CPD 1 2019: Welsh slate

Introduction

Slate has long been used as a building material in the UK. Quarrying has been taking place at Penrhyn Quarry, North Wales, since the 13th century, when farmers would dig up slate to roof farm buildings. While it has been used for roofing for hundreds of years, in recent times slate has also been used for cladding, walling, paving, flooring and landscaping, with its aesthetic qualities being recognised by designers, developers and clients. 

This CPD will look at the origins of slate, how it can be used, how the material should be installed and the regime of regulatory standards which those working with slate should bear in mind.

What is slate?

Most slate available today was formed from marine deposits of clay called aqueous sediments, created millions of years ago by heat and pressure within the Earth. Beds of slate or layers of rock were formed by the settling of aqueous sediments – these are mostly comprised of clay and lie parallel to the surface of the water in which they were formed. Clay deposits that split along the bedding plains are known as shale. 

Some clay beds were subject to tectonic forces and became inclined or broken. The compression of some of these beds produced cleavage plains perpendicular to the original bedding, whereupon slate was formed.

In the UK, slate deposits are found throughout the Scottish Highlands, in Cumbria and Cornwall, and across Wales.

Sustainability

As its name suggests, “natural” slate is a naturally occurring product that does not require any additional reprocessing, therefore it does not have the high levels of carbon and energy expenditure associated with other products such as concrete.

Indigenous slate also has a smaller transport impact than imported materials. However, granites used in facades are imported, resulting in higher transport costs and a larger carbon footprint. Other indigenous facade materials such as sandstones and limestones are less durable than slate. The by-product of slate production is also used to manufacture landscaping products, decorative and construction aggregates and industrial minerals.

Slate quarrying can impact a landscape; however, this is being minimised by the reuse of disused quarries for leisure activities, and restoring and remodelling sites close to the original landform.

Quality criteria for selecting slate

Slate has a compact structure with the ability to produce uniform, straight blocks. It is free from deleterious inclusions such as carbonates, clay minerals and oxidising metal sulphides and features a thickness in proportion to the strength of the rock. Good-quality slate does not fade, even after being subjected to a range of climatic conditions.

It is worth noting that there are issues with some forms of slate and users can encounter problems with poor-quality varieties – while it can be used internally with few issues, problems can occur when it is used externally.

The four main potential problems are:

• Ferrous metallic inclusions
• High carbonate content
• Low strength
• High rate of water absorption

Slate with poor strength is not durable enough for use in building environments as it will have a shorter life expectancy than good-quality slate, leading to rising repair costs. Some lower-quality slates can absorb relatively large amounts of water. 

Some sedimentary stones are incorrectly referred to as slate. These types of rock have a higher rate of water absorption than “true” slate, which can cause problems when the water freezes, potentially causing delamination of the stone.

Ferrous metal inclusions can be reactive and begin to oxidise when exposed to wetting and drying. Oxidising will result in unsightly staining at best, and at worst the metal inclusion will expand as it breaks down, cracking the surrounding material.

Exterior uses for slate

Slate is increasingly being used for exterior purposes such as walling and cladding as clients and designers recognise the material’s value-added visual qualities. It is also utilised in flooring and paving situations. 

Walling

For walling, slate is generally available in four finishes: rustic, pillared, cleaved and sawn.

Rustic walling – quarried and manufactured from rustic bed seams. Rustic walling generally gives a rugged natural finish, which is graded and selected to enhance its natural look and usually requires laying by a skilled stonemason.                                  

Pillared walling – slate has two workable planes: a cleavage plane and a pillaring line. The pillaring line is similar to the grain in a piece of wood. In good-quality slate, the pillaring line is at 90° to the cleavage plane. Pillared walling is usually produced with a pillared face. The back can be pillared, sawn or gauged.

Cleaved walling – split in the same plane as a roofing slate or paving tile with a lightly textured riven surface. In general, cleaved walling is used in either fixed or patterned course heights as any adjustments must be cut with diamond blades.

Sawn walling – has a manufactured sawn face finish. Sawn walling is produced to higher tolerances, making it easier to lay, and can be fixed with tighter joint lines than traditional stone walling. 

It is important to bear in mind that good-quality slate is suitable for use as an outer or inner leaf, while the highest-quality slates have water absorption rates and compressive strength equal to engineering bricks.

Mortar, which should be applied as with other natural stone walling, must comply with BS EN 998-2 Specification for mortar with masonry – part 2 masonry mortar. However, adhesive can be used as an alternative to mortar in order to provide tighter joints. If using standard wall ties, these should comply with BS EN 845-1. 

Cladding

Another exterior use for slate is cladding, which can enhance the visual qualities of a building. Stone cladding can be divided into three groups:

• Handset cladding
• Precast cladding
• Rainscreen cladding

Handset cladding – typically this method would see slate panels fixed to the existing structure. It can be used internally or externally. Panels can be wet- or chemically-fixed up to a height of 3m. Above 3m in height a mechanical fix must be used.

Precast cladding – as the name suggests, this system comprises precast concrete panels, which are manufactured off site. Fixings and slate facing are also installed in the factory. The modular system allows for quick and – crucially – safe installation. There must also be an effective water management system for joints between panels.

Rainscreen cladding – a typical rainscreen cladding consists of a non-load-bearing external cladding material, a ventilated cavity, vapour barrier and an inner leaf. Cladding material, fixed to a supporting metal grid, protects a building’s structure from water and features a ventilated cavity behind. Joints are left open, which equalises pressure in the rainscreen cavity. With the pressure equalised within the system, water is not drawn into the cavity.

