Slate Roof Facts

What is Slate?

Slate is a type of metamorphic rock, meaning a form of existing rock which has undergone transformation due to intense heat and pressure. It is a fine grained, frequently grey coloured rock however, slate occurs in a variety of colours even from a single locality. For example slate from North Wales can be found in many shades of grey from pale to dark and may also be purple, green or cyan. Slate is not to be confused with shale, from which it may be formed, or schist which has undergone much more intensive amounts of heat and pressure.

How Your slate Roof is laid

Slate is laid on the battens in a 'brick bond' pattern, i.e. with the joints between them aligned with the centre of the slates above and below, and with about 3mm between the sides.

This layout gives a double lap covering, (i.e. the top part of each slate is covered by two slate, the centre of the slate of the next row and the lower part of the slate above that). The overlap will depend upon the slope of the roof - the steeper the slope, the smaller the overlap required.

The lowest row of slate at the fascia board is made up of shorter slate to provide the double lap for the first row of full ones, the lower edges of these eave slate and the first row of full slate should be the same

The top row of slate at the ridge is also made up of half slates to give the next row down a double lap. The top edge of the second row down must be at a level where it will be covered by the ridge tile.

How roofing slate is cut

Slate is by nature a layered material which can tend to flake.

Using a pair of slate cutters is obviously the preferred option, (these support both sides of the cut underneath while the cut is made by a blade coming down between the supports).

Alternatively if the slate is not too thick, hold the slate face down overhanging the edge of a flat surface along the line to be cut. Working towards you, use the edge of a bricklaying trowel to chop along the line and remove the waste.

How Slate roof tiles are fixed

Aluminium nails should be used in preference to galvanised nails as aluminium will not corrode whereas the galvanised coating will tend to. Alternatively copper or stainless steel nails can be used.

Slate can be either nailed at the centre line or near the top. When slate is nailed at the top, the nails are covered by two slates and so are less exposed, but, being secured at only one end, they are at risk of being lifted by the wind and are more likely to be broken. Nailing slates at the centre line reduces the protection to the nails but reduces the risk of them being lifted by the wind. Generally, it is better to nail them at their centre line using non-corroding nails.

One needs to be very careful not overdrive the nails, just pinching the surface of the slate is enough - overdriving the nails risks damaging the slate (from the head of the hammer hitting the surface) and putting unnecessary stress onto the slate which will make it more likely to break.

Most artificial slate come with the holes already drilled, where necessary nail holes should be drilled at least 30mm in from the edge and 25mm in from the top.

Start aligning and nailing at the lower row of half slates at a verge or other appropriate place - work away from this point along and up the roof. Keep an eye on the position of the holes in relation to the battens, this should prevent you from going off line.

Different types of Roofing Materials including slate

The outer part of a roof shows great variation dependent upon availability of material. In simple vernacular architecture, roofing material is often vegetation, such as thatches, the most durable being sea grass with a life of perhaps 40 years. In many Asian countries bamboo is used both for the supporting structure and the outer layer where split bammboo stems are laid turned alternately and overlapped. In areas with an abundance of timber, wooden shingles are used, while in some countries the bark of certain trees can be peeled off in thick, heavy sheets and used for roofing.

The 20th century saw the manufacture of composition shingles which can last from a thin 20-year shingle to the thickest which are limited lifetime shingles, the cost depending on the thickness and durability of the shingle. When a layer of shingles wears out, they are usually stripped, along with the underlay and roofing nails, allowing a new layer to be installed. An alternative method is to install another layer directly over the worn layer. While this method is faster, it does not allow the roof sheathing to be inspected and water damage, often associated with worn shingles, to be repaired. Having multiple layers of old shingles under a new layer causes roofing nails to be located further from the sheathing, weakening their hold. The greatest concern with this method is that the weight of the extra material could exceed the dead load capacity of the roof structure and cause collapse.

Slate is an ideal, and durable material, while in the Swiss Alps roofs are made from huge slabs of stone, several inches thick. The slate roof is often considered the best type of roofing. A slate roof may last 75 to 150 years, and even longer. However, slate roofs are often expensive to install – in the USA, for example, a slate roof may have the same cost as the rest of the house. Often, the first part of a slate roof to fail is the fixing nails; they corrode, allowing the slates to slip. In the UK, this condition is known as "nail sickness". Because of this problem, fixing nails made of stainless steel or copper are recommended, and even these must be protected from the weather.

Roofs made of cut turf (modern ones known as Green roofs, traditonal ones as sod roofs) have good insulating properties and are increasingly encouraged as a way of "greening" the Earth. Adobe roofs are roofs of clay, mixed with binding material such as straw or animal hair, and plastered on lathes to form a flat or gently sloped roof, usually in areas of low rainfall.

In areas where clay is plentiful, roofs of baked tiles have been the major form of roof. The casting and firing of roof tiles is an industry that is often associated with brickworks. While the shape and colour of tiles was once regionally distinctive, now tiles of many shapes and colours are produced commercially, to suit the taste of the purchaser.

Sheet metal in the form of copper and lead has also been used for many hundreds of years. Both are expensive but durable, the vast copper roof of Chartres Cathedral, oxidised to a pale green colour, having been in place for hundreds of years. Lead, which is sometimes used for church roofs, was most commonly used as flashing in valleys and around chimneys on domestic roofs, particularly those of slate. Copper was used for the same purpose.