Roof covering

The roofing material is primarily designed to shed water. The covering is also a major element of the architecture so roofing materials come in a wide variety of colors and textures. The primary roof covering on houses in North America are asphalt shingles, but some have other types of roof shingles or metal roofs. Tile and thatch roofs are more common in Europe than North America. Some roofing materials help reduce air conditioning costs in hot climates by being designed to reflect light.

Asphalt shingles is the most used roofing material in North America, making up as much as 75% of all steep slope roofs. This type of material is also gaining popularity in Europe due to lower installation costs. Asphalt shingles dominate North American residential roofing market because they typically less expensive of other materials

In the southern US and Mexico, clay tile roofs are also very popular, due to their longevity, and ability to withstand hurricane winds with minimal or no damage.

In Europe, slate and tile roofs are very popular. Many slate roofs in Europe are over 100 years old, and typically require minimal maintenance / repairs.

Ventilation and insulation

Roof space ventilation is needed to combat condensation within the roof space, leading to interstitial condensation within the roof fabric; this can lead to serious structural damage, wet or dry rot, as well as ruining the insulation in the roof spec.

Condensation within the roof space is much more of a problem today due to: much less fortuitous ventilation due to tighter building envelopes with high performance windows and door and no chimneys leading. This tighter envelope means the air temperature in buildings has risen, the warmer the air in the building is, the more water vapour the air can carry.

As the occupied part of building has become warmer, the roof space has become colder, with high performance insulation and roofing membranes leading to a cold roof space.

When the warm, moist air from below rises into the cold roof space; condensation begins as the air temperature drops to the ‘dew point’ or as the warm air comes into contact with any of the cold surfaces in the roof.

Most building materials are permeable to water vapor; brick, concrete, plaster, wood and insulation all can fall victim to interstitial condensation, this is why UK Building Regulations require roofs to be ventilated, either by the use of soffit vents, ridge vents, or replacement ventilation slates or tiles.

A common method of ventilating a roof is to make openings in the soffit and ridge to allow natural airflow. This example also has ventilated exterior walls called rainscreen construction.

Ventilation of the roof deck speeds the evaporation of water from leakage or condensation and removes heat which helps prevent ice dams and helps asphalt shingles last longer. Building codes in the U.S. specify ventilation rates as a minimum of 1 sq. ft. of opening per 150 sq. ft. (1:150) with a ratio of 1:300 in some conditions. Warm air rises so ceiling insulation is designed to have a higher r-value and the insulation is often installed between the ceiling joists or rafters. A properly insulated and ventilated roof is called a cold roof. A warm roof is a roof that is not ventilated, where the insulation is placed in line with the roof pitch. A hot roof is a roof designed not to have any ventilation and has enough air-impermeable insulation in contact with the sheathing to prevent condensation such as when spray foam insulation is applied directly to the under-side or top-side of the roof deck or in some cathedral ceilings.

A more recent design is the installation of a roof deck with foil-backed foam along with a second deck that is air-gapped away from the foil-backed foam to allow air to flow vertically to a ventilation outlet at the peak of the roof—it is a double-deck design with an air gap. This design improves efficiency.