The roof is the top covering of a building intended to shed water off the building and to prevent it from accumulating on top. To achieve this goal, roofs may be highly pitched (sloped) or low sloped in form. Low sloped roofs are commonly found on industrial/commercial type structures. Pitched roofs are the primary design found on residential homes. The most common type of low sloped roof for industrial/commercial structures are the conventional built-up-roof (BUR), modified bitumen, or single plies (EPDM, PVC, TPO). Metal standing seam roofs are becoming increasingly common on low sloped roof assemblies.

Residential roof designs in mild climates or where there is little precipitation tend to exhibit lower pitched roofs and drainage at the exterior wall; an example of residential flat roof is that of the adobe construction in the American Southwest. In northern climates, where temperatures drop below freezing, or in southern climates with high temperatures, the need for greater thermal resistance will dictate the type of roof design as well as the waterproofing membrane specification. In areas that have a high accumulation of snow, such as Buffalo USA or Montreal Canada, a minimum slope of 6 inches in 12 inches (6/12) of roof surface is required. Roof pitches may exceed 45 or 50 degrees.

Cool roofs and green roofs are becoming increasingly popular, and in some cases are mandated by local codes. Cool roofs are defined as roofs with both high reflectivity and high emissivity. Green roofs have soil placed over the waterproofing membrane in which plants and even trees are planted.

Some building styles, for example, geodesic and A-frame, blur the distinction between wall and roofs. Pitched roofs are often covered with asphalt shingles (in the US and most of eastern Canada) although thatch, wood shake, steel, corrugated galvanized iron, slate and tile roofs are used elsewhere.

Terminology:

• An attic is the story under a non-flat roof.
• Galvanised roofs are normally used on sheds.
• Zincalume roof sheeting is used widely in Australia for commercial and residential roofs.
• "Pavilion" is a term used by architects to refer to any autonomous, self-contained structure, regardless of size or roof configuration.
• A simple ridged roof consists of inclined rafters that rest on horizontal wall plates on top of each wall. The top ends of the rafters meet at the horizontal ridge plate or ridge beam. Horizontal purlins are fixed to the rafters to support the roof covering. Heavier under purlin are used to support longer rafter spans. Tie beams or ceiling joists, are connected between the lower ends of opposite rafters to prevent them from spreading and forcing the walls apart. Collar beams or collar ties may be fixed higher up between opposite rafters for extra strength.
• The pitch of a roof is the angle that the roof surface is to the horizontal. In the UK and Australia the angle or pitch of a roof is given in degrees. That is a roof with a 5 degree pitch is flatter than a roof with a 60 degrees pitch.

  
• The pitch of the roof in North America is usually called the slope, it is defined as a ratio of the height over the run, most commonly in inches. The run is usually stated as 12 inches. So a slope of 1" in 12" is a low pitch and a slope of 18" in 12" is a steep pitch.