Regular spaced drips are placed within the system to aid drainage of water from behind the facade. Spacing the drips will depend on system design for any individual project.

Fixing systems for rainscreen cladding

Natural slate is suitable for use with all common fixing systems used with natural stone. Popular fixing systems for natural stone panels used in cladding include:

• Dowel systems
• Anchor systems

Dowel system – this is known as a body anchor. It is fixed directly to the building structure and requires panels of at least 40mm in depth. Material costs are lower than other systems, as fewer components are used, but increased labour costs are incurred as the setting-out of anchors is crucial to maintain tolerances. It is also worth bearing in mind that loadings on panels are concentrated around fixings.

Anchor systems – anchors must be secured to a framing system, which is fixed to the building structure. This means material costs can be higher than other fixing systems. However, with the anchor fixings being inboard (for example, fixing is placed away from all vulnerable edges), good-quality slate panel thicknesses can be reduced to 20mm. Anchors can allow for adjustment to maintain clearance of joints, meaning quicker and easier installation.

Anchor fixings (pictured) spread loadings across the panel and in the event a fixing fails it will not come out of  the panel. In some cases, the system as a whole may be more cost-effective. 

Flooring and paving

Flooring – good-quality natural slate is an excellent material for flooring because it is durable and waterproof. In pedestrian-only or domestic areas where tiles are fully bedded in adhesive, a 12mm tile is suitable. In a commercial or public environment where a large amount of foot traffic and heavier loads are expected, a 20mm thickness is recommended. 

Slate flooring is usually available in four finishes: honed, riven, flame-textured and bush-hammered. The honed finish has a smooth surface that provides a low risk of slipping in dry conditions but a moderate risk of slipping in wet conditions. However, riven, flame-textured and bush-hammered finishes have a low risk of slipping, even in wet conditions. A bush-hammered finish is typically only used in commercial installations. 

Paving – when used externally, the thickness of the tiles needs to be increased. In a domestic situation a 20mm thickness is suitable. Thicker tiles are required in commercial applications or areas where heavier loads are expected. Typically, a 50mm thickness is suitable in an area of heavy foot traffic and/or light vehicle loadings.

Where there are likely to be heavy vehicle loadings, a minimum 80mm thickness would be required. This will depend on loadings and the sub-base. BS EN 1341 gives guidance for calculating the required thickness of natural stone paving and on slab thickness where the paving cannot be fully supported.

External paving slates are usually available in three finishes:

• Riven
• Flame-textured
• Bush-hammered

Laying and installation

As with other natural stone, slate is laid on a suitable level sub-base. For internal spaces, it should be fixed with suitable proprietary adhesive. For external spaces, it should be fixed with a sand and cement mix. Joints are internally sealed with proprietary grout. Externally, a sand/cement mix is typically used.

Unlike some stones, good-quality slate does not need sealing with a chemical sealant. However, some sealants can be used to enhance the colour.

Roofing

Natural slate has been used as a roofing material for more than 500 years. Natural slate is a double lap roofing product and has a minimum roof pitch of either 20° or 22.5°, depending on the location of the building and the size of the slate. There is a UK code of practice that covers slating and tiling.

The slating process: basic steps

BS 5534 Code of practice for slating and tiling

Setting out the roof – hole the slates, then sort into groups of equal thickness where required. Underlay should then be laid as specified and the roof marked out to correct the battening gauge. Battens should be marked out with perpends.

Slating – slates should be loaded out on the roof and under eaves courses fixed “bed up”. The first short course of slates trimmed on site that are fully lapped by the first full slate are fixed with the chamfered dressed edges facing downwards. Slates should be fixed to perpend lines using two nails.

Slate nails – the choice of nail is dependent on the prevailing local atmospheric conditions.

• Aluminium to BS 1202: Part 3 – suitable for most environments away from coastal locations.
• Copper to BS 1202 : Part 2 – suitable for all locations apart from those affected by extremely high levels of corrosive pollutants.
• Silicon-bronze. For use in areas exposed to high levels of corrosive pollutants.

Cutting slates

Slates should be cut no less than 150mm wide. Consideration should be given to slate-and-a-half widths where required, particularly at valleys and hips, which require slates to be cut diagonally to closely fit. Cutting of slates is normally with either a slater’s “knife and bar” or a site guillotine to provide a consistent “dressed” edge.

Fixtures and fittings

Calculations for nail fixings have changed in recent years and there is now greater emphasis on the effects of dynamic wind pressures at junctions and ridges. For standard sized slates in areas of moderate exposure with roof pitches of 30° and over such new calculations will have minimal impact. 

For larger sized slates and those in areas of severe exposure, longer or improved nails may be required at perimeters and junctions. Producers should be able to offer advice or provide calculations.

Ridge and hip tiles can no longer rely only on mortar bedding for fixing, but are required to have a mechanical fix as well. This can be achieved using a proprietary dry-fix system. If mortar bedding is used, then a purpose-made clip or screw must be used in conjunction with the mortar. A suitable sealing washer must be used if the ridges or hips have an exposed fixing.

Underlays must now be tested and sold with a label showing their suitability for use in different applications and areas of the UK. Natural slate is generally fixed to a batten gauge of 250mm or less. A membrane with a wind uplift resistance should be suitable for all areas of the UK.  However, when using slates in excess of 600mm in length, advice should be sought from a manufacturer.