BCD 2019

BUILDING CONTRACTORS 2 47 C AT H E D R A L C O MM U N I C AT I O N S T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 1 9 TIMBER ROOF STRUCTURES and their assessment ED MORTON T   HE FUNCTION of the roof structure is primarily to support roof coverings of slate, tile or thatch, which protect the building from rain, snow and wind. In addition, the space within may provide accommodation for the building, either by the original design or often through later adaptation. When assessing a roof structure it is important to initially inspect the building both internally and externally to better understand its layout, structural form, the likely phasing of its construction and its general condition. It also provides an initial understanding of any existing health and safety risks which could affect or limit the assessment. In particular, for roof interiors with a floor/ceiling, it must be determined if it is safe to walk over or if the insulation at this level is making this impossible. It is often helpful to sketch out a plan of the roof to sub-divide it into bays, where appropriate, and produce a cross section marked up with the names of the various members which can then be used in the detailed inspection. This exercise will also highlight major and obvious positions of structural defects such as deflections, bulges or perhaps failures, as well as the most vulnerable areas, principally where water is more likely to become trapped such as gutters, abutments and valleys. While the roof is the target for the detailed inspection, the walls or structure which support it also need to be inspected as they may have been affected by the performance of the roof, or alternatively could have affected the roof through settlement or other defects. This initial inspection will help to determine the scope of the detailed survey. Moving into the more detailed assessment, firstly it is useful to try and establish how the structure works, by asking questions like ‘how are the loads applied to the roof transferred through to the walls or structure below?’ The roof orientation can be checked in relation to the prevailing weather directions, and the form can be verified by observing if the roof ends in gables, hips, half-hips with gablets or abutments of valleys with adjoining roofs. Structurally, it is important to understand that pitched roofs generally want to thrust out at eaves level due to the weight applied through roof coverings, finishes and imposed or live loads such as snow and wind (see diagram overleaf). The thrust which is developed must be constrained, with the most efficient form being to introduce ties (of timber or steel) at eaves level across the building. The ties may occur on every rafter or, through the use of trusses, at intervals. However, the architectural design may require a situation with no ties, such as a hammer beam roof truss, or to provide increased height for an attic floor, or for the floor below to have a raised tie or collar. As a result, historic roofs come in many different forms and styles, and indeed different timber types. Broadly pitched roofs can be sub-divided into two categories: purlin roofs (top illustration) and rafter roofs (lower illustration). In the first, trusses support purlins (horizontal timbers) which in turn support the common rafters. In rafter roofs there are no purlins and it is the common rafters that are the primary An example of a purlin roof at Lacock Abbey: in this case the rafters are connected to trusses by two pairs of purlins and the wall plate, while the trusses are tied at eaves level and their midpoint. (Photo: Jonathan Taylor, by kind permission of the National Trust) An example of a rafter roof with crown posts: each pair of rafters is collared at the crown and braced, but there are no purlins. The tie beams provide the main lateral constraint at eaves level and are supported at the midpoint by the crown posts to stop them sagging. (All photos: Ed Morton, The Morton Partnership unless otherwise stated)