Slate Roof Detailing and Design

Slate roofs have been part of British architecture for centuries. You see slate roofs in all forms from small village cottages to large country estates. In this post we will explore some of the key factors to consider when designing a slate roof. We aim to share practical tips and useful resources along with a selection of slate roof detail examples to illustrate some of the different junctions required for detailing a pitched slate roof.

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01 Header image
Slate roofs are built to last, combining durability and timeless British charm.

Types of Slate

Slate for roofing comes in various types, each with its own characteristics and suitability for different applications. We mention a few here but there are of course other slate producing countries beyond the selection listed below.

Welsh Slate

Welsh slate is quarried primarily in North Wales, and is renowned for its exceptional quality and durability. It has a fine grain and uniform texture, with a distinct blue-grey colour. Welsh slate is often used for high-end roofing projects, particularly in heritage restoration and conservation work where authenticity is important. 

 

Spanish Slate

Spain is a major producer of slate, with regions such as Galicia and Catalonia known for their high-quality slate deposits. Spanish slate is exported worldwide and a popular choice for roofing materials on both residential and commercial projects where cost effectiveness is a priority. Spanish slate varies in colour and texture depending on the specific quarry. It typically has a coarser texture and may exhibit shades of blue, grey, green or purple. 

 

Burlington Slate 

Burlington slate is quarried in the Lake District in Cumbria, renowned for its unique blue-black colour and exceptional quality. It has a smooth texture and uniform colour, and is often considered one of the finest roofing slates in the world. It is usually used on prestigious roofing projects, including historic buildings, stately homes and luxury residences. 

 

Chinese Slate

China is one of the largest producers of slate in the world, which is exported globally and known for its affordability. Chinese slate quality varies widely depending on the quarry, with some exhibiting inconsistencies in colour, texture and durability. Chinese slate is often used for budget-conscious roofing projects where cost is a primary consideration. 

 

Artificial Slate

Artificial or synthetic slate is made from a mixture of cement, fibre and additives, moulded to replicate the appearance of natural slate. Some artificial slates can incorporate recycled materials. Artificial slate closely mimics the look of natural slate but is generally lighter in weight and more uniform in appearance. It offers benefits such as easier handling, lower cost and consistent quality. 

Artificial slate is suitable for a wide range of roofing applications, particularly where weight restrictions, cost considerations or design flexibility are important. It is worth noting that artificial slate does not have the same sort of textured surface as a natural slate therefore not providing the same level of ventilation between the slates as natural slate. This is an important consideration when detailing slate roofs.

 

Slate Roof Terminology 

02 Slate Terminology

Head – the upper edge of the slate when laid

Tail – the lower edge of the slate when laid

Face – the upper surface of the slate when laid

Lap – the distance that the tails of the slates overlap in one course, the heads of the slates to the next course below

Bond or Side Lap – the horizontal distance between the side of a slate and the side of the slate immediately above it

Gauge – the vertical distance between the tail of one slate and the tail of the slate immediately above it

Holing Gauge – the distance between the tail of the slate and the nail hole

Batten Gauge – the distance between the battens

Design Considerations

Some of the main factors to be taken into account when designing a slate roof include site exposure, roof pitch, type of slate and slate lap.

Site Exposure

One of the first considerations when designing a slate roof is the climate conditions of the site.

The level of exposure a building faces from driving rain influences the required minimum lap specification. Additional factors such as tall buildings, structures situated on slopes or hilltops, and coastal locations may elevate the exposure rating necessary for a particular project.

03 Exposure map UK

The roof must be designed with adequate resistance to wind uplift. BS5534, BRE Digest 346 and BS6399 can help with design calculations for wind uplift and wind load.

Roof Pitch for Slate Roof

The pitch of the slate roof plays an important role in its performance and aesthetic appearance – but also has implications on the design.

Traditional natural slate roofs typically feature steeper pitches, ranging from 20 to 45 degrees. The steep slope facilitates effective water drainage and helps prevent water infiltration. BS5534 recommends a minimum pitch of 20 degrees.

However, you do see slate roofs with a shallower pitch, some artificial slate suppliers can guarantee their slates can be used on pitches as low as 10 degrees.

