Repairing Clay-tiled and Slated Roofs

Jonathan Greenough

 

Welsh slate roof with conical slated fleche at roof junction  
Hand-cut Welsh slates laid to diminishing
courses on the Church Institute, an Arts &
Crafts building of 1912 by Herbert Luck
North in Llanfairfechan, North Wales 
 

There is evidence of the use of slate, stone and clay tile roofing materials in Britain dating back to Roman times. Although for much of the early medieval period thatch of one form or another predominated, by the 12th century the most important buildings tended to be roofed in more permanent materials once again. Church and cathedral architecture in particular demanded roofing materials of the very highest quality, and slate, stone and fired clay fixed to timber battens or sarking boards quickly emerged as the standard roofing system from the later Middle Ages to the present day.

Nationally, the appearance of roofs varied widely, from large stone slabs in the lake district, to the smaller rough stone-slates of the Cotswolds, from the fine blue and purple slates of North Wales, to the green-hued slates of Edinburgh and Cornwall, and from the plain clay peg tiles of the South East, to the pantiles of Lincolnshire and Humberside. Detailing too was affected by the materials used as well as by local traditions, until the advent of the canals and then the railways brought access to new ranges of materials.

SLATE

Prior to the mechanisation of Britain’s slate quarries and the laying of the railway systems in the early 19th century, natural slate was generally produced in varying sizes which provided a ‘random’ slate roof. This meant that all sizes produced by the quarry in the process of cleaving the rock from the face were used, eliminating unnecessary wastage. Roofs were fixed with the largest slate at the eaves diminishing to the smallest slate at the ridge. This ensured that the area of the roof that carried the most water made use of the slates that provided the most lap.

Once the quarries were furnished with machines and linked by rail, it became far more efficient to transport slate in standard sizes, despite the fact that this produced much more waste (evidenced by the huge slate tips we see today). Fixing standardised slate also saved many man-hours on site and didn’t require the same level of expertise.

The vast majority of slate produced around the world today is sized slate, but it is still possible to source random slate from UK quarries.

Stone-slates have been used in many areas across the UK for centuries and were generally laid in a similar fashion to random slate. However, these tend to be formed in much thicker and heavier pieces than true slate to allow for their relative weakness and porosity.

TILE

Double-lap clay tiles were the main alternative to slate in most areas of the UK. Produced in the Midlands and the South East in particular, these tiles were laid in much the same way as slate, being double lapped. In other words, because plain tiles do not overlap their neighbours on either side, there needs to be a double thickness of tile over every part of the roof to keep it water-tight. The tiles therefore overlap not only those in the course below but also those in the course below that.

Many tiles were produced with ‘nibs’ at the top in order to hook them onto the fixing battens. This lessened the need for additional fixings such as nails, and made them relatively quick to install given their small size, which was generally around 10" x 6".

Clay tiles are still produced and used on a large scale throughout the UK, and a few specialist firms can still produce tiles by hand, using clays and firing techniques to match the less regular appearance of older tiles.

Single-lap clay pantiles were traditionally seen all down the eastern coast of the UK. This is because they were originally produced in countries such as Holland and Belgium and were imported across the Channel and North Sea, tiles being an ideal ballast material for ships returning from exporting British goods to the continent. Bridgwater, Somerset was another centre of production.

Because they overlap neighbouring tiles in the same course, single-lap pantiles only need to overlap a single course below them to keep the roof water-tight.

Church roof in which many slates have been re-fixed with tingles Partially laid clay tile roof with unlaid tiles stacked at intervals
Tingles (metal clips) pepper the roof of a church at Llangollen in North Wales, indicating widespread failure of the nails fixing the slates. New handmade nibbed clay tiles being laid across new battens on breathable felt (Photo: Jamie Moore, Recclesia)

MY CHURCH ROOF IS FAILING: WHAT ARE THE LIKELY CAUSES AND IS IT REPAIRABLE?

Slate, stone and fired clay are all extremely long-lasting materials, and fixed to modern standards can last for centuries. However, one weakness is common to all types of traditional pitched roofing: fixing failure.

Metal fixing nails are particularly vulnerable. Once exposed to atmospheric conditions, corrosion causes ferrous fixings to fail far more rapidly than the roofing material itself. This is why slipped slates and ‘tingles’ are a common sight on old slate roof slopes. Tingles are narrow strips of lead, copper or other metal which are nailed to the batten at one end and hook under the lower edge of the slate at the other to form a simple, if unsightly, temporary fix. However, It isn’t just the slate nails that can fail prematurely, batten nail failure can lead to whole courses of slate or tiles slipping down the roof as gravity slowly takes over.

  Sample tile courses laid next to originals which weathered to mottled grey
  Two samples of tiles being compared with the weathered originals to ensure the most suitable match (Photo: Jamie Moore, Recclesia)

Clay nibs and riven oak pegs, although more durable than galvanised steel nails, also present a weak point. Clay nibs can fracture due to thermal stresses in the roof covering, and oak will eventually decay.

However, after nails it is the failure of metal flashings that is the most common reason for roof failure on historic buildings. Usually of lead but sometimes of other materials such as copper, these flashings were designed with prolonged exposure in mind, and well-detailed leadwork can last for centuries. The most common reasons for failure are not defects in the material itself but poor design, bad installation and inadequate maintenance. Roof failure will normally make itself known where the rainwater is most concentrated such as the lower portions of valleys, parapet gutters and outlets. Internally, problems will generally become evident on the outer walls. It is vital to investigate the source of these problems at the earliest opportunity if outbreaks of dry rot and extensive structural damage are to be avoided.

