Collyweston Roofs

Janine Dykes


	The newly renewed Collyweston slate roof of the 18th century watermill at Sacrewell, Cambridgeshire (Photo by Rob Harvey for Messenger BCR)

Collyweston stone slate takes its name from the village of Collyweston in Northamptonshire. It is a traditional roof covering which has been used in the East Midlands since the Roman era. Indeed, during excavations on Roman sites at Great Casterton and Godmanchester hexagonal shaped slates with a single peg hole at the top have been found (Clifton-Taylor, 1987) and similarly, excavations at the Roman villa south of Apethorpe Palace and the settlements at Barnwell and Collyweston have uncovered further examples of Collyweston slate (Burgess, 1991).

GEOLOGY

Collyweston slate is a sedimentary fissile oolitic limestone from the Jurassic period (140–190 million years ago). Sedimentary rocks are formed by the gradual accumulation of layers of sediment, such as on the floor of the sea, and may include seashells (calcium carbonate), sand (from the decay and disintegration of pre-existing rocks) mud and/or other mineral sediments. The layered formation is retained in the new rock as bedding planes, affecting its physical characteristics. As Collyweston slate was formed by precipitation of calcium carbonate into the sand bed, it is not true slate, which is a metamorphic rock formed from a fine-grained sedimentary or igneous rock. However, like slate it is fissile: in other words, it splits easily along planes of weakness into sheets, albeit less thin than those of a true slate.


	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. (Photo by Janine Dykes)

	Stone slates can last for many years and can be reused, redressed and re-laid. (Photo by Janine Dykes)

Like all limestones, Collyweston slate is high in calcium carbonate, but about 30 per cent of the slate is quartz grains (essentially silicon oxide), and other particles include fragments of fossil up to 4mm in size, iron
oxide and white mica (a hydrated silicate of aluminium or potassium). Calcite is the cementing agent and at the bedding planes it is free of quartz particles, which causes the thin horizontal divisions known in Collyweston slate as the cliving planes (Sharp, 1873).

THE IMPORTANCE OF LOCAL BUILDING MATERIALS

Prior to the railways it was very difficult to transport heavy materials. As a result, most buildings were constructed with materials available near to the site, and only the most wealthy owners and the patrons of ecclesiastical buildings could afford to transport the materials they wished to use from further afield. This is why historically places were built from a small palette of materials, which created a strong local character. This has been diluted somewhat since the 19th century but is still strong in many areas across the country, including the use of flint stones in Norfolk, hard granite stones in the Lake District and limestone in the Cotswolds. Physical context strongly affects the image of a place in people’s minds and local materials are a fundamental element of what gives different places their character and identity. There is a close relationship between traditional materials and geology, which architectural historian and writer Alec Clifton-Taylor described as ‘the pattern of English building’ in his book of the same title. Local materials like Collyweston slate have distinctive physical and aesthetic qualities which carry on hundreds of years of traditional skills and detailing, and create strong connections between people and place. To allow their sense of pride and connection with the locality to continue for future generations, it is therefore important to replace local materials and their traditional detailing on a like-for-like basis.

METHODS OF COLLYWESTON SLATING

Collyweston slates are often nailed to battens, but traditionally they were fixed by a peg hooked over riven lath and with a dab of lime mortar at the head to secure it (head bedding). The newly laid Collyweston slates were then pointed with lime mortar (tail bedding) including the tail and the perpendicular joints, but no higher than the tail of the slate above. Internally, the underside of the slates was draft-proofed with a coat of lime plaster. Torching, as this practice is known, has largely been replaced by the use of roofing under felt, or underlay, but this can trap moisture where slates are fully bedded in mortar (see common failures, below).


	A traditional laced valley detail. (Photo by Janine Dykes)

Collyweston slates are laid in diminishing courses. This means that the slates get smaller with each course, with the smallest slates at the ridge line and the largest at eaves level. The stone slates are laid using the double lap slating method. This means that the slates should overlap the slate in the course next but one below by two or three inches (head lap) and each slate should also overlap the slate in the course below laterally by about half of its width (side lap). To be sure that the roof is waterproof both laps are required, but unfortunately they are sometimes reduced so that less slates are needed and more money can be saved. This is known as stretching the slates. Unusually large slates may be an indication that the side lap has been reduced which can compromise the 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.


