The roof is the covering structure on top of a building. It keeps rain, snow, and sun out. It also completes the building's form, defines its silhouette, and often provides space for mechanical equipment or outdoor terraces.

Roofs come in two broad types. A pitched roof slopes to shed water. It is common in residential architecture and works well in rainy or snowy climates. A flat roof does not mean perfectly flat. It has a slight slope for drainage. Flat roofs are common on commercial buildings because they are easier to walk on and provide space for HVAC units.

Roof assemblies include the structure, insulation, waterproofing, and sometimes a drainage layer and vegetation. A green roof adds soil and plants, which reduce stormwater runoff and improve insulation. A cool roof uses reflective materials to reduce heat gain. Each system has different maintenance requirements.

Roofs fail more often than any other part of a building. Water finds every weakness. Flashing, drains, and penetrations are the usual suspects. A good roof design details these areas carefully and provides access for maintenance.