Function
A roof structure has basic functional requirements that have to be fulfilled, these can be broken down as follows:-
Climate barrier
The main basic requirement is to keep the weather out and the warmth in, enabling it to maintain a comfortable environment for its inhabitants to live in and carry out the social activities that the building was designed for. A roof should provide adequate insulation as it is the main area of a building where heat loss may occur.
Strength and stability
A roof has to have the ability to carry the self-weight of the roof covering and structure and be able to resist forces from winds and the applied load of snow. The structure should be built as light as current technologies allow to keep imposed loads on the supporting walls to a minimum and finding the most economic means of carrying the roof structure and its load over spans of varying degrees.
Fire protection
The roof must also prevent the spread of fire to and from adjacent or adjoined properties.
Aesthetics
The type of roof used can alter the appearance of a building. There are many types of coverings with different colours and textures which may add to the appeal of the finished building. The slope of the roof also has a major impact on the aesthetics.
Pitched roofs
The Mono Pitch Roof
The mono pitch roof was commonly used to form extensions in Victorian times and is still used in a similar fashion today. It comprises a series of rafters fixed to plates at the top of a wall and the rafter feet are nailed to a wall plate, which distributes the load evenly across the supporting wall.
Joists were fitted to form level ceilings, and could be raised to give more height. They were commonly supported by struts, which stopped the rafters from sagging. Victorians often built the timber joists into the wall, creating a risk of timber rot and cold bridging. This could potentially compromise the stability of the wall. This is no longer considered good practice and joist hangers are now commonly used which give the same support, while not affecting the wall.
The Couple Roof
Often referred to as the simplest form of pitched roof, the couple roof comprises two lengths of timber (rafters) leaning against each other, tied where they meet at the top.
The rafters sit on a wall plate - a length of timber usually 100 x 75mm bedded on mortar on top of the wall. The wall plate provides a fixing point for the feet of the rafters, and is an efficient means of spreading the load exerted by the roof structure down through the walls without creating pressure points where each rafter meets the wall. As the mortar does not bond the wall plate to the wall, steel straps are used to ensure that the roof structure remains secure.
The Couple roof has a very limited span - approximately 3.5m. Historically the problem with this type of structure was that the weight of the roof created natural deflection in the supporting walls by pushing them outwards at the top. Walls could be reinforced but this would require extra brickwork, adding unnecessary expense. In order to support their load, rafters needed to be thick to prevent twisting under pressure, making them difficult to handle on site and expensive.
The Closed Couple Roof
By adding ceiling joists, a length of timber running horizontally in-between the rafter feet, typically 75mm x 50mm to the Couple Roof form, the structure became much more secure. The joist acted as a tie preventing the outward deflection of the wall and increased the potential roof-span to approximately 5m.
Joists are secured to the rafter feet rather than the wall plate to negate any potential deflection. A secure connection between the rafter and ceiling joist is therefore critical. Ridge boards provide restraint for the top of the rafters, preventing lateral movement. The timber used for ceiling joists was commonly thinner than the rafters, so they required support. This was often provided by internal load bearing walls, although in their absence 'hangers' and 'binders' were used. Often built into the gable wall for extra support, binders were nailed to the centres of each ceiling joist parallel to the ridge board. Problems associated with this method were damp penetration, timber rot and also the risk of compromising the supporting wall.
Binders were often supported by hangers (lengths of wood attached to the ridge board). The use of hangers and binders reduced the need for large ceiling joists.
Collar Roofs
By raising the height of the ceiling joists the Collar roof allowed any upper rooms to be constructed partly in the roof space, leading to some economies by slightly reducing the height of the external walls and therefore the amount of brickwork needed.
The problem with this method was that lifting the ceiling joist reduced its restraining force, therefore increasing the instability of the supporting walls and decreasing the span to approximately 4m. In order to maintain the required stability,