Aled Williams, SIG Roofing Northern Slate Business Development Director, discusses slate selection, certification, and sustainability in National Parks and conservation areas with Architecture Today’s Technical Editor John Ramshaw.

Bisbrooke Village Farm in Rutland comprises four barn-style houses, which are intended to evoke the site’s agricultural origins. SIGA 13S slates were chosen for the roofs and laid by specialist roofing contractor Sandall Roofing (photo: Hereward Homes).

Natural slate is often considered a benchmark material for protected environments, such as National Parks and conservation areas, due to its durability, longevity, and strong visual connection to local vernaculars. But specifying slate in these contexts requires careful consideration of planning expectations, technical performance, and heritage sensitivities. In conversation with Architecture Today’s Technical Editor John Ramshaw, SIG Roofing Northern Slate Business Development Director Aled Williams explains how the company supports architects and conservation officers in meeting planning requirements, and why traceability, Code for Construction Product Information (CCPI) assessments, and robust standards remain central to quality assurance.

Why is natural slate often regarded as the benchmark roofing material for sensitive or protected environments like National Parks?
Natural slate is frequently specified because of its historical continuity, aesthetic compatibility, and long-established use across areas such as Snowdonia, the Lake District, and the Peak District. In many cases, indigenous slate quarries still exist within or near National Parks, reinforcing its status as a vernacular material. In places like Snowdonia, SIG Roofing supplies selected slates that match the appearance and performance of locally-quarried slate, such as Welsh Ffestiniog, and have been formally approved by the park authority.

The Cosmopolitan Hotel in Leeds is a Victorian heritage building that has been reroofed with SIGA 112S slates (photo: AccuRoof).

How does the SIG Roofing Slate Specification Team work with specifiers and conservation officers to ensure its products meet local vernacular and planning expectations?
We offer a wide range of slates to suit regional preferences in terms of colour, thickness, size and texture. Traceability is built into every product, from quarry to roof, and SIG Roofing works closely with architects, specifiers, and conservation officers to ensure products align with planning authority expectations.

What are the key performance characteristics of natural slate in exposed or rural environments?
Natural slate’s key performance indicators include low-water absorption, high-frost resistance, and minimal porosity. These qualities make it ideal for exposed and/or rural sites. Under BS EN 12326, slates with water absorption below 0.6 per cent are rated T1 (suitable for exposed areas), while those above this threshold are classified T2. SIGA slates meet or exceed T1 classification for use in UK protected environments.

Can you explain how SIGA slates are tested and classified according to BS EN 12326 and BS 5534?
BS EN 12326 governs the physical properties of slate, including water absorption (A1), weather resistance (T1), and structural integrity (S1). All SIGA slates are tested at source – both by the quarry and independently by SIGA’s on-site operations. BS 5534 relates to installation and wind uplift, while BS 8000 provides guidance on handling and site practices. Every SIGA pallet is delivered with full performance data, ensuring compliance with BS EN 12326. The conformity of a slate to the requirements of the designated standard is stated in the associated Declaration of Performance (DoP). This lists the values from testing which the supplier declares in support of the products use and performance for a given application.

Measuring 500x250x7mm and holed at 90mm, the 112S slates specified for The Cosmopolitan Hotel were installed by specialist roofing contractor NRA Roofing (photo: AccuRoof).

What role does third-party verification play in ensuring long-term performance?
Third-party verification is essential to SIGA Slate’s quality-assurance process. As part of a PLC, SIG Roofing has to adhere to strict traceability and marketing standards. Independent testing at the quarry ensures that declared performance levels are accurate and repeatable. Each slate pallet is fully traceable and labelled with test results, which are also made available on the SIGA Slate website.

What challenges do architects and developers typically face when specifying roofing materials in National Parks or Areas of Outstanding Natural Beauty (AONBs)?
A common issue is the proliferation of lookalike or unapproved slates that offer lower up-front costs but lack the technical performance and traceability of genuine graded products. Additionally, misunderstandings around classification, such as confusing borderline T2 slates with T1, can lead to costly long-term failures. SIG Roofing seeks to support architects with transparent data and direct site references to help avoid these pitfalls.

Could you explain the SIGA Slate grading system and how it helps specifiers make informed choices?
SIGA has a numerical classification system to differentiate slate sources and grades, reflecting regional availability and performance characteristics. For example, SIGA 32S is widely used in the north of England and available in both 5mm and 7mm thicknesses. Other ranges, such as SIGA 112S, are grouped by quarry origin and grade, helping specifiers match slates to local styles and performance requirements.

Macclesfield’s Grade II listed King’s Gate Library has been converted to residential use and reroofed with SIGA 32 slates. The specialist roofing contractor was Nevson Roofing (photo: AccuRoof).

What are the main environmental benefits of using natural slate over alternative roofing materials?
In addition to a long life span (SIGA Slate offers warranties of up to 100 years), natural slate quarrying and processing involves comparatively minimal energy use, while the slates themselves require no firing or chemical treatment. The only significant carbon input is transport. Spoil from extraction is either returned to the ground or landscaped, making it a resource-efficient and inherently sustainable material.

What typical pitfalls should be avoided during slate selection and installation?
Documentation, not marketing gloss, should be the basis for comparison. In December 2024, SIGA Slate was the first natural roofing slate to pass the Code for Construction Product Information. This achievement confirms that all product information provided for SIGA natural slate meets the CCPI’s acid tests of being clear, accurate, accessible, up-to-date, and unambiguous. On Site, slates should always be graded into at least three thicknesses and sorted at ground level – never on the roof. Poor grading can lead to lipping and compromised aesthetics. In addition, thicker slates may require more careful sorting, which is often overlooked.

SIGA 32 slates on the King’s Gate Library development in Macclesfield (photos: AccuRoof).

What advice would you give to architects working in protected areas who are unsure how to specify compliant slate?
Speak to SIG Roofing early on in the design process. We can provide in-situ examples in your region, so specifiers can see weathered slates in context. This approach can be more powerful than simply viewing lone product samples. We also provide project-specific advice, including size, colour, texture, and fixing requirements to ensure a good fit with local character and planning expectations.

How can the SIG Roofing Slate Specification Team support a project from early design stages through to construction?
We provide NBS specifications, contractor liaison, quarry coordination, and site visits. Our technical team checks deliveries, reviews workmanship, and offers aftercare support. Finally, SIGA Slate’s and SIG Roofing’s end-to-end project service ensures quality is maintained from concept to completion, giving you clear traceability from rock to roof.

Contact details
For more information, technical support, or to request specification assistance, please visit www.sigaslate.co.uk