Domestic roof construction: what’s involved?

Domestic roof construction serve the same common purpose, yet they come in different styles and designs - image courtesy of APS
Domestic roof construction serve the same common purpose, yet they come in different styles and designs - image courtesy of APS.

The construction of domestic roofs refers to the frame construction and covering that is found on the majority of detached houses in temperate or cold climates.

These roofs tend to be built with timber, and can take on a wide range of different shapes and covering types. Modern timber roofs are typically framed with pairs of prefabricated trusses or common rafters that are fastened together with truss connector plates.

Roofs can also be designated as either warm or cold, depending on how they are designed and built with regards to ventilation and thermal building insulation. Along with that, the steepness or pitch of a roof is primarily determined by the material covering the roof and other aspects of the aesthetic design.

Roof construction and the weather

In high-wind areas, for example where a hurricane or cyclone could occur and take landfall, the main consideration for engineering is to ensure that the roof is held down during severe storms. Each component of the roof, along with the rest of the structure, must be able to withstand the strength of severe winds and storms. In order to accomplish this, metal ties are fastened to each truss or rafter.

Rafter roof construction

A basic rafter roof is made up of rafters which rest on horizontally placed plates at the top of each wall. The top ends of these rafters typically meet at a ridge beam, however in some cases they can butt directly to another rafter, forming a couple (pair of rafters). Depending on the material used to cover the roof, either horizontal purlins, laths or battens are attached to the rafters. In other cases, boards made from oriented strand or plywood from the roof deck are used in order to support the roof covering. Tie beams, which sometimes also double as ceiling joists, are usually connected to the bottom ends of opposite rafters which prevents them from forcing the walls apart by spreading. Collar beams or ties may be fixed at a higher level by roofing contractors to add further strength.

Truss roof construction

Truss roofs are usually pre-manufactured, and they come in a wide variety of styles. They are designed by the manufacturer to fit each specific building. Timber trusses are also built in a wide range of styles and can be built using either wooden or metal joints. Heavier timber rafters, which typically space eight to twelve feet apart, are known as principal rafters. These rafters may be used with common rafters, and can be found carrying common purlins.

Design loads

The design of a roof framing must be done in such a way as to hold up a total structural load, including what is known as dead load, its own weight, the covering material weight, and any additional, environmental loads for example wind or snow. This total load and the distance between supports, known as the span, determines the size and spacing of trusses and rafters.