Domestic roof construction – Wikipedia, the free encyclopedia

This article is about the type of roof construction that consists of footings and an outer weatherproof skin, as found on most domestic architecture. Such roofs may take a number of different shapes and be constructed of and covered with a variety of materials.

A simple ridged roof consists of declined rafters that rest on vertical 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 rafters, tie beams and joists serve to transmit the weight of the roof to the walls of the building. There are a number of structural systems employed to facilitate this, including the use of wall-plates set at the top of the wall, hammer-beams, which spread the weight down the wall and create an equilibrium between outward and upward thrust, king posts which transfer the weight of the roof ridge, and various types of trusses. The through section shows the specification for roof sheeting. If the specification applied to the UK with a concrete tiled roof, the rafters would be at 600mm centers the roof battens would be at 300mm centers and the ceiling joists would be at 400mm centers.

In cyclone and hurricane prone areas the main engineering consideration is to hold the roof down during severe storms. Every component of the roof (as of course the rest of the structure) has to withstand the uplift forces of high wind speeds. This is not normally a problem in areas not prone to high wind.

Modern roofing technologies, apparent in the accompanying photo of a house under construction in a region of Northern Australia, include the purpose-made steel hook bracket which is bolted to the truss with M16 bolt. The bracket is bolted to an M16 bolt cast in situ, embedded 300mm into the reinforced concrete block wall. This system is typically in place every 900mm around perimeter

A steeply pitched roof under construction

Ridge and rafters- tie or joist visible in foreground

A strongly reinforced roof structure in a cyclone-prone area

A hip roof construction in Australia showing multinail truss construction. The blue pieces are roll formed metal roof battens or purlins

Applying corrugated iron sheeting to a house under construction. Shows two gables and external cyclonic fastenings

Read more:
Domestic roof construction - Wikipedia, the free encyclopedia