Scottish slate roofs have a number of characteristics which make them well suited to both the local climate and the nature of the material produced by slate quarries in Scotland. These include features such as diminishing courses, random lengths and widths of slate, single nailing and laying onto sarking board rather than battens (Figure 1).
|Figure 1 – Scottish slate roofs, constructed to traditional patterns and details, have proven to be durable.||Figure 2 – A squared and shouldered Scottish slate with a single nail hole|
When repair work is being carried out to Scottish slate roofs it is important that these differences are understood and work specified accordingly. It is also important to treat the roof as an integrated whole rather than a series of distinct elements.
As well as the slate covering, the sarking board onto which the slate is fixed, lead flashings, ridges and a range of other features are all integral to the proper functioning of a roof. All of these elements should be considered when repairs are being specified and carried out.
SCOTTISH SLATE AND SLATING
While some Scottish quarries produced slates in standard sizes, most produced slates in random sizes and thicknesses. The latter were generally intended for the domestic market although some were exported. Slates of all sizes and grades, including those of random width and length were, on occasion, also imported from other parts of the United Kingdom which saw Scotland as a market for this output, although this was never common practice.
Variations in the size of slates from Scottish quarries led to the practice of laying the material in ‘diminishing courses’, ensuring the most efficient use of the quarry output. This means that the largest slates are laid at the base of the roof with the smaller slates laid nearer the ridge. The slates were fixed in place using a single nail in the centre of the slate head. Slates were trimmed at the shoulders of the head to make it easier to move slates aside to access any that were broken or damaged. This type of slate is known as ‘squared and shouldered’ (Figure 2).
The pattern of a Scottish slated roof is dictated by the size of the slates and although it has a distinctive character, this varies from one building to another.
A further difference in Scottish slating practice is that Scottish slate roofs are almost always laid onto sarking boards rather than onto battens. Such boards were laid with a ‘penny gap’ between each board – a gap of about the thickness of an old penny. The gap performed the twin function of allowing for any expansion and contraction within the wood, and keeping the roof space below well ventilated and allowing the dispersal of occasional wind-driven rain from under the slates, keeping the adjacent timber dry and free of decay.
SURVEY AND INSPECTION
Figure 3 – Scottish slate has decorative as well as functional applications, as the treatment of this turret illustrates.
|Figure 4 – A detailed inspection is key to correct specification of repairs.|
As with any building element, regular inspection of Scottish slate roofs will allow emerging repair issues to be tackled early (Figure 4). Repairing small problems as they arise will help avoid the need for larger, more costly repairs in the future.
Figure 5 – New slate should match the characteristics of Scottish slating practice as shown on this new roof with its distinctive diminishing courses.
A detailed roof survey should be carried out prior to any repair work taking place to ascertain whether localised patching or full re-roofing is required. This will include taking photographs to ensure that the pattern of the roof is maintained in any required work. The scope of work should then be agreed with a contractor, allowing for the likelihood that when work starts it may be discovered that slightly more slates than first anticipated will need to be replaced.
A common cause of a need to re-slate a roof is ‘nail sickness’, the rusting away of the nails which hold the slates in place. This might be due to poor quality or simply age. Depending on the quality and durability of the slate, decay and softening of the top of the slate can also take place around the nail hole. As a general rule, if more than 25 per cent of the slates on a roof are loose or defective then re-slating will be required (Figure 7).
When considering repairs, adjacent areas and elements should be investigated, including lead flashings, lead valleys and related details as well as masonry work, chimneys and skew copes (the coping stones that cap a pitched gable parapet). If there are external signs that a flashing has becoming loose or misaligned, repairs should be carried out as soon as possible.
Flashing defects can allow significant amounts of water to penetrate into the fabric leading to decay of timber and a build-up of moisture in masonry. An inspection internally of the underside of the roof will also show whether there is any water ingress and aid the identification of problems which may not be obvious externally.
SOURCING SLATE FOR REPAIR WORK
As no Scottish slate has been produced for over 50 years, sourcing suitable replacements for use in repair work can be challenging. While salvaged or reclaimed Scottish slate may be available in some instances it is likely that new slate will be required for many jobs due to the scarcity and cost of reclaimed material.
When procuring new slate, care should be taken to match as far as possible the dimensions, thickness, texture and colour of the original slates (Figures 5 and 6). It is also important to replicate the original pattern as far as possible, using the same number of courses, sidelap and treatment to elements such as skews and dormers.
Several producers offer a range of sizes to allow the replication of a Scottish slate roof with its characteristic diminishing courses. However, mechanised production means that the range will not be a full mixture of random widths and lengths.
|Figure 6 – In this case a good match has not been achieved.||Figure 7 – Where a roof has suffered the loss of many slates and is showing signs of further deterioration, re-slating is likely to be required.|
SMALL SCALE REPAIRS TO SLATE ROOFS
Roof access and forming a working platform should be considered carefully to ensure the safety of the operative and to avoid damaging adjacent slates. The replacement of single slates in a Scottish slate roof is a relatively easy task as they are normally single nailed (Figure 8). The broken off head of the slate is removed with a slate ripper (Figure 9) and a new slate is dressed to size and nailed in its place.
