The detriments of standing water on the roof surface are well documented and quite extensive. The severity of the defects ranges from structural damage and membrane degradation to temperature changes and the growth of vegetation and fungi. Standing water is such a deterrent that most low-slope roof system manufacturers require that the roof surface is free of all rain water within a specified period of time for their warranties to be valid.
The most serious defect of standing water on the roof surface is structural damage. Structural damage can occur if water is allowed to pond in large areas for any length of time. Standing water can be deceptively heavy, and if the added weight of the water causes the roof to exceed the design load, the roof could collapse.