18 CONTEXT 185 : SEPTEMBER 2025 does acknowledge that it is written for modern buildings and may not be appropriate for old roofs. Slavish adherence to a British Standard is not a requisite for specifiers, provided they can show that other methods will ensure a satisfactory outcome. Historic buildings have the cardinal benefit of demonstrating effective longevity as a way of justifying ‘non-compliant’ traditional methods and materials. Modern methods BS 5534 generally discourages the use of mortar on roofs and now recommends dry fixing for ridges and hips, following failures of cement mortar used on new-build houses. Lime mortar has been used for centuries on slate and stone roofs and placed appropriately it provides good performance and longevity. It was applied judiciously with minimum exposure on ridges, hips and verges, unlike today where the common practice is to use a thick layer of mortar exposed to the elements. Lime mortar was essential in some regions where dabs of mortar were used to provide an even bed for the slate above and to provide additional support to strengthen the fixings for peg-hung slates. Lime mortar could be removed easily during the next re-roofing, which meant that stones, slate, ridges and the like could be re-used, unlike those bedded in cement mortar, which often cannot be detached without breakage. The standard does contain a table showing mortar mixes using hydrated (used with cements) and hydraulic lime, but all mixes are proposed in proportions of 1:3. Nowadays, experienced traditional roofing contractors use a variety of mixes depending on their application. These tend to be more lime-rich, particularly if using a quicklime, which also can provide significant adhesion and therefore can be very beneficial against wind-lift. In current standards, battens are recommended to be preservative-treated softwood with a minimum size of 50 mm x 25 mm for natural slates. This is to counter bounce in modern truss roofs and to allow roofers to safely stand on them. Historically, riven hardwood (usually oak) laths were used. While they were much thinner, because they are riven rather than sawn, they are sufficiently robust to cope adequately. Historic slate roofs use more closely spaced laths, compared to single-lap tiles, so any imposed shock loads (such as a slater falling on them) is spread over many laths, but the standard makes no distinction between the size of battens needed for triple-lapped slates, for example, or single-lapped tiles. There are many other examples of modern recommendations being applied to historic roofs which are resulting in a continuing loss of vital vernacular detailing, much of it unrecorded. The code of practice and technical advice note To address these problems, Historic England has commissioned Terry Hughes (author of the article on Melin Daron on page 18 in this issue) and the author of this article to write a code of practice and technical advice note. The code of practice seeks to encourage best practice for any works on stone and slate roofs. It is intended to provide concise recommendations on dealing with the conservation and repair of roofs, ranging from setting out the roof to repairing individual details such as the eaves, valleys and verges. Unusual and innovative roofs are also covered, such as curved slating and patent slating. All these are illustrated with sketches and images. Allowance is made for changes to existing or historic detailing if these are either not performing well, or the anticipated changes in climate (for example, increasing instances and severity of driving and deluge rain) may overwhelm the existing drainage details. The need to improve energy efficiency is also included. Heritage values and significance are explained, along with all the legal issues that need to be considered before repairing or changing historic roofs. Advice on specifying repairs is also a key component. The code of practice is complemented by a new technical advice note, which will replace Stone Slate Roofing: technical advice note, published by English Heritage in 2005. It has been updated and expanded to include advice on metamorphic slates as well. This document is being aligned closely to the code to provide further background and information on its recommendations. A shale valley being recorded. As each slate is removed information is lost. Video recording allows the investigative process to be reversed and re-examined. The proposed code of practice will recommend a full condition survey being carried out by suitably experienced individuals, particularly on more complex details such as this valley. (Photo: Terry Hughes)
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