A stitch in time… Makes good sense and saves money

Natural Slate and Stone

What to look for:

  • Cracks and splits
  • Slippage
  • Missing slates
  • Moss growth
  • Vegetation
  • Colour change
  • Delamination
  • Creepers

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A roof overdue for repair. Many of the stone slates could be salvaged and re-used, preserving the historic integrity of the building and saving the cost of replacement materials

Many vernacular buildings around the country were roofed in locally sourced slate or stone and many areas can be identified by their particular slates or slating techniques. Stone slate is an incredibly durable material and a well maintained slate roof should be expected to have a lifespan of 150 years or more.

Slates should be inspected at least once a year ideally before the onset of winter and after any storms or periods of extreme weather. Roofs should be checked for damaged, missing, or slipped slates by running your eye along the courses and working down the roof from the ridge to the eaves. Any vegetation growth or colour change should also be noted.

Not all colour changes, minor cracks or delamination (flaking slates) mean that the roof is in poor repair or needs replacing. As a general rule if more than approximately 20-25% of the slates are damaged or missing it is likely that a full strip and re-slate will be necessary.

Inspections should be undertaken in the roof space, ideally during wet weather; it is often here that most leaks can be identified.

In making repairs attempt to re-use as many existing sound slates as possible. Slates removed from the roof can be checked for soundness by lightly tapping the slate with a small hammer, sound slates should ‘ring’ when struck with dull sounding slates indicating delamination or other problems with porosity. In some parts of the country where different sized slates are used together on the same roof, some damaged slates can be resized and redressed for use on other parts of the roof. Even where this is not common slates can often be salvaged for redressing for use at abutments and flashings where cut slates are required.

With any replacement slates it is important that they should be of matching colour, sizes and thickness, and, where possible, from the same source as the original slates.

In historic building repair, if the traditional slate is no longer available second hand slates are often specified, however because the trade in these materials has led to the theft of roof materials, it is recommended that you should salvage what you can from the existing structure and use new stone slates for supplementary repairs.

When replacing slate it is imperative that the slating pattern on the roof is maintained in regards to lengths, widths, laps and coursing and these should match those on the original roof. Particular roofing details such as diminishing courses and any cut slating patterns (such as diamond or fishtail patterns that were common in the late 19th century) should be replicated. Texture is often as important as colour with, for example, a smooth Welsh slate will look out of place on a riven Scottish slate roof or vice versa.

If matching slates are proving difficult to source it is generally accepted to use a lighter colour slate of a similar hue as a replacement. Weathering and environmental soiling will in most cases dull the surface of the slate and add patina helping to visually match the slates. Darker replacement slates or incongruous colours are likely to remain noticeable for a long time.

When making repairs it is advisable to ask the roofing contractor for an additional supply of the slates to be used. These can be kept in the home and used for any emergency repairs without having to quickly source an appropriate match or make a temporary repair.

If many slates are loose or have slipped, check the condition of nails and battens or roof boards (known as ‘sarking’ in Scotland). Replace rusted iron nails and fixings with copper or stainless steel equivalents, galvanised nails should be avoided as they can be damaged when fixing or through rubbing with movement of the slate in winds, increasing their susceptibility to rust. Replace any battens or boards that are defective or weakened by previous nailings or water penetration.

In some cases ‘Tingles’ (narrow strips of copper or lead, or galvanised steel wire) can provide an acceptable temporary repair for slipped slates. The tingle is nailed to, or hooks over, the batten and is bent back over the bottom edge of the slate, holding it in place at the centre. Do not use sticky bitumen-coated fabric (applied over the roof) as a temporary repair, and avoid spray-on coating systems (on the underside of the roof): these will make it difficult to trace any further leaks to their source. They will also hinder re-use of the slates.

Moss can harbour damp and cause slates to deteriorate (as can wind blown dust, leaves and general detritus between the slates). Moss-removing washes are available from garden centres, however if access to the roof is readily available the brushing of the slates with a stiff bristled nylon brush with warm water and a little detergent or bleach may be all that is required.

Ivy and other climbing plants can grow branches thick enough to crack slates. Their stems should be cut as close to the root as possible and they should be allowed to wither before being gently pulled free. Various herbicides are available to treat the roots to prevent re-growth however care should be taken in their specification and application.

It is common practice to introduce a layer of roofing felt under the slates when replacing whole roofs, but it is not essential and not recommended if the roof has been close boarded. It is very important to ventilate the roof adequately and roofing felt if tightly applied can often impede this.

The introduction of insulation into any roof can bring with it unexpected problems; ventilation must be considered very carefully if damp and mould growth are to be avoided.

Geotextile (paper) underlays are often considered to be a good substitute for roofing felt and their use could be considered if a full strip is undertaken. Geotextile membranes are selectively permeable allowing for the passage of water vapour from inside the house whilst maintaining a weatherproof covering. However they are more suited to modern house construction where the aim is to create an impermeable outer shell. For traditional buildings that need to breathe a geotextile may not bring any major advantage.

Substitute materials, such as artificial slates made of fibre resin, concrete or ‘reconstituted stone’, are not a match for natural materials, and should be avoided on historic buildings. A specialist local roofing contractor is essential, as materials and traditions vary from place to place.