These notes are not intended to form part of any specification document but are designed to help inform the specifier of the issues, and options.
It is the policy of English Heritage and all local authorities in the Collyweston area that:
- Where ever possible, new slates should be used rather than second-hand
- Reclaimed slates should be used only on the building or group of buildings from which they were removed
The Collyweston Slaters Trust fully supports this policy.
The objective of such policies is to discourage the loss of Collyweston roofs in the area and to encourage the overall increase in the production of new slates. This is in order to ensure that the craft has a long-term future and the character of the area is sustained.
Slates which have been stripped from buildings which were formerly animal shelters or barns can be unsuitable for re-use in domestic buildings. There is some evidence that salts and acids have been absorbed and this may shorten the life of the slate on the roof.
Where a mixture of colours and profiles are present, i.e. "white" and terra- cotta colours and half round and "hogs back"profiles, the sourcing for extra tiles would need to be agreed. It is quite common for the ridge to be a mix of the older pale "hogs back" shape and other newer replacements. Occasionally blue hard tiles with an angular profile have been substituted in the past. These are not suitable or traditional and the opportunity can be taken to remove them.
Traditionally a Collyweston roof is ventilated naturally by the flow of air between the slates and at the eaves. Today there can be a conflict between the modern desire for high levels of insulation and the functioning and appearance of a traditionally laid stone roof. In fact stone slate roofs are thermally efficient, but with increasing pressure to convert former roof voids to living accommodation, the traditional ventilation is not possible. Every effort should be made to ensure that modern insulation and ventilation materials and methods do not have a detrimental effect on either the life of the slates themselves or on the overall appearance of the finished roof. This is particularly important on buildings which are listed.
Not all contractors and building professionals are completely confident in their use of natural lime. Up until WW2 it was in common usage and all craftsmen would have been knowledgeable about its characteristics and properties. All the buildings which now have Collyweston roofs were constructed and had their roofs laid without the use of portland cement.
English Heritage, all the local authorities in the area and the Collyweston Slaters Trust wish to encourage the re-introduction of cement free mortar for the repair and laying of Collyweston roofs.
There are frequent courses and training days held locally on the use of lime.
Collyweston roofs where the slates are pegged over the battens are becoming an increasingly rare survival. In the recent past they have all been routinely replaced with nails as they have come up for repair.
Where a pegged roof is being repaired or re-laid on a listed building this method of fixing needs to be replaced like for like unless specifically agreed otherwise by the local authority. Oak pegs are available.
A pegged roof will need torching on the underside.
Where some of the battens have failed, it is recommended to replace the whole area.
Traditionally the battens are made from riven chestnut or oak. It is not normally feasible to salvage these as the nail fixing has usually weakened the ends too much. In special circumstances, for example, where an historically important pegged roof is being replaced, consideration can be given to using oak or chestnut battens. As they are naturally more waney than modern sawn softwood they add to the character of the finished roof, as the lines of the tails of the slates are not as even.
Riven battens are available.
As with the note on ventilation, modern standards are being applied to traditional roofing materials and methods. It has become standard practice in recent years for roofing felt to be laid under the Collyweston slates when a roof is stripped and re-laid. This was never done in the past and there has not been any research into to the long-term effects of this practice on the life of the slates.
For most roofs it is advisable to use only the best quality breathable felt on the market, to reduce any risk of harming the slates to a minimum. However where the roof is over an open barn or where the underside of the roof can be readily seen and appreciated roofing felt should not be introduced, and indeed is an unnecessary expense. For buildings which where formerly stables, coach houses, lytch gates and domestic barns keeping the traditional appearance of the underside of a Collyweston roof is a worthwhile contribution to the character of the building.
The eaves of many Collyweston roofs are extended, by means of sprockets, so as to form a deep overhang. Before the general introduction of guttering this served to ensure that rainwater fell well clear of the building. Today guttering is mounted on long rise and fall brackets. Many examples are elegantly shaped and almost certainly made by a local blacksmith. Such examples should always be retained and any replacements made to match. There are blacksmiths who can do this work.
These brackets are usually spiked straight into a mortar joint in the wall as a method of fixing.
Soffitt and facia boards should never be introduced.
Replacement timbers need to be restricted to those whose condition threatens the structural soundness of the roof as most Collyweston roofs are supported by structures of high historic importance.
Any necessary replacement structural timbers to be on an exact like for like basis unless there are sound conservation reasons otherwise.
In general weak or failing timbers to be reinforced by "doubling up" or splicing in sound wood.
The introduction of steel should only be considered as a last resort.