D10 Covers: Rain, Snow, Wind and Stability

Topic

A roof must simultaneously withstand three climatic forces that rarely act in isolation—snow, wind, and rain—with each testing a different aspect of the roof's performance. The snow load a roof must bear depends on both the characteristic snowfall for the climate zone and the roof's shape: a flat roof retains more accumulated snow than a steeply pitched one, from which much of the snow slides off rather than piling up.

Wind, for its part, exerts both pushing and pulling forces: across much of a flat roof or the leeward sides of a pitched roof, wind generates outward suction rather than inward pressure. This suction explains why lightweight roofing materials—such as sheets or panels—require particularly secure anchoring at their edges and corners, the very areas where suction tends to be strongest. The intensity of this suction depends on the area's characteristic wind speed and the extent to which the height and roughness of the surrounding environment amplify that speed.

Finally, the drainage system for a flat roof must be sized to handle the peak flow expected during a design-basis heavy rainfall, requiring a sufficient number of downpipes and drains. Furthermore, a flat roof lacking a minimum slope will retain water for extended periods; this accelerates the deterioration of the waterproofing membrane, causing damage far more serious than mere visual unpleasantness.

Thus, this module—which begins by addressing three climatic forces that seldom occur in isolation—concludes by demonstrating that none of the three can be managed independently: a roof designed solely to withstand snow may fail due to wind suction, while one designed solely for wind resistance may accumulate water that ruins its waterproofing.

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