BCD 2019

50 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 C AT H E D R A L C O MM U N I C AT I O N S scribed to the deflected shape and then coach screwed to the top of the existing at close centres (illustrated bottom right). Although this is quite significant alteration work, it improves the structural capability of the rafter considerably. A different option is to install a partner timber alongside and bolt fix through, but this may not be practical where the rafters are exposed. However, if the foot of the rafter is decayed, a short partner timber may well be invisible and uncontroversial. For a deflected purlin, additional struts can be added from the support structure below, or between opposing purlins, but this may affect how the space is utilised, or not be possible at all. A partner member alongside may be possible, but visually perhaps not appealing, or a steel plate could be run over the back of the rafters and then connected through, creating a diaphragm action as illustrated above. Where a tie beam or principal rafter end is decayed, it may be possible to cut a slot and insert a steel plate, known as a flitch plate into the member to replace the function of the affected area. Full access and temporary works are likely to be necessary to install this (illustrated right and top right), with bolts set through both the timber and repair plate so they act compositely, with the bolts countersunk and pelleted if required. Where flitches are not possible, steel plates, channels or angles may be used to fix alongside the defective members. This is perhaps better suited to members which are concealed within floor voids or similar, although where visible the visual impact can be reduced by using specially fabricated steelwork. In some situations a combined approach of both new timber and steel is required. Care is needed where green oak is used as the tannic acids in the green timber can cause mild steel to corrode, and the use of stainless steel is necessary. This applies equally to fixings with green oak repairs. A steel flitch plate repair to the end of a roof beam The end of the roof beam with decayed timber cut away, ready for the installation of the flitch plate illustrated above Rafters with 12 inch firring pieces Steel plates to strengthen rafters are placed over the back of the rafters and bolted Very occasionally a roof can be found to be in such a condition that more major intervention is required. One example of this was a church medieval roof of rafter form with a double eaves plate where the deflection of the rafters was approximately 300mm (12 inches) (illustrated below, bottom right).Layers of firring timbers had been applied to reduce this effect but the rafter foot must have lifted a considerable distance off the wall plate, to the point where they had been cut over the ashlar post (see lower diagram and photo, p48), leaving the main roof in a state of tenuous equilibrium and extremely vulnerable to collapse. Here an option of inserting new rafter trusses between the existing to support the roof coverings was used, and therefore also the imposed load from wind and snow. The existing structure was retained and the internal finishes (lathe and plaster) applied to this. Although significantly interventionist, it did retain all the historic fabric and was clearly reversible, in that the added truss rafters could be removed at a later date if required. In conclusion, when assessing historic roofs, ensuring that there is a clear understanding of the significance, the form and geometry, the condition and the defects, is essential. This will allow a clear methodology and philosophy for the works required to be implemented. ED MORTON is the managing director of The Morton Partnership, consulting civil and structural engineers and historic buildings specialists (see page 42). Ed is accredited in conservation (CARE), sits on the CARE Panel, IHBC technical committee, the ICOMOS UK Timber committee, and was a principal contributor to the republished English Heritage Practical Building Conservation on Timber . He is engineer to six cathedrals and honorary engineer to four dioceses and The Ancient Monuments Society.