Some flat roofs are covered with only a single layer of waterproof rubber or thermoplastic compounds. In these cases, roofers roll the sheets over the roof's insulation and seal the seams. They apply adhesives or mechanical fasteners to keep the sheets in place. On pitched roofs, roofers lay, cut, and tack strips of roofing felt to the roof. Then, starting from the bottom edge of the roof, they nail overlapping rows of shingles.
Roofers measure and cut felt and shingles to fit around vents, chimneys, and corners.
They nail strips of metal or shingle over spots where shingles meet an object, such as a vent. Finally, roofers cover exposed nailheads with roofing cement or caulking to prevent water leakage. Some roofers work on both flat and pitched roofs while other roofers specialize in one type. In addition to repairing roofs, some roofers also waterproof concrete walls and floors. They start by removing any rough spots from the area to be waterproofed.