Ask the expert: Slate

Natural slate roof at the Windsor Court care home in Worcestershire. Photograph courtesy of Maria Mallaband Care Group

Ged Ferris, answers readers’ questions on natural slate

Is slate suitable for all types of roof and wall build-up?

Natural slate can be used on all pitched roofs and vertical walls. When combined with the Easy Slate system from Permavent, it can be laid to the same pitch as interlocking tiles.

Do slate roofs and walls need to be ventilated?

This depends on the build-up of the roofing or cladding element. Specifiers should always consult BS 5250, the code of practice for the management of moisture in buildings, and BS 5534, the code of practice for slating and tiling. Ventilation should always be provided beneath the sarking, and the roof space should be ventilated in accordance with BS 5250. Natural slates are considered to be permeable when used as a roof covering.

What type/grades of slate are commonly used on building projects and what should architects look out for?

Being a natural product, slate has more variation than factory-produced roof coverings. Thickness and flatness, together with the inclusion of ‘contaminant’ minerals and consistency, will vary from source to source (quarry or mine) and even slate to slate. Quarries or mines will generally produce two or three slate grades with corresponding price points. The more consistent the slate the higher the price. There is a premium for slates that meet specific criteria, usually associated with heritage projects.

Slates are categorised in BS 12326 according to their performance characteristics. The best performing slates are classified W1, T1, and S1, and are readily available throughout the UK. Slates with a T2 classification can exhibit streaks of rust, which though unsightly, will not affect the weatherproofing properties of the roof.

Architects should always consult the Declaration of Performance (DoP), which provides details of the slate’s classification. Labels on the slates delivered to site should correspond to the DoP. If this is not available or forthcoming, it is likely that something is being hidden. Natural slates can pass through several hands between source and site and It’s been known for less principled suppliers to disguise the provenance of their slates in order to gain a competitive advantage or disguise the supply chain.

How important is provenance when it comes to specifying slate?

Provenance or traceability is crucial as it will determine the legitimacy of any statements regarding durability or guarantees. Slates at the extremes of the quality spectrum can fail prematurely. Rectification will only follow if the original source can be identified. This includes invoices from the supplier so that the entire supply chain can be traced back.

What types of finish are available with natural slate?

Slate textures vary from smooth to rugged. Generally, the thicker the slate the more rugged the surface texture. There is a directionality to the surface texture which can be influenced by the bedding of the mineral components within the slate and the location where the slate was split from the rock. Edges are usually dressed creating a bevelled surface. Some sources will provide sawn edges. The colour of the slate is determined by the minerals within it. Slates with higher carbon and graphite content tend to be blacker, whereas greater quantities of iron and chlorite will result in redder and greener slates respectively. The most commonly available slates come in the range of tones from dark grey to light grey, as well as blue grey and black.

Why do slates need to be sorted prior to installation and how is this done?

Slates overlap, so they need to be sorted to ensure that each row or course is the same thickness. Failure to do so will result is an uneven ‘gappy’ roof with reduced wind-uplift resistance. For the same reason thinner slates should be laid first at the bottom of the roof slope. Sorting is usually achieved by grouping the slates into three lots by thickness. This can be done by viewing the layers of slates in the crates upon delivery to site, and by feel and weight as the roof is loaded out from the scaffolding.

How long do slate roofs and walls typically last, and how should they be maintained?

Best quality products, such as Welsh and Cumbrian slates, are known to last hundreds of years. Typically, the fixings and battens decay before the slates. Slates tend to come with some form of guarantee or statement of service life. 75- and 100-year guarantees are the most common, which correspond with the standard design life of modern buildings. T2 and T3 slates (as mentioned above) are often given shorter warranties.

Can slate be reused and/or recycled at the end of its life?

Slate can and is reused. There is a market for salvaged products, which accounts for between two and five percent of the total UK market volume. Reused slates are particularly prominent in conservation areas where locally-sourced products are no longer available.

How can I check the quality of reclaimed slates?

There is no standard for recycled products. Slates will often need to be re-holed and/or cut down to a smaller size because the original fixing hole has become enlarged. The act of removal can cause damage and of course weathering may also cause weakness or thinning of the slate – all of which will affect its future performance. The surest way to guarantee quality is to inspect each slate. This is best done at the time of installation by a slater who is used to working with reclaimed slate. Fading and other staining will inevitably change the colour of the slate.

Source: Architecture Today

Author: Architecture Today

Photography: Maria Mallaband Care Group