Five examples of the system for waterproofing an outside deck, thereby improving the usefulness of the space below the deck, are disclosed. Both inflexible and flexible panels are used. All panels may be made of fire retardant materials. Examples of both types of panels are installed at the time the deck is constructed and fit between the joists. They consist of panels which may be used in conjunction with each other in order to protect decks of varying widths. The other examples are added to previously constructed decks. One example includes a clip strip which is attached to the bottom of the joists and panels which are attached to the clip strip. A trim piece completes the installation. A second example involves the flexible panel with provisions for connection to a mounting bracket installed between the joists. Another example involves a flexible panel which may be installed by the homeowner along the bottoms of the joists. In each system there are provisions for insuring that the system remains waterproof.