Your house or commercial building is not a boat or a submarine and even they take on water.
When Hurricane Sandy like rains come, many roofs may leak and never leak again. This type of abuse only happens every few years or so. Ernesto was a named storm in late summer 2006 that dumped 10″ of rain in approx 3-4 hrs in Hampton Roads , VA. I was on a low slope TPO single-ply membrane roof in downtown Portsmouth, Virginia in that rain and there was an 1-2 inches of running water from up slope to the interior roof drains. This water covered the entire roof surface. If those drains wouldn’t have performed properly, that roof would have folded under the water weight and leaked badly or even collapsed. Water weight temporarily sags the roof structure and can cause seams and flashings to fail by pulling them loose. Keep your interior roof drains, suppers, conductor heads, gutters and downspouts clear of debris so water runs off and does not back up causing leaks. Also remember that fiber glass asphalt shingles, shake, slate, tile, metal roofs are lapped for water shedding – not for water proofing. There is a big difference. Too much rain water just can’t shed fast enough and your roof may leak even though you don’t know it. Keep it maintained regularly and draining well and the first most important thing is to put the right product in the right place. For example, you would not install a shingle on a slope lesser than 2″ in 12″. It will leak if you do.
Do not let debris like pinestraw pile up in roof valleys, behind chimneys, etc. because heavy rain will make the roof leak and this lack of attention also shortens the life of the roof system.
With the appropriate product for the application and with excellent workmanship and maintenance your roof will handle most of the even “perfect storm” rain scenarios.
Robert Andrews ll , President Andrews Roofing email@example.com