Slate-and-a-half roofing: what it is and why it matters
Have you heard of the “slate-and-a-half” technique but aren’t quite sure what it is or how to use it correctly? Don’t stress! In this article, we’ll break down everything you need to know about this essential slating method.
The slate-and-a-half solution is a roofing technique designed to maintain the proper bond and alignment of slates, particularly near verges or other areas where a standard-width slate isn’t sufficient.
It involves using slates that are one and a half times the width of a standard slate, preventing the creation of small, weak pieces that could compromise both the durability and appearance of the roof.
The regulations
In recent years, roof designs have become significantly more complex, with dormer windows, false chimneys, flue penetrations and solar panels all becoming more common. As a result, greater consideration must be taken to ensure that the roof slate around these types of features is both secure and also adheres to the UK regulations.
In the UK, contractors must conform to BS5534, which is the code of practice for slating and tiling and is a requirement of most roofing product manufacturers’ warranties. With regards to verges, the standard states:
Verges […] should be detailed with a slate and either a slate-and-a-half or half slate in alternate courses (not less than 150mm in width).
The standard does give allowance for both the use of slate-and-a-half sizes and manually cut half width slates. However, it is important that the half widths being cut by hand are wide and strong enough for the holes and fixings required at verges and abutments, and in other exposed areas of the roof.
If a half slate is too small to fix with an adequate number of copper clouts this could lead to water ingress or allow wind lifting.
This means that it is not possible to manually cut a half slate from a standard 500 x 250 mm slate – the size most commonly used in the UK – as it would not be wide enough to install effectively and also meet the UK requirements.
Slate-and-a-half
As such, for many contractors, the logical solution for verges and abutments is to rely on the slate-and-a-half option, which is guaranteed to comply with the British Standard.
By using slate-and-a-half, contractors can offset roofing patterns precisely and accurately, easily creating the broken bond look so that there are no straight lines running vertically across courses.
Furthermore, by sourcing slate-and-a-half sizes from the same manufacturer, and the same quarry where possible, it is possible to maintain the same aesthetic throughout.
However, as roof designs have become more complex the demand for-slate-and-a-half has increased, impacting both cost and availability. Despite growing demand, it is not a simple task for manufacturers to increase capacity to match – as it is far harder to produce when compared to standard slate sizes.
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Like regular natural slates, slate-and-a-half products are extracted from specific quarries and roughly cut into large blocks that are later hand split into individual slates. Yet extracting larger blocks of slate is a far more challenging as it can crack during the extraction and cutting process and it can also be difficult for quarries to find outcrops of rock big enough to extract blocks of the required size.
As a result, most manufacturers only supply a set number of slate-and-a-half pallets per order and it is not possible to buy additional pallets separately.
Use it wisely
It is crucial that contractors carefully analyse the design and build of a roof to make sure that they use their slate-and-a-half product efficiently and without waste.
Where possible, consider where slates can be hand cut and still conform to the British Standard, using this method in order to conserve slate-and-a-half for where it is needed most.
Alternatively, the NFRC suggests in its Guidance Note 60 (GN60) to utilise other larger slates at the verge alongside compliant self-cut halfs. For example, when using standard 500 x 250 mm slates you could alternate between larger 500 x 300 mm slates and self-cut 500 x 175 mm slates at the verge.
This method maintains the required side lap without affecting the appearance of the verge, while ensuring that each slate is compliant with BS5534. This method is also suitable when using 400 x 250 mm slates, where 400 x 300 mm slates are available.
The difficulty with this methodology is that it relies on the use of additional wider slates, such as 500x300mm and 400x300mm wide slates, which for slate producers are equally as difficult to produce as regular 500x375mm slate-and-a-half.
To ensure an optimum finish when using any of these methods, it is important to select products from a trusted manufacturer that can supply a range of slate sizes that are guaranteed to be sourced from the same quarry to provide an ideal quality and colour match.
We own all 24 source quarries from which our slates are produced and as such, we provide a unique bar code on every pallet that details key information about the slate including product name, selection, technical data, quarry of origin and production date.
This ensures that if a roof needs to be replaced or fixed in the future, the exact slate from the same quarry can be sourced and used again, but it is also useful in scenarios where various sizes of slate that match is required.
As roof designs continue to become ever more ambitious, it has never been more important to carefully plan and to consider how to effectively and efficiently use slates. This is not only to make sure that an installation is compliant with the British Standards – and therefore weatherproof – but to ensure projects do not run out of crucial slate-and-a-half sizes mid-way through.
