ASCE Mean Roof Height - An Introduction
A building’s height plays a large role in calculating the magnitude of wind pressures on a structure.
Wind will affect a building and its components differently if it can travel overtop of the structure, or if wind travels around the building sides.
The ASCE 7 standard uses a structure’s ‘Mean Roof Height’ as a factor in calculating the positive and negative wind pressures acting against that structure.
Per ASCE 7, the Mean Roof Height (MRH) is defined differently for “flat roof” structures (Roof angles less than or equal to 10 degrees, or 2:12 slope) and sloped roof structures (greater than 10 degree slope).
The mean roof height only affects the negative (suction) components & cladding wall pressures of ASCE 7. Positive wall pressures may use the elevation of the opening in question.
Flat Roofs
For flat roof structures, the MRH is permitted to be taken at the highest roof eave height. This considers both the highest roof eave, as well as the lowest elevation at the structure’s base. This measurement is applicable for structures with multiple eaves, gables or sloped roofs, or with structural parapet walls – if the rood angle is not steep, the MRH will be calculated to the highest point.
(source: AAMA TIR-A15-14)
Sloped Roofs
For roofs with slopes or gables greater than 10 degrees, the standard allows a reduction in the MRH to account for the wind being able to pass more smoothly overtop of the structure.
Instead of using the highest point on the roof, for sloped roofs, the MRH may be calculated as the average between the maximum roof height and the eave height.
Why don’t they just call it the average roof height?
The term ‘mean’ indicates the center of 2 data points (highest and lowest) whereas average could span over many data points.
Calculating Wind Pressures
If you need to calculate wind pressures acting on windows, doors, or other building components, you may choose to use our pre-calculated wind charts or our online calculator.
Both resources require information about your project, including the Mean Roof Height – MRH, which is a significant factor in the wind pressure calculation.
Generally, the larger the Mean Roof Height – MRH, the higher the wind pressures on the structure will be.
Why Purchase Engineer Certified Calculators & Charts?
See this Guide To Permitting Windows & Doors which explains the window & door selection process for houses in Florida & across the US.
Watch how winds create vortexes around building corners & over roofs, multiplying the wind-velocity to wind-pressure formula.
Wind pressures are higher in building corners than the rest of the building for this reason.
Last Update: March 17, 2025
