Why is this retained water critical?
The most immediate problem occurs when the weather is warm and your family is spending the most time out of doors. The retained water is stagnant. It’s a warm and cozy breeding ground. When you’re on your upper deck, there’s a mosquito farm just under your feet.
Later on, leaf litter and other debris will find its way to the draining surface of the waterproof ceiling and sit in the pooled water.
The litter sits, becomes waterlogged and sinks. It’s now harder for the next rain shower to rinse away. As this repeats the litter will accumulate, creating a dam. When the water level rises enough, the ceiling will begin to leak between the panels .
Leaks are not the worst problem caused by retained water.
The litter accumulation will get heavier and heavier. Eventually, after some years perhaps, one or more panels, or a gutter will collapse. The repair in some cases can be as labor intensive as the original installation. A system that does not retain water allows litter to dry out between showers so it can float and be washed away in the next rain.