Repairing Collyweston slate roofs
1. When to repair a roof
Collyweston stone slate roofs can last hundreds of years. The slates themselves are extremely durable and failure of a roof is usually due to the rotting of the oak pegs or battens on which they are hung. Due to the weight of the slates, slippage can occur quickly once is starts. Slippage of nailed slates can occur when the small area of slate between the edge and the hole erodes. In such cases, the slates are redressed to form a smaller slate and reused in the re-roofing. As with other materials, failing roofs are also susceptible to wind damage. In the majority of cases, it is the corrosion of the nails fixing the battens or laths that cause the slippage.
Traditionally, slates were pegged (now nailed) with a little lime mortar used at the top of the slates, known as 'spot bedding'. Some slaters also point up the roofs, using lime mortar to fill joints and gaps under the Front edges of the slates. Internally, more lime mortar was applied directly to the underside of the slate, a technique referred to as 'torching. When re-slating a roof, this practice has generally been replaced by the use of roofing underfelt.
If a roof is relatively sound and is not suffering from large areas of slate slippage or batten failure, minor repairs will prolong the life of the roof by several years. Small areas of failing woodwork should be repaired and the roof pointed (or repointed if this has been done before) to improve its weatherproofing properties. The occasional slipped slate can be spot-bedded back in place. This will also allow any torching to be retained and repaired if necessary.
If the battens have decayed and the fixings are rotten or corroded, there is little alternative but to strip and re-slate the roof. This will unfortunately result in the loss of any torching, as, whilst the roof could be re-torched, modern requirements are such that underfelt tends to be used. A flow of air is needed around the roof timbers and slates to keep the roof ventilated and prevent condensation. Where felt is used, therefore, the slates should not be fully pointed.
2. What does it involve?
The work should comply with the Specification for Collyweston
Before work starts, the contractor will need to erect a scaffolding to access the roof.
The existing stone slates and ridges will be carefully stripped from the roof and set aside for parting up, either on the ground or on the scaffolding, where space allows.
The old battens are removed, roof timbers brushed down and repairs to the roof structure are carried out. New timbers should always be inserted alongside the existing roof timbers, to retain the historic roof structure. The removal of roof timbers will normally require the agreement of the Conservation Officer, and Listed Building Consent may be needed if the building being repaired is listed. The roof may also be treated against decay and infestation at this point.
This Note has been prepared by Christine Leveson and Jon Burgess, in association with the Collyweston Stone Slaters Trust