BCD 2019

77 C AT H E D R A L COMMU N I C AT I O N S T H E B U I L D I N G CON S E R VAT I ON D I R E C TO R Y 2 0 1 9 ROOFING 3.1 aesthetic of the roof and cause it to leak. The roof is finished with lead around the chimney and abutments which are usually covered by lime mortar. This means that lead chimney aprons and top abutments are not usually visible. Handmade clay ridges called clay hogs-back are fixed along the ridge line on the slates with a lime mortar bedding (Collyweston Stone Slaters Trust, n.d.). THE LOSS OF TYPICAL DETAILING All the Collyweston slate quarries had closed by the 1960s as a result of several factors, including the availability of cheaper mass produced products. Developments in farming led to the machinery being too large for traditional farm buildings, which caused many to became redundant, and many were demolished or stripped and replaced with other materials, which provided an abundant supply of reclaimed Collyweston slate. The production of new slate was labour intensive and costly and the growth of the salvage industry to service the heritage market produced far cheaper material. From the 1960s until recent years the Collyweston slate supply came purely from the reclaimed slates, often stripped off other buildings when being reroofed with an alternative material. Fortunately several local construction and roofing firms are now producing new slates by mining or quarrying fresh log and splitting it by a freeze-thaw process using freezers. The new slate has been used on several prestigious buildings including a Cambridge college and Apethorpe Palace. Unfortunately, the decline of Collyweston slate and its less frequent use on unlisted and new buildings has resulted in a loss of knowledge and expertise in traditional detailing and construction. The 20th century has had a damaging effect for several reasons. Firstly, the push to meet modern performance standards has resulted in modern details and materials which have often proven detrimental to both the fabric and historic detailing. An example of this is the use of spray-on sealing foams which have been applied to the underside of the roof. This traps moisture causing the battens and pegs to decay, and it makes the reuse of the original material extremely difficult. Secondly, traditional details are often overlooked or deliberately ignored when roofs are repaired or renewed. For example, leaded valleys are often used because they are quicker and easier to make, and because it is wrongly assumed that the vernacular detail is inadequate. Historically, laced valleys were used, which is where the stone slates on either side of the valley are rotated to fit against a lozenge shaped slate in the centre. Historic England (then English Heritage) sought to address the use of inappropriate materials and detailing in vernacular roofs with their Stone Slate Roofing Technical Advice Note in 2005 (revised and to be reissued in 2018). An increased interest in historic detailing and new slate since the note has led to a greater need to look at correct specifications. THE CONSERVATION AND REPAIR OF COLLYWESTON STONE SLATE ROOFS Many Collyweston roofs have been in place for at least 100 years and some may have remained undisturbed since the 17th century, proving beyond doubt that traditional details really do work. Nevertheless, there will come a time when even the best roofs reach the end of their life and need to be reroofed. When approaching the project it is important to recognise that these roofs would have been constructed by expert craftsmen and although they may have received some patch repairs over the years, they are likely to retain evidence of the traditional roofing details that were used right up to the 19th century. Stone slates are reusable and can last for hundreds of years, so all of the slate would be removed, redressed and re-laid, making up the shortfall with new or reclaimed slates. Pre-works and considerations Because of the age of the roof fabric, the evidence it contains and the links to past craftsmen and the local area, any work undertaken should be carefully considered on a case by case basis and carried out with respect for the historic roof detailing. It is essential to record details of the roof covering and supporting structure both before it is disturbed and as it is being dismantled, as this information will help conservators and craftspeople to identify and understand how the roof was originally constructed, providing essential information for conservation Collyweston slates are laid in diminishing courses using a double lap method, with the smallest slate at the ridge line and the largest at eaves level. When Collyweston slate roofs are renewed leaded valleys are often used instead of traditional details because they more convenient to make. A traditional laced valley detail (All photos on this page by Janine Dykes) Stone slates can last for many years and can be reused, redressed and re-laid.