Roof pitch design will vary according to site conditions, for instance, in areas with heavy snowfall, a steeper pitch will be necessary to prevent the build up of snow and potential roof collapse.

In general, the lower the pitch of the roof, the greater the lap between tiles should be. On steeper roof pitches with free flowing drainage, smaller slates may be used.

Lap of Slate Tiles

Lap can be described as the distance by which the tails of slates in one course overlap the heads of the slates in the next course but one below. The required lap of slate tiles is calculated by taking into account the wind uplift, exposure to driving rain and the roof pitch.

Siga Slate Provide an example of exposure zones and associated lap recommendations on their website.

04 Siga Slate Exposure Map

Image credit – Siga Slate

05 Siga Slate Lap Table

Image credit – Siga Slate

The gauge can be defined as the vertical distance between the tail of one slate to the tail of the slate immediately above it. 

Ventilation

It is important to provide suitable ventilation to a slate roof to control the condensation and moisture build up that can occur. In order to comply with the Building Regulations and BS5250, ventilation must be provided at eaves level on both sides of the roof. The ventilation gap requirement varies according to the type of roof and insulation positioning.

Ventilation must also be provided at or near the ridge as a continuous vent, again dependent on type of roof, insulation positioning, pitch and more.

We have written a guide on Pitched Roof Ventilation and Membranes, you can check it out here.

The ventilation path, whether for a cold roof or a warm roof, will need to be above the line of insulation.

This means that the detailing of a slate roof will be different depending on whether the roof has insulation at ceiling level (cold roof) or insulation at rafter level (warm roof).

It is important to note that natural slate provides more ventilation than artificial slate due to its more uneven surface allowing for some airflow between the surfaces of each slate. Therefore the ventilation and membrane requirements of a natural slate roof will vary from an artificial slate roof.

Structural and Fixing

The roof structure must be designed to withstand the loads of the roof  itself, as well as other factors such as wind uplift, and other conditions.

All slate roof designs should be in consultation with a structural engineer to ensure suitable roof framing material, spacing of rafters, connections and structural elements. 

Fixing nails should either be aluminium alloy or copper, silicone bronze or stainless steel in coastal areas. Nails must be driven to the correct depth to avoid overexposure and potential water ingress, while ensuring they do not puncture the membrane or felt below.

Detailing Components

A slate pitched roof comes with a few junctions and areas that can be challenging to detail. Below we will explore some of the common junctions and illustrate some detailing solutions we have produced on the Detail Library. All of the details featured below can be downloaded from the Detail Library. Save yourself hours of time, get a head start on your detailing – check out our membership options.

Slate Roof Verge

A verge on a pitched roof is the edge of the roof where the slates meet the gable walls. They serve to seal the roof edge and prevent water ingress.

Slates can be secured at the edges using lead, zinc or GRP flashings, cement mortar or mechanical fixings to secure the slates along the verges.

Common issues with roof verges can be slippage, cracking or mortar degradation. Correct installation and regular maintenance can help mitigate these issues.

Below we show an example of a slate roof verge detail.

10 DL302 Gable verge detail to cavity wall - traditional slate roof - insulation at ceiling level
11 DL302 - Gable detail traditional slate roof masonry cavity wall 3D copy

Image of DL302 2D and 3D

DL302 shows a verge detail to cavity wall with traditional slate roof and insulation at ceiling level. To download this detail along with the detail notes follow the links at the end of this post.

Slate Roof Ridge

The ridge is the highest point in the roof where the two sloping surfaces meet. Methods for securing ridge tiles or slates include mortar, clips or mechanical fixings, depending on the roof design and material used.

The roof often requires ventilation at ridge level, this can be provided with roof vents towards the ridge or breathable ridge systems to allow airflow and prevent condensation build up in the roof space. The ventilation requirements at ridge level will depend on factors such as cold vs warm roof, roof span, membrane specified and type of roof covering.

Below we show an example of a slate roof ridge detail.

12 DL338 Roll vented ridge detail - refurbished slate
13 DL338 Roll vented ridge detail - refurbished slate 3D

Image of DL338 2D and 3D

DL338 shows a roll vented ridge detail for a refurbished slate roof with insulation between and under the rafters. To download this detail along with the detail notes follow the links at the end of this post.