Repair or replace? In many instances it will be quite possible to carry out localised repairs in order to prolong the life of an entire roof. However, if nail failure is advanced then a complete re-roof may be the only answer. If water ingress has gone undetected or ignored for long periods then structural work may also be necessary.

Simple maintenance Whether there is internal evidence of water ingress or not, it is always prudent to carry out regular visual inspections of all roof areas and attend to any minor defects such as the odd slipped slate or tile, damaged flashing or blocked rainwater channel. Parapet gutters, valleys and rainwater channels must be regularly cleared of leaves, plant growth and even dead pigeons. Keeping up with simple regular maintenance in this way can prolong the life not just of the roof, but the entire building.

REPAIR AND REPLACEMENT OF SLATES AND TILES

Individual slates can be successfully replaced but it is not usually quite as simple as repairing clay tiles, which can often be slotted into place without too much technical know-how. By far the best solution is to remove all the slates above the offending area and re-fix in same manner as they would have been fixed originally, usually with two clout nails.

Incomplete courses of polychromatic clay tiles with colour variation used to form pattern  
Church roof with banding and diamond-shaped patterning formed using colour variation in polychromatic clay tiles  
New polychrome clay tiles made by Dreadnought for the chancel of All Saints’, Nocton near Lincoln (by George Gilbert Scott, 1862). Clay which was naturally the
right colour was sourced to match the originals, rather than dyed, and those original tiles found to be still serviceable were reused on the south face of the nave.
(Photo: Dreadnought Works, Hinton, Perry Davenhill)
 

It may not always be practical to remove large areas of roof just to replace one or two slates near the eaves, so a little skill can be required to make a successful replacement. Unfortunately, many individual slate repairs have been carried out in the past using inappropriate methods such as adhesives and tingles, neither of which can be considered a permanent fix.

Successful, permanent and invisible slate repairs can be achieved in a number of ways. For example, copper straps can be threaded through pre-made slots in the slate and fashioned to create a wedged spring, which is flattened while the slate is being pushed up into place, then springs out behind the fixing batten when arriving at the correct position. Because the slate is not secured into the batten, as with nailing, it wouldn’t be appropriate to fix a whole roof in this way but as an individual repair it is effective because the surrounding nailed slates will prevent uplift.

There are a number of products on the market that work in a similar way such as the ‘Jenny-Twin’ fixing, which consists of a pair of aluminium clips that clasp the sides of the slate and are secured by a tongue that passes through a drilled or punched hole in the slate and a swivel ‘tail’ that rests on the batten. Installation can be a bit fiddly but the clips are very effective once in place.

While tingles have had a relatively useful role in signalling whether a roof needs replacing, they are only a temporary solution and need to be replaced by something more permanent. As long as repairs are recorded it will still be possible to monitor the overall condition of the roof and determine when replacement is necessary.

With double-lap clay tiles, the first indicator of the end of the roof’s useful life is finding whole rows of tiles slipping due to corroded batten nails. These tiles generally have either ‘nibs’ or oak pegs which allow them to hang onto the batten even if the tile fixing nail has corroded. Only a complete re-roof should be considered when this defect is identified as it will not otherwise be possible to carry out an effective, lasting repair. As with slates, it may well be possible to reuse some proportion of the existing tiles if the aesthetics and historic interest of the roof are to be preserved. However, specifiers and clients should be realistic when considering reusing roofing materials and should not expect to save more than 50 per cent in most instances even if the material appears to be in good condition.

If renewing a roof completely, it is usually best to replace materials like-for-like in order to maintain the authenticity of the building. Fortunately we are blessed in the UK with a stable supply of traditional materials such as Welsh slate and clay tiles and therefore replacing these can be relatively simple logistically. However, difficulties may be encountered finding some stone slates, and Scottish slate is now rare. In such cases it may be necessary to use material of similar geology.

Clay tiles can be made by specialist manufacturers to match almost any profile, colour or texture. Some may be available from standard ranges, but specials are often required to match regional variations, particularly where pantiles and romans (flat roofing tiles with one or more rolls) are concerned.

Beneath the slates and tiles themselves things can be quite different where a roof covering has been renewed, as modern fixings and methods have evolved to last far longer than their predecessors. Also the introduction of insulation and ventilation systems to comply with modern regulations can have a major effect on detailing. It should be remembered that it is now mandatory to install insulation to strict current specifications if replacing 25 per cent or more of any roof area.

MODERN VERSUS TRADITIONAL FIXING METHODS

There is no practical reason to install slates and tiles using historic fixing methods. Materials should be fixed according to current best practice. This shouldn’t mean that the roof will look any different to the original, but it will last longer. Laying slates or tiles using the most appropriate modern fixings will also allow future generations to date the work and assess its suitability.

 

 

 

Historic Churches, 2012

Author

JONATHAN GREENOUGH is a Heritage Roof Master and a Fellow of the Institute of Roofing with over 27 years’ experience. His heritage roofing expertise has been called upon for projects throughout the UK and beyond, including at Buckingham Palace.

Further information

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