Waterproofing a bunker roof could be done with plastic and/or canvas. Plastic with no tears kept out all water but the bunker no longer exchanged air with the outside, so it got very humid and hot inside. Canvas kept out most water early in a rain but as the storm lasted, it leaked. The bunker in this picture has plastic protection under the sandbags. Canvas worked better when on top of the sandbags but risked blowing away in the wind or letting rain blow under it. I no longer care for the smell of wet canvas.