Roofers typically do the following:

• Inspect problem roofs to determine the best way to repair them
• Measure roofs to calculate the quantities of materials needed
• Replace damaged or rotting joists or plywood
• Install vapor barriers or layers of insulation
• Install shingles, asphalt, metal, or other materials to make the roof weatherproof
• Align roofing materials with edges of the roof
• Cut roofing materials to fit around walls or vents
• Cover exposed nail or screw heads with roofing cement or caulk to prevent leakage

Properly installed roofs keep water from leaking into buildings and damaging the interior, equipment, or furnishings. There are two basic types of roofs: low-slope and steep-slope. Solar and vegetative features are sometimes incorporated into both low- and steep-slope roofs. Roofers may specialize in the installation and replacement of one or more of these roof systems.

Low-slope. Low-slope roofs rise less than 3 inches per horizontal foot and are installed in layers. Low-slope roofs make up nearly three-quarters of all roofs, as most commercial, industrial, and apartment buildings use this type. 

Many of today's low-slope roofs are covered with a single-ply membrane of waterproof rubber or thermoplastic compound. Most previously installed low-slope roofs, however, use several layers of roofing materials or felt membranes stuck together with hot bitumen (a tar-like substance).

Steep-slope. Steep-slope roofs rise more than 3 inches per horizontal foot and most commonly use asphalt shingles, which often cost less than other coverings. Steep-slope roofs make up most of the remaining roofs, as most single-family homes use this type.

Although roofers most commonly install asphalt shingles, some also lay tile, solar shingles, metal shingles, or shakes (rough wooden shingles) on steep-slope roofs.

Traditional roofing systems may incorporate plants and landscape materials, and these features are becoming more common. A vegetative roof is typically a waterproof low-slope roof, covered by a root barrier. Soil, plants, and landscaping materials are then placed on the roof.

Solar features are increasingly popular on roofs. These systems include solar reflective, which prevents the absorption of energy; solar thermal, which absorbs energy to heat water; and solar photovoltaic, which converts sunlight into electricity. Roofers install some photovoltaic products such as solar shingles and solar tiles, but solar photovoltaic (PV) installers typically install PV panels. Plumbers and heating, air conditioning, and refrigeration mechanics also may install solar thermal systems.