The present invention relates to a watertight sub-floor system for balcony or roof, in the form of a semi-rigid rollable plank of corrugated polymeric type installed in juxtaposition to form a waterproof flooring between boards and beams. Under-plank are unwrapped and cut at a desired length to avoid transverse junction with respect to water flow. Under-planks overlap parallel to water flow without loss of sealing. The corrugated under-plank has a design of crests and dips, crests widened and sloping to make possible the receiving of screws from deck board, thereby permitting the descent of water trickling from around the damaging screwing of the under-plank, and dips which bear no perforations from screwing and merely allow water to run off to protect wood structures of patios, balconies or terraces against moisture causing premature rot and to protect objects (bike, mowers . . . ) under a deck.