	The newly renewed Collyweston slate roof of the 18th century watermill at Sacrewell, Cambridgeshire (Photo by Rob Harvey for Messenger BCR)

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 and repair work. This should include a comprehensive photographic record with a scale in the photos where necessary.

It should be noted that many buildings with Collyweston slate roofs are listed, and that protection extends to curtilage listed buildings within its grounds, such as barns and other ancillary buildings or structures. Any modifications that could alter the building’s original fabric or character will need listed building consent. The local planning authority should be enlisted to help decide if consent is necessary for the works proposed.

Common failures in Collyweston slate roofs


	Stone slate roofs can be repaired if they have minor slippage, but regular maintenance is needed to stop the damage worsening. (Photo by Janine Dykes)

	Inadequate ventilation and cracking occurs when inappropriate materials are used for repairs to stone slate roofs. (Photo by Janine Dykes)

The most common reason for reroofing is the failure of the fixings – usually the nails or pegs, but lath or batten decay is also common. The slates themselves, by and large, do not fail, but the hole at the top can be a weak point depending on the thickness of the slate and how they have been dressed. If a slate does fracture at this point, it can be redressed and used as a smaller one higher up.

Poor ventilation can prevent the roof from heating and cooling by effectively joining the slates together into one mass. Thermal expansion can lead to cracking of the slates and trapped moisture can greatly accelerate deterioration of the roof in general, including its timber supporting structure. Poor ventilation can be caused by the use of underlays with fully bedded systems, trapping moisture in the batten space. This is especially bad with vapour permeable underlays which encourage moisture into the batten space. If underlays are used there must be ventilation through the batten space, but this normally means raising the roof on counter battens, causing detailing problems at the verges and abutments.

In the worst cases poor ventilation and other ill-considered alterations can cause extensive decay to the main structural timbers due to insect or fungus attack and leaks.

Repair

Localised repairs can extend the life of a roof with some slate slippage. However, regular maintenance is crucial as areas of slippage can rapidly worsen due to the weight of the slate. In order to understand whether simple localised repairs would be sufficient or if more comprehensive repairs are required, the roof would need to be inspected from the underside. According to guidance on the Collyweston Stone Slaters Trust website, more comprehensive repairs can include the replacement of small areas of failed battens and the re-laying of local areas of slate to the same specification as the rest of the roof. If the roof was torched it can be re-torched from the underside.

Poor detailing and inappropriate materials can be very harmful to the slates themselves and result in failure of the roof. Some materials not only detract from the visual qualities of the roof but can also lead to poor ventilation and result in cracking and accelerated deterioration of the roof described above: these include spray-on sealing foams and bituminous products to stick the slates down. In addition any wholesale pointing up of the roof with a mortar which contains any cement will effectively end the life of the slates as it is almost impossible to clean and reuse them.


	It is inadvisable to use mortar containing cement when installing a stone slate roof, not least because it makes the slates very difficult to reuse. (Photo by Janine Dykes)

Other improper practices include pushing slipped slates into position and securing them with mortar along with repointing whole slopes and/or large areas of slates by pushing mortar into all of the joints. Both methods have been used frequently to repair and prolong the life of the roof, although neither should be recommended. (Collyweston Stone Slaters Trust, n.d.)

In conclusion, Collyweston slate is an important vernacular material that contributes to the local distinctiveness and character of towns and villages in the East Midlands. Traditional detailing is a vital part of this, particularly aspects that affect the roof’s visual qualities like valley treatments. Many lessons can be learnt from recording and replicating successful historic roofs. To ensure knowledge and detailing is continued along with conserving the historic environment, Collyweston slates roofs should always be re-laid according to their earlier layout and detailing, with the only exception being if unsuitable and unsympathetic methods have been used.

Author

JANINE DYKES is an inspector of historic buildings and areas for Historic England in the East Midlands. She joined HE in 2013 after working as a conservation officer in several local authorities. Collyweston slating has been a lifelong interest which has carried on into her professional life.

Further information

Roof Insulation and Ventilation

Sourcing Slates