The issues outlined above regarding the sourcing of slate for repair work are as relevant for small patch repairs as for larger scale re-slating work.
RE-SLATING SCOTTISH ROOFS
Assessing whether a slate roof needs repair or complete replacement has to be made based on the amount of repair, the condition of the slates and other plans for the building. In some cases the existing sarking boards may be in sufficiently good condition to allow the new slates to be fastened onto the existing boards.
If the sarking boards are damaged and require renewal they should be re-laid with a small gap to allow for ventilation. The use of bituminous under-slate felt or other vapour barriers under the slates which prevent water vapour movement can lead to high humidity in the roof space and can have detrimental effects on the roof in the long term.
If a membrane or vapour control layer is being specified it should be of a type which allows moisture to move through the sarking board (Figure 10). Modern proprietary roofing papers are designed for this purpose, and if correctly specified remove the need for obtrusive roof vents on the pitches.
|Figure 8 – Individual missing slates can be patch-repaired fairly easily using single nailing. The double lap characteristic of Scottish roofs is also evident.|
Figure 9 – Removing the head of a broken slate using a slate ripper
In Scottish practice, slates are double lapped so that each slate covers part of the slates in two courses below (Figure 8). The head and side lap must always be large enough to prevent wind driven rain penetrating underneath the slate.
The head lap or ‘cover’ is the distance by which the leading (bottom) edge of a slate overlaps the nail hole of the slate two courses below and is usually 50-75mm, decreasing to under 50mm at the top of the roof to allow the smaller slates to lie properly.
|Figure 10 – These sarking boards are in sufficiently good condition for replacement slates to be laid onto them with the addition of a breathable membrane.|
Figure 11 – Lead detail at a gable parapet
The varying widths of slate mean that an even side lap cannot always be maintained. (The side lap is the distance between the edge of a slate and the edge of the slate which it partially covers in the course below.) Generally the side lap is worked out by placing the perpendicular joint between two slates in a course approximately centrally to the slate in the course below.
In practice, this is difficult to maintain with slates of varying sizes so it is usually assumed that the side lap will be at least 50mm. One and a half slates are often used on the edges of the slopes, while narrow bachelor or ‘in-bands’ slates are used mid-row to regulate the side lap.
Slates should be single nailed as in traditional practice. Fixing with non-ferrous nails is likely to prove more durable in the long term and copper nails are often specified. Traditionally it was usual to ‘cheek nail’ (or side nail) every sixth course to help keep slates in place and T-shaped nails were manufactured specifically for this purpose. In exposed areas or on turrets this can be increased to every three courses. This practice is likely to be beneficial in repair work. In some cases slates in exposed areas are bedded in mortar to keep them in place, particularly when working with smaller sizes nearer the ridge.
REPAIR OF LEADWORK ON SCOTTISH SLATE ROOFS
The correct repair of leadwork is a subject in its own right but it in the context of slate roofs it is an important part of the works and should always be done at the same time. As mentioned, lead is often used in situations such as valleys and at points where masonry such as parapets and chimneys meets slate roofs. Where such leadwork is being repaired it is important that it is correctly detailed and secured and that lead of a sufficient thickness (or ‘code’) is used to ensure a durable repair. For example, Code 7 lead is recommended for valleys.
Relevant trade association guidance should be consulted and Scottish practice in this area followed when repairs are being carried out. On many traditional roofs mortar skews were used, and while their replacement with lead is likely to result in a more durable repair, the change in material, and the angles in which the slates sit will change the appearance of the roof (Figure 11).
Figure 12 New slate should match the characteristics of Scottish slating practice
The ridges of slate roofs are an area of vulnerability and are treated in a number of ways. Lead is often employed along ridges, applied over a timber former called a ridge roll. Where cost is an issue zinc ridge of standard length can be used. In some cases ridges are formed of terracotta or stone. Any repair or replacement should be carried out using like-for-like materials to ensure that the visual integrity of the roof is maintained. Whatever material is used it is important to ensure a firm fixing or bed to aid the securing of the smaller slates at the top of the roof.
WHY DOES IT MATTER?
As with all natural building materials, the use of slate changes regionally, according to the properties and characteristics of the material, according to climate, and according to fashion. Often seemingly minor variations in construction technique or material have a disproportionate impact on colour, texture and pattern, shaping the appearance of a building or structure, and contributing to distinct regional styles of architecture.
Slate roofs are an integral part of our heritage, and a detailed understanding of the material and its traditional use is a prerequisite of successful and sympathetic repair work. Both the replacement material and the pattern in which it is laid must match the original as far as possible to preserve not only the roof’s character but also its function.
Source : www.buildingconservation.com – Moses Jenkins
Photo : WD Cameron Slaters