Abstract: A method of waterproofing inner surfaces of tunnels, channels and mine galleries in which sheets of material are unrolled and cut in situ and applied to the surfaces of the wall to obtain the desired fit and so that the sheets are overlapped. Holes are cut into the walls through the sheets and anchors are attached to the walls. The anchors applied where the sheets overlap are on top of one of the sheets and are covered by another sheets. The overlapping regions are welded by thermal fusion. The anchors in the intermediate zones (where the sheets do not overlap) are covered with a circular part made from the same material as the sheets and more than 6 cm. greater than the diameter of the anchor heads. The anchors are applied in a greater density at the starting and ending zones and a sealing bead is applied between the sheets and the wall surface.