significant increase in the load applied to both the roof and the exterior walls.
Other factors in building calculations follow:
- The type of roof deck the structure has.
- Whether minimum standards are met by the existing deck.
- The height of parapet walls, if there are any.
The above information is needed to calculate uplift resistance as well as wind uplift ratings. Such calculations are more complex matters well-understood by such experts as skilled roofing contractors. The evaluations are used in the risk reduction design process.