ROOF SLATES

Natural Slate: Slate is a metamorphic rock, formed under great pressure from what was originally mudstone between 350 and 500 million years ago.  Traditionally slates for the UK were produced in Wales although slate mines are found all along the west coast from Cornwall to Cumbria. Today, slates are imported from a wide range of countries in Europe and places varied as Chile and China. All slates mines produce slate with their own distinct colour variations, Welsh slates are available in with grey, blue/ black, reds,  purples and green tones. Natural slate is available in wide range of sizes although smaller sized slates should not used on low pitch roofs. A standard specification for fixing slate would call for natural slates fixing each slate with two 30mm aluminum alloy nails to and including 25x50mm tanalized battens fixed with galvanized nails over BS 747 1F roofing felt.

Man-made Slate: Fibre cement slates are a recent development compared with other roofing materials. Originally they were made from Portland cement reinforced with asbestos fibers. Health concerns lead to the development of today's modern slate which uses synthetic fibers and fillers to replace the asbestos. Available in a variety of sizes, the surface finish can be a plain matt surface or molded to form a textured appearance similar to that of natural slate. All leading manufactures produce a wide range of accessories and fittings which can give different effects and appearances which may not be available with natural slate. It is important when fixing the slates that a copper disc rivet is used to  secure the bottom of the slate which will prevent the slate from distortion. A standard specification for fixing slate would call for fibre cement slates fixing each slate with two 30mm aluminum alloy nails and one copper disc rivet  to and including 25x50mm tanalized battens fixed with galvanized nails over BS 747 1F roofing felt.

Prior to any work at the eave consideration should be given to installation of a ventilation system. The roofing underlay should drain any rainwater into the gutter. Any water traps behind the fascia should be avoided. The first tile batten is positioned so that the first full slates drain water into the centre of the gutter. The first course of tiles to be laid are called eave slates. These are normal slates cut shorter to the required length, which are to maintain a headlap on the first course of roof slates and should be fixed with two tile nails. The first course of slates can now be laid and  again should be fixed with two tile tiles. When fixing fibre cement slates an addition course of eave slates are required to be fixed under the eaves course. These are require to support the copper disc rivet.

The ridge tiles are usually the last part of a roof to be completed. The last course of tiles to be laid are called top slates. These are normal slates that are cut shorter in length, which are to maintain a headlap on the last course of roof slates and should be fixed with two slate nails. The ridge tiles are then bedded into a mortar which can be coloured to match the colour of the roof slates and pointed to give an even appearance. Ridge tiles should not be solid bedded. The tiles should only be bedded at the edges of the tiles to prevent shrinkage of the mortar.

The roof battens should extend on to the outer wall. A fibre cement undercloak is inserted between the top of the wall and the battens. The purpose of the undercloak is to close of the cavity wall and to support the bedding material. The undercloak should extend over the wall between 38-50mm.  The roof slates can be bedded in a sand cement mortar that should be coloured to match the colour of the roof tiles. Slate and half are used in alternate courses to maintain the broken bond of the roof slates. All slates at the verge should be fixed with two nails. Upon completion the excess mortar can be struck off and pointed for a neat finish.