Chapter 10 case study 1 Roofs: structures and coverings, Formation of pitched roofs
Most roofs to modern houses are formed in trussed rafter construction. The trusses are formed by fixing sections of timber together using galvanised steel gang-nail plates as seen above. The rafters are then secured to the wall plate using saddle fixings at around 600 mm centres.
The trusses are made up of a series of struts and props to transfer loading as needed. They do not rely on intermediate support from partitions etc. Once erected they must be braced longitudinally and diagonally to resist overturning or failure by twisting or buckling.
Bracing is required at the hipped corner of this roof to prevent the spread of the corner due to roof loads. The diagonal tie seen here is referred to as a dragon tie.
Roofs are subjected to considerable uplift forces as a result of wind loadings. In order to combat these the wallplate to which the trusses are fixed is secured to the external wall using galvanised holding down straps.
The storage of roof trusses on site is an important element in their long-term performance. Here they are seen stored appropriately so as to avoid flexing and twisting, which may affect the security of joints in their construction.
It is common to form details in the roof shape to add interest to the building’s appearance. The creation of this gabled roof section is achieved by first forming the main roof, then building the smaller section on top of it. In small-scale works of this type it is common to create a small ‘cut roof’ section on site. In larger examples trusses of diminishing size may be used.
Pitched roof coverings
The completed roof structure is covered with roofing felt or ‘sarking felt’. The felt is secured with timber battens that are used as the base for fixing of the tiles or slates. The distance between battens is termed as the gauge and is dictated by the size of the tiles, the pitch of the roof and the level of exposure.
In order to avoid the risk of condensation it is important to ensure that the roof void is ventilated. This is achieved here by installing a continuous vent fitting in the soffit of the roof. The grill fitting allows air to pass into the roof void, but excludes insects and vermin.
This roof features a hipped corner where two roof slopes meet. The hip is made weathertight by fixing hip tiles along the junction. The metal hip iron at the base stops them from sliding down the hip.
A feature of this roof is the lead valley at the junction between two roof sections running perpendicular to each other. The valley must remain weathertight when subjected to the run-off rainwater from the two sections of roof.
Where sections of roof meet with external walls as seen here there is the risk of moisture penetration at the junction. This is avoided by using a lead abutment flashing. The lead is dressed from the wall down and over the roof tiling as an overflashing. If the tiling had been plain rather than interlocking a lead soaker would be needed to each course of tiles in addition to the flashing.