Secondary Water Resistance is a technique used to protect the interior of the building when the roof cover and underlayment blow off during a storm. It can be applied to plywood roof decks and is a self-adhering polymer-modified bitumen roofing underlayment (thin rubber sheets with peel and stick underside located beneath the roof covering and normal felt underlayment) with a minimum width of 6" meeting the requirements of ASTM D 1970 installed over all plywood/OSB joints to protect from water intrusion. These photos show this "rubber sheet" being applied to a plywood deck.

Very few homes in Florida have Secondary Water Resistance (SWR). A qualified construction professional is needed to verify the installation of SWR. It is an economical and highly effective means of reducing losses in windstorms, since a majority of losses are caused by water damage. All secondary water resistance products must be installed per the manufacturer's recommendations.

Note that roofing felt or similar paper based products (30lb/90lb hot mop systems) are NOT acceptable for secondary water resistance. Hotmop felt is often used under tile roofs as the primary water resistance layer, and are often confused with secondary water resistance products.