CPD 03 2022: Specifying Natural Slate

This CPD sponsored by Cupa Pizarras will examine the considerations in specifying natural slate for roofing and cladding. Deadline: 10th May 2022

Natural slate is a good ecological option to consider for any architectural project, as the production process uses relatively little energ

Introduction 

This CPD will look at the strengths of natural slate, its suitability for roofing, together with its sustainability, thermal and rain-cladding credentials. The feature will take readers through the steps necessary to ensure quality is maintained from sourcing the material to its use on a construction project, while adhering to the required building standards regime. 

Sustainability 

Natural slate is the most sustainable product for roofs and facades. Shaped by nature over millennia, it acquires properties that no artificial product can imitate. From the quarry to its final destination, natural slate goes through a simple production process, one where no chemical products or industrial combustions are employed. 

It uses far less water in its sourcing and extraction than other materials such as fibre cement (11 times less), zinc (135 times less) and clay (half as much). It also takes much less energy to extract – six times less than fibre cement, four times less than zinc and half as much as clay. Its extraction also causes less atmospheric pollution. 

Slate is usually surface quarried

A lifecycle analysis, which evaluates the environmental impact of a product from extraction to the final disposal, confirms that natural slate is a good ecological option to consider for any architectural project. Furthermore, the Inventory of Carbon and Energy, an independent database located at the University of Bath, highlights natural slate as the material with the lowest number of adverse impacts on the environment, and Dr Joan Walsh of Glasgow University says that while there is a large variation in the embodied carbon of natural roofing slates, its embodied carbon value is at most 16% of that of other roofing materials. 

Sourcing and quality  

Slate is usually surface quarried, and sometimes mined where geological conditions make this activity preferable. Once the purity and viability of the slate composition has been ascertained, development of the extraction site begins. Once earthworks and other initial tasks have been finished, the slate is removed in large blocks using whatever technique is best suited to the individual quarry.  

The large blocks are sorted and laser-sawn along their natural cleavage planes into smaller blocks, which are then hand-split by skilled workers into slates of the correct thickness. The edges are finished to give a natural chiselled look, which contributes to the aesthetic effect once installed. They can also be pre-holed, if required, to minimise the risk of splitting when fixing. 

Traceability 

Checking at every stage in the production process is vital to overall quality control. A good manufacturer will employ a process of bar coding which ensures superior traceability, enabling slate matching if additional slate is required for a particular project.  

Checking at every stage in the production process is vital to overall quality control

Barcodes contain the slate’s origin, type, dimensions and the results of tests to ascertain the required quality threshold. They also provide information about where and when the slate was processed and who worked on that particular batch. This helps to meet standards, as under BS EN 12326 all slate consignments should now have an “accompanying commercial document”. 

Production standards 

There are several international and national standards governing the quality and origination of slate. It is important that the slate being specified meets – or preferably exceeds – such standards in order to ensure the quality and longevity of the structure they are a part of. 

When slate is specified, it is vital that it meets or exceeds current national/international standards. The level of quality is certified by Switzerland-based certification body IQNET with the ISO 9001:2015 standard. Each quarry should have quality approval from the national laboratories of the relevant country. 

In the UK under BS EN 12326-1:2014 a hundred samples are selected by the company commissioning the test. According to the European standard BS EN 13501-1:2018 Slate and natural stone products, slate is class A1, a non-combustible material. The reaction to fire of stone and slate products must be classified according to EN 13501-1 and declared as class A1, without the need for testing. 

When specifying roofing slates, the National Housebuilding Building Council’s advice is to ensure they comply with BS EN 12326 and that they are graded ‘W1, S1, T1’.  

W1 indicates the slate’s water absorbency and how it reacts when it gets wet; S1 shows how much carbonate the slate contains – important since carbonate can quickly dissolve causing material loss and structural weakness, and T1, which covers reactive metallic minerals.  The Thermal Cycling Test indicates the stability of any reactive metallic minerals (often called pyrites) that may cause staining, pitting or delamination. 

In the UK, slate on all homes must meet T1, S1 and W1 criteria to be compliant with the various UK standards. 

Only the sample being tested is deemed to have complied with the quality standard achieved. Testing is carried out every two years unless production exceeds 12,000 tonnes, when it would be required to be conducted annually. 

Installation and accompanying standards 

Slate’s durability means it has a lifespan of 100 years or more, leading to low maintenance costs

Adherence to quality assurance procedures in the production of slate is vital. It is important the slate being specified meets or exceeds the variety of relevant standards in order to ensure the quality and longevity of the structure they form part of.  

Slate durability is a result of its being a non-porous material, making it resistant to heat, cold, sunlight and acid rainfall. Its durability means that it has a lifespan of 100 years or more, leading to low maintenance costs. 

It is crucial that roof coverings and cladding are fixed in accordance with British Standard for Slating and Tiling (BS 5534) to comply with UK building regulations. Numerous important changes have been made, with the aim of improving roof security and structure.  

The most recent standard for calculating fixing specifications for roof slates and slate cladding facades came into effect in February 2015. 

One of the most important changes is the mechanical fixing requirement of slates, increasing the safety on roofing and facade cladding. All slates must be mechanically fixed, installing dry ridge and hip systems. Mortar should be used in verytraditional roofing and can be used in conjunction with mechanical fix options such as clips, nails or screws. 

The British Standard BS 5534 identifies two installation methods: nailing and hook fixing. Slates are usually fixed to a batten in England and to a sarking board in Scotland, especially in areas of severe exposure. 

