SIG Roofing Natural Slate Trading Director Robert Edwards discusses how architects can achieve best practice design for slate roofs.

Gallegas 39 slates from SIG Roofing Natural Slate were specified for the Windsor Court care home in Malvern, Worcestershire (ph: Maria Mallaband Care Group)

Natural slate roofing is a perennial favourite among architects for reasons of appearance, small carbon footprint, durability, weather resistance and longevity. The use of good quality materials, combined with considered specification, rigorous detailing, and correct installation, are crucial to ensuring slate roofs not only look good but last for generations to come. Robert Edwards, Natural Slate Trading Director at SIG Roofing Natural Slate discusses how architects should approach the design of slate roofs, from initial site considerations and product selection to fixing methods and key details.

General guidance on slate roof design can be found in BS 5534, the Code of Practice for Slating and Tiling. This useful document provides recommendations for the design, performance, and installation of new-build pitched roofs, including vertical cladding, and for normal re-roofing work, including repairs using slates, tiles, shingles and shakes, and their associated components. It’s worth noting, however, that the document may not be appropriate for re-slating of old roofs, particularly where traditional and/or reclaimed materials are used.

Slate splitting techniques from 100 years ago and today (historic photo courtesy of Welsh Slate)

SIG Roofing Natural Slate’s best practice guidance is based on BS 5534, and takes into account seven interrelated factors or steps, designed to produce slate roofs that are not only fit-for-purpose but also long lasting. Specifiers working in Scotland should refer to SIG’s comprehensive Traditional Scottish Roofing Practice for Natural Slate Roofs in Scotland document.

The first step for architects looking to design slate roofs is to use SIG’s Product Finder to choose which natural roofing slate is suitable for their specific project. The finder enables specifiers to select the geographical location of the scheme; choose the project type, for example housing; and then employ a selection of filters to find the most suitable slate from the products available.

The next step is to check the project’s site exposure by referring to the Annual Driving Rain Index. This divides the UK into four exposure zones for wind-driven rain, ranging from Sheltered (less than 33 litres/m² per spell) to Very Severe (100 or more litres/m² per spell). The degree of exposure determines the minimum headlap (or ‘lap’) which should be specified on a natural slate roof. Localised factors, such as tall buildings, long rafter lengths, structures on tops of hills and coastal sites, can increase the exposure grading which should be applied to the project.

Gallegas 39 slates from SIG Roofing Natural Slate can be cut to accommodate curves

Once the degree of exposure is determined, specifiers can use our Recommended Headlaps Table to determine the minimum recommended headlap for the site exposure and roof pitch. In general, the lower the pitch of the roof, the greater the lap should be. Longer laps will be more resistant to capillary action and wind uplift. Smaller slates can be used on steeper roof pitches with free-flowing drainage. For exposed sites, wide slates with a greater lap should be employed. In sheltered areas, roof pitches as low as 20 degrees can be achieved using a hook fixing system. For even shallower pitches and exposed locations, the combination of Gallegas 39 500x250mm natural slate and the Easy Slate low-pitched roofing system have been proven to be effective on 12-degree slopes!

Gallegas 39 slates are suitable for low-pitched roofs down to 12 degrees 

The fourth step is to determine the coverage (number of slates per square metre) using SIG’s Coverage Tables. The values shown in the tables are calculated using nominal sizes and incorporating 3-5mm joint gaps in line with BS 8000: Part 6. We recommend the addition of at least five per cent wastage allowance.

Following this, architects should confirm their fixing method of choice. BS 5534 specifies two ways of installing slates: nailing or hook fixing. These fasteners fix the slates to either battens, which is the most common English method, or to sarking board, which is commonly used in exposed areas in Scotland.

Natural slate is a versatile and long lasting roofing and cladding material 

Nailing has an aesthetic advantage, in that the nails are hidden beneath the rows of slates above. Nails should be copper (to BS 1202 part 2) or aluminium (to BS 1202 part 3). In corrosive or marine atmospheres, copper nails are preferable and in severe conditions silicone bronze nails should be used. While hook fixings remain visible on the finished roof, this fixing method offers considerable design freedom, better wind uplift resistance, has the potential to reduce labour costs by up to 25 per cent, and can greatly reduce slate breakages. Furthermore, should slates need to be replaced, hook fixing offers a faster, less invasive and neater finish.

Aerial view of Windsor Court showing slate-clad hips, valleys and dormers (ph: Maria Mallaband Care Group)

The next step is to reference SIG’s Batten and Holing Gauges Table. This will enable specifiers to confirm the correct batten gauge (distance between battens) for their chosen slate length and lap, determine the holing gauge (distance from hole to tail of slate), and the number of linear metres of batten per square metre of roof.

The final step for architects is to decide on their preferred choice of details for the ridges, valleys, eaves and verges. We recommend the use of slate-and-a-half slates on both verges and valleys to ensure secure fixing. A shorter top slate is preferred at the ridge to compensate for the gauge and to ensure adequate ridge overlap. Eaves course slates should be installed dressed edge face up, with the tail aligned to the under eave slate. The slate should extend 50mm over the fascia or to the centre of the gutter.

Contact Details
For more information, please visit the SIG Roofing Natural Slate website.