A building’s integrity is heavily reliant on its roof, which acts as a vital defence against environmental forces. When the roof fails, the impact is felt throughout the entire structure. In addition to shielding the interior, the roof’s construction must support the weight of the protective materials, transferring these loads to the structural skeleton of the building. Therefore, a well-constructed, resilient roof is crucial for ensuring the building’s durability and safety.
Up to the 1970s, the construction of pitched roofs adhered to traditional ‘cut timber’ methods. Rafters were fundamental to this technique, tasked with distributing the weight of the roof covering and structure to the perimeter walls. In larger roof constructions, collars were introduced between rafters to prevent outward expansion and maintain structural integrity. Purlins were incorporated to enhance the roof’s rigidity and provide additional support to the rafters, while struts were sometimes utilised to transfer loads to internal structural elements like walls or joists.
From the 1970s onwards, trussed rafters have become the dominant choice for low-rise domestic pitched roofs, taking over from the traditional cut timber structures. These rafters are valued for their cost-effective production and straightforward installation, reducing the need for skilled carpentry work. They also feature enhanced spanning capabilities, making internal structural walls for load distribution unnecessary. Binders are integral to this design, fastening the trussed rafters together to ensure a stable and robust structure.
Representing the pinnacle of modern engineering, this ‘Fink’ trussed rafter can span up to ten meters without internal support. As technological innovations continue, more elaborate truss designs are being developed, facilitating even longer spans.
Structural failures frequently result from inadequate design or specifications, a vulnerability that is especially pronounced in older buildings built before modern building standards. This problem is particularly evident in the speculative housing projects of the 19th century, where rapid urban expansion often led to widespread design deficiencies.
When rafters, purlins, or other vital structural parts are insufficiently sized or missing, the building’s strength is compromised. This deficiency becomes crucial when the roof covering’s weight, along with external forces such as wind or snow, is factored in. Without adequate lateral restraint, the roof structure can spread, exerting vertical and horizontal pressures on the rafters. This outward force can displace the external walls, significantly jeopardising the building’s overall structural integrity.
In their efforts to improve their homes, homeowners sometimes inadvertently weaken their roof structures by removing collars or struts to make room for water tanks or additional storage. The most significant and harmful change, however, is replacing traditional slate roofing with modern concrete tiles. Roofs originally built for slate may not be strong enough to support the much heavier concrete tiles, causing gradual deformation and potential structural issues