Slate Roof Valley

A roof valley is the internal angle that is formed where two roof slopes meet. Types of valley include open valleys, where the slates are laid directly onto a valley lining, or closed valleys where a metal valley trough is used.

Lining and flashing are essential to prevent water ingress, materials such as lead, copper and other proprietary valley lining materials may be used to create a watertight system.

Below we show an example of a slate roof valley detail.

14 DL344 Valley gutter lead and vent detail - refurbished slate roof
15 DL344 Valley gutter lead and vent detail - refurbished slate roof 3D

Image of DL344 2D and 3D

DL344 shows a valley gutter lead and vent detail – refurbished slate roof with insulation between and under rafters. To download this detail along with the detail notes follow the links at the end of this post.

Slate Roof Abutment

A slate roof abutment is the point where the roof meets a vertical surface such as a wall or chimney. They are vulnerable to water ingress and require careful detailing to maintain weather resistance.

Flashing, sealants and mortar are commonly used to create a watertight seal between the roof and abutting surfaces.

Below we show a couple of examples of slate roof abutment details.

16 DL304 Typical party wall masonry cavity wall to lean-to abutment detail - traditional slate roof - insulation at ceiling level
17 DL304 - Party wall lean to abutment detail traditional slate roof 3D copy

Image of DL304 2D and 3D

DL304 shows a typical party wall / masonry cavity wall to lean-to abutment detail with a traditional slate roof insulation at ceiling level. To download this detail along with the detail notes follow the links at the end of this post.

18 DL309 Typical party wall solid masonry wall to sloped roof abutment detail - traditional slate roof - insulation at ceiling level
19 DL309 - Party wall sloped slate roof abutment detail 3D copy

Image of DL309 2D and 3D

DL309 shows a typical party wall solid masonry wall to sloped roof abutment detail with a traditional slate roof and insulation at ceiling level. To download this detail along with the detail notes follow the links at the end of this post.

Compliance with the Regulations

It is important to ensure compliance with the regulations when designing and detailing a slate roof. Requirements will vary according to project, location and other factors. Below we look at some of the key considerations.

 

Building Regulations Approval

Approved Document Part A: Structural Safety

This document sets out the structural design requirements for buildings, including roof structures. It covers aspects such as loading, stability and resistance to weather and ground movement.

Approved Document Part C: Site Preparation and Resistance to Contaminants and Moisture

This document addresses measures to prevent moisture ingress into buildings, including roofs. It covers topics such as damp-proofing, ventilation, and drainage.

Approved Document Part L: Conservation of Fuel and Power

Energy efficiency is a significant consideration in modern building design. This document outlines requirements for thermal insulation, air permeability, and heating systems, all of which impact the design and detailing of slate roofs.

Approved Document B: Fire Safety

Fire safety regulations dictate measures to prevent the spread of fire within and between buildings. They may include requirements for fire-resistant materials, compartmentation, and means of escape, which can influence the choice of roofing materials and construction methods.

 

British Standards

BS5250 Management of Moisture in Buildings and BS5534 Slating and Tiling for Pitched Roofs and Vertical Cladding contain guidance relevant to pitched roof design. 

 

Product Certifications

Using roofing materials that carry relevant certifications, such as British Standards (BS) or eEuropean Technical Assessments (ETA), to verify their compliance with performance standards.

 

Installation Certificates

Obtaining certificates of compliance from qualified roofing contractors or installers to confirm that the slate roof has been constructed in accordance with approved plans and specifications.

Download the Guide

31 Slate Roof Detailing and Design - Resource

Download the Details

Sign up to the library to download any of our large and growing collection of slate roof details. You can purchase individual details, sign up for monthly membership to access a limited selection of details, or sign up for annual membership to get unlimited free downloads of all the details.

06 Slate Roof Details P1
01 Slate Roof Details P2
11 Slate Roof Details P1

Related Reading

Pitched Roof Ventilation and Membranes - A Guide
06 Refurbished Slate Roof Details Part 1

Author

Written by Emma Walshaw, Architectural Technologist. Emma is the founder of First In Architecture and the Detail Library. She has written a number of books on construction and detailing.