Pitched Roof Structures: Common Property Defects #7
Tuesday, 17th May 2016
Common Property Defects | Surveying
The roof is one of the most important parts of a building. It is responsible for protecting the structure beneath from weather, and its failure can have a serious effect on other elements of the building. The roof structure is responsible for supporting the weather-tight roof covering, and distributing its loads (which tend to be considerable) to the structural walls of the building. In order to perform this function satisfactorily, the roof structure must be strong, stable, and durable.
Traditional ‘cut timber’ pitched roof structures were in common use up until the 1970s. The rafters distribute the load of the roof covering and structure to the external walls. In larger roofs, lateral restraint, typically in the form of collars spanning between the rafters, prevents the structure from spreading outwards. Purlins are included to provide extra rigidity and support to the rafters, and struts may be also introduced to distribute some of the load to internal structural walls or joists.
A traditional cut timber roof, incorporating a collar for lateral restraint, purlins for rigidity, and struts to distribute some of the load to an internal load bearing wall.
From the 1970s onwards, most low-rise domestic pitched roofs have been formed with trussed rafters. Trussed rafters have the advantage of being relatively cheap to manufacture, and easy to install on site without the use of skilled labour, as opposed to traditional cut timber roofs, which require the use of skilled carpenters. They can also span much greater distances than traditional roof structures, without the need distribute load via internal structural walls. Binders tie the trussed rafters together, and keep the structure ridged.
The simple ‘Fink’ trussed rafter shown above can span up to 10 metres without taking support from the internal structure. More intricate truss designs can span even greater distances.
Failure can be the result of poor design or specification. This is more common in older properties, which were not subject to today’s strict building regulation controls. Design deficiencies are particularly prevalent in the large swath of speculatively built housing constructed during the 19th century’s period of rapid urban expansion. Where rafters, purlins or other elements of the structure are undersized, or omitted entirely, the structure may be too weak to satisfactorily support the loads imposed by the roof covering, especially when combined with external elements such as wind or snow. If insufficient lateral restraint is provided, the roof structure will spread, due to the vertical and horizontal pressure on the rafters, pushing the external walls outwards and potentially compromising the entire structure of the building.
The ‘dishing’ to this Victorian mono-pitch rear addition roof (excuse the foliage) is likely the result of the rafters being undersized. Whilst the slate roof covering is relatively light, the load will become significantly heavier during periods of heavy snow, resulting in gradual distortion over the years.
It has been known for the unwitting homeowner to compromise their roof structure by undertaking alterations, such as removing collars or struts, be it to create extra space for items such as water tanks, or simply for storage. By far the most common problematic alteration is the replacement of slate roof coverings with modern concrete tiles. Roof structures designed to carry slates will often be insufficiently strong to accommodate concrete tiles, which are significantly heavier. This can lead to severe distortion of the roof structure.
The original slate roof covering of this Victorian terraced house has been replaced with interlocking concrete tiles, causing the rafters to bow under the additional weight. Internally, secondary purlins and additional struts have been added to the roof structure in an attempt to remedy the issue. Problems caused by the replacement of slate roofs with concrete tiles have become so common that, since 1991, it has been compulsory to submit structural calculations to building control whenever a roof covering is changed.
Timber roof structures are also susceptible to damage from moisture penetration, which increases the risk of rot and beetle infestation. This will often be due to the failure of roof coverings, flashing details, or box and valley gutters, but can equally result from internal sources of moisture, such as leaks from water pipes or tanks. Timbers built into solid masonry external walls can be at risk of penetrating dampness. Where roof voids are insulated to modern standards, but not ventilated (as is often the case) a further risk is present in the form of interstitial condensation.
Staining to the valley boards of this traditional cut timber roof suggests that the valley gutter lining has failed. If the issue is not remedied, rot is likely to set in, and will spread to effect the surrounding sarking boards and rafters.
Roof structure defects may be in existence for some time before being discovered, especially when the occupier rarely has cause to go into the roof void, as is often the case with tenanted properties or those owned by older people. The potential for defects in the roof structure is just one of the many reasons to commission a full RICS HomeBuyer Report or Building Survey before committing to the purchase of a property. Alternatively, if you are worried about the condition the roof structure in your current home, a Specific Defect Report can be commissioned to detail the cause of any defects and the required remedial work. Whatever your requirements, our team of Chartered Surveyors will be happy to help. You can call the team on 020 7471 8932 or contact us by email.
