Modern cut roofs

Although it is rare to find traditional roofing techniques used in modern estate developments, individual new properties are sometimes built with traditional cut roofs, particularly where there is extensive use of the roof for accommodation. The principles outlined earlier have changed little, although the Building Regulations guidance document published by TRADA is more onerous with regard to the sizing, spacing and spans of timbers. It is not uncommon to observe sound, undistorted timbers in an older traditionally cut roof whose dimensions are significantly smaller than those prescribed by such tables. The structural quality and durability of timber used in the past, particularly pre-1914, is generally superior to that available today.

Most modern houses are built with cavity walls and a typical eaves detail is shown below. The cavity is normally closed before positioning the wall-plate. This acts as a cavity barrier in case of fire and prevents moist air from the cavity entering the roof space where it can condense and lead to conditions which encourage rot to occur. Closing the cavity is not necessary if the cavity is filled with insulation. However, recent guidance from the National House-Builders Council (NHBC) suggests that there have been some incidences of horizontal cracking just below the wall-plate caused by differential movement of the internal and external leaves. They attribute this to the use of lightweight blocks in the internal leaf. The NHBC, therefore, recommends that cavities should be closed with a flexible mineral wool positioned inside the top of the cavity.

Current Building Regulations guidance documents allow the use of traditional solid timber purlins, although their acceptable spans are limited. Trussed timber purlins (factory fabricated and engineered specifically for their intended purpose) or steel sections are a modern alternative. Steel sections can be built into party walls, although trussed purlins are usually bolted to the inside face of the walls or supported on steel ‘shoes’. The party wall is important in terms of fire protection and if timbers are built into this structure this will reduce the wall’s effective fire resistance.

The Building Regulations Approved Document A ‘Structure’ recommends the use of straps in roof construction. These ensure that the gable walls cannot buckle or move as a result of wind pressure, by linking the walls to the roof bracing and structure, so tying the roof structure to the external walls. The requirements relating to strapping of wall-plates are primarily for shallow pitched or lighter non traditional roofs rather than their heavy traditional counterparts. Consequently, details of strapping are contained in the section on modern trussed rafters in Section Two.