A building’s environment and the weather conditions to which it is exposed are essential factors to consider when designing a roof

Pre-holed slates are a time-efficient option for those who wish to use the nail fixing method. Additionally, slate nails should be either aluminium alloy or copper, or silicone bronze or stainless steel in coastal areas. 

Hook fixing has a number of advantages, including reduced labour costs, fewer breakages or a simpler replacement process. This fixing method is very common in Europe, particularly in France and Belgium. 

Once installed, natural roofing slate is subject to the forces of nature in which light, rain, ice, wind, hail, pollution and other atmospheric agents test its properties. It is vital that the slate quarried meets the rigours of these elements, so the whole production process has to  be carefully managed and stringent quality control measures need to be in place.  

It is important to understand the composition of natural slate in order to identify and differentiate between the high- and low-grade variants of the material. Natural slate can have a widely varying chemical and mineral composition, which is determined by the geographical and geological characteristics of the quarry from which it was extracted. Certain constituents can have an adverse effect on the appearance and integrity of natural slate over time.  

Ventilation 

Roof space ventilation is the most effective and economic method of keeping harmful condensation to a minimum, and it can be achieved by providing eaves through to the ridge ventilation.  

Ventilation also depends on the construction method used; this can be either cold roof or warm roof. A warm roof has the insulation at rafter level, while a cold roof has insulation at the ceiling level.  

It is important to consider adding some kind of ventilation to slate roofs. This will help reduce costs and dangers attributed to winter weather. Even in summer, without proper ventilation, heat will rise and cause moisture to build up. 

Impact of environment and site exposure 

The type of slate used depends on the pitch of the roof

A building’s environment and the weather conditions to which it is exposed are essential factors to consider when designing a roof. The form and technical details will differ depending on whether, for instance, a project is located in a sunny, dry region or a rainy area. The main factors to consider here are exposure to wind and rain. 

The specific location is important as well, since it will affect the level of exposure. For example, buildings positioned on slopes, hills or coastal areas are classified as high exposure.  

The type of slate used depends on the pitch of the roof. For example, small slates are more suitable for steep roofs with free-flowing drainage whereas larger slates are a better choice for lower roof pitches with more exposure. In southern England the most common slate size is 500mm x 250mm, while in the north of England and Scotland it is 400mm x 250mm. 

The head lap – exactly where the head of a slate is overlapped by the slate two courses above it – is calculated by measuring the roof pitch against the wind and rain exposure. The thickness of the slate must also be considered, as it reduces the pitch by 3° or more. For example, in lower pitches, the lap must be increased to help resist wind uplift and capillary action, especially if they’re highly exposed. 

Slates should be laid so that the thickest ones are installed at the eave. The rest of the slates should then be laid in diminishing thicknesses until the thinnest ones are at the top. 

New innovations 

A new trend in slate specification is to exploit the properties of natural slate which enable it to convert sunlight into energy, to produce heating, hot water or pool heating. Product manufacturers have created innovative solar thermal collector systems that adapt to all types of construction requirements and are imperceptible once installed. 

Another end use for natural slate is as a rain cladding system. These can vary across the spectrum of design, from traditional to contemporary, offering architects and specifiers a wide choice. 

Some natural slate rainscreen cladding systems consist of natural slates fixed onto proprietary aluminium horizontal profiles, mechanically fastened to an aluminium subframe support. These systems are maintenance-free, since they do not require any additional treatment and their colour and properties remain unchanged over the years. 

Slate economics 

There are a number of advantages and disadvantages to consider before using natural slate. On the plus side, natural slate has an aesthetic appeal and will stay looking good for many years. Other types of roofing slates such as concrete will lose colour and not wear so well visually. This means that slate is less likely to need replacing, making it more cost-effective.  

One downside of slate is its colour limitation, as it can only be provided in naturally occurring shades of grey. While it tends to complement most environments, performing well in rural locations and heritage sites, there will be some developments where it might not fit aesthetically.  

The geographical and geological characteristics of the quarry from which the slate is extracted determines its composition, surface texture and appearance. Over time, some elements such as iron or copper pyrites can have an adversely impact the appearance and integrity of natural slate. A quality supplier will provide slate that has passed W1, S1 and T1 tests. Pyrites exist in all slates, but can be unstable, or stable, which is classified T1. 

Any good roof covering should be maintenance-free. Natural slate does not require cleaning and ideally should be left alone. Apart from storm damage, foot traffic on a roof to carry out unnecessary cleaning is probably the most harmful thing that can occur. Reusability is one area where natural slate out performs most clay roof coverings.  

Natural slate does not require cleaning and ideally should be left alone

A slate roof will generally last as long as the fixings that hold it in place. With a natural slate roof, the same slates can be refixed onto new battens, whereas concrete tiles will need to be replaced with new ones, and clay tiles will also probably require substantial or complete replacement.  

While the cheapest and most versatile way of covering a pitched roof is with a concrete interlocking tile that can be used down to low pitches, concrete will lose its colour due to ultraviolet light within a few years and will cease to be aesthetically satisfactory. Clay tiles weather considerably better, but can be more expensive. 

Lighter than other roofing slate materials such as concrete, natural slate performs to a high standard and the slates’ roofing materials can be reused on new battens.  

For reasons of traceability it is important to select slate from a company that can guarantee where it comes from on each batch of slate it supplies, so that replacement and matching do not become an issue.  

Responsible producers will supply a health and safety product data sheet relating to working with natural slate. This will cover all matters relevant to the prevention of accident or injury in the workplace.  

In addition, there are health and safety regulations for pitched roofing, but since these are general to all types of roofing material they are not exclusive to natural slate.