Advantages of Slate
Slate benefits from a number of highly practical properties. It is exceptionally durable, unaffected by normal extremes of temperature and is highly resistant to acids, alkalis and other chemicals. It retains its colour, even in UV light and is impermeable to water.
In addition, it is non-combustible and compatible with all other building materials. Slate is easily maintained, making it an economical choice for all building purposes and is readily available In a variety of sizes, colours and textures.
A slate roof should last over 100 years - failure is usually caused by corrosion of the nails rather than of the slate itself. Thus, re-roofing recycled slates is a good option - cheaper than new slates and giving a far more attractive result than modern concrete tiles. Slate can, however, delaminate, becoming absorbent. Slates in this condition must be replaced.
When restoring a slate roof the options are:
New Welsh slates
These are very expensive, but the best option. They can last for more than 100 years. They come in a range of grey colours, and with plum and blue tints.
Second Hand Welsh slates
These tiles are a good compromise between cost and longevity
A good alternative to Welsh slate is Canadian slate.
Spanish slate is too soft and has an unpredictable grain.
Chinese slate does not have a British Standard kite mark. Furthermore it sometimes goes rusty due to iron oxide, and is brittle and therefore hard to fit without cracking.
New Spanish slates
These give an authentic look but because of irregular grain, they can split early in their life. They are likely to last 40 or 50 years, being softer than Welsh slate. They are blue/black in colour.
This is a modern material; a cheap solution but it does not weather, is very smooth, and does not have a long life. It looks very shabby in 10 years.
Although they are available in grey, they are not authentic looking, and because they are heavier than slates, the roof timbers typically need to be strengthened. When in an interlocking design, they look very different from other slates and tiles.
Here to help you through every step
In rare cases, a roof may have Westmoreland green slates. These are irregular in size and usually laid in diminishing courses with the smallest at the ridge and the largest at the eaves.
Slates may be nailed at the top or, more usually, in the centre.
When repairing an old slate roof, ensure that you match the size, texture, colour and detailing of the existing roof.
When seeking a roofing contractor, get references and visit their previous work. Slate should be sorted into three grades of thickness, with the thickest laid at bottom of the slope, the thinnest at the top. Battens need to be set out horizontally and vertically with string line before slating. Copper nails should be used for fixing. Check the lines of slates: good slating when viewed from the ground should have straight, perpendicular courses from the base to the top. There should be no kickers, tiles which are lifted up to leave a gap underneath. There should be no thick slates laid in amongst thin slates.