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Low-Pitched Roofs: Measuring and Calculating

Learn how to accurately determine roof pitch with our essential guide. Get practical tips and insights to make informed decisions. Read more now.

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Are you planning a roofing project and wondering how to calculate and determine the right roof pitch?

Whether you’re dealing with a modern extension or a traditional pitched roof, understanding the correct measurements and calculations is crucial for success. Picking the right roofing material and being aware of the compliance requirements are essential for ensuring durability and helping facilitate efficient rainwater drainage.

In this comprehensive guide, we’ll explore basic pitch calculations and BMI’s innovative product range.

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Low-Pitch Roofs: Definition and Considerations

Low-pitch roofs are defined as roofs with a pitch of less than 20°. They are often used for domestic extensions and modern contemporary developments due to their unique aesthetic and anti-view-blocking properties. However, low-pitch roofs can be more susceptible to leaks and water damage due to slower rainwater runoff.

When designing a low-pitch roof, it’s essential to consider the type of roofing material used, the roof’s drainage system, and the building codes and regulations. A low-pitch roof may require additional features, such as a waterproof membranes or sub-roof weathering systems, to ensure its integrity and durability.

A long shot image of Welsh Celtic Manor featuring a lake with a fountain.

Understanding Roof Pitch in UK Construction

The roof pitch refers to the steepness of a roof, as measured by the rafter pitch, which is the angle in degrees that the inclined rafters create to the horizontal. Various aspects of roofing, such as appropriate material selection, adequate rainwater drainage and roof durability, link to the pitch of a roof. When determining the appropriate pitch for a roof, several critical factors must be considered.

Structural Considerations

The building’s room plans and structural design create the foundation for pitch selection. Higher-pitched roofs can provide additional structural benefits, as well as room for internal loft modifications, though modern architectural trends increasingly favour lower pitches, particularly in contemporary developments. Traditional builds often maintain steeper pitches for historical consistency.

Regulatory Framework

Local Planning Authorities maintain specific requirements regarding roof design, particularly in conservation areas or Areas of Outstanding Natural Beauty (AONBs). These requirements often dictate acceptable pitch ranges, materials, or aesthetic considerations to match other roofs in the area.

How Roof Pitch Determines Roof Tile Suitability

Material Compatibility:

Different roofing materials have different minimum pitch requirements to ensure proper installation and rainwater resistance. Our Camrbian Tile is suitable for pitches as low as 15° at a 75mm headlap and 10m max rafter length. This tile can be beneficial for low-pitch roofing projects where double-lapped natural slate cannot normally be used.

Weather Resistance:

Low-pitch roofs are more likely to experience slower rainwater runoff and wind-driven rain penetration, making the material choice, tile type and correct headlap installation critical for durability and leak prevention
Steeper pitches facilitate faster rainwater runoff, reducing the risk of water ingress.
Granular or rougher surface finishes slow the rate of rainwater runoff, also increasing the risk of water ingress

Building Design Constraints:

Structures with height restrictions often require low-pitch roofs
Aesthetic preferences or architectural styles may also dictate roof pitch selection

Legislative Framework & Requirements:

The successful implementation of low-pitch roofing systems demands meticulous attention to detail during installation. Considerations include the suitability of roofing materials, the importance of headlaps and sidelaps, and rainwater runoff efficiency.

British Standard recommends specific roofing underlay requirements for pitched roofs where roof tiles are being used:

All penetrations to the underlay should be adequately sealed to prevent water ingress.
Minimum unsealed horizontal underlay laps can vary between 100 mm and 225 mm, depending on whether the underlay is fully supported or unsupported and whether the rafter pitch is below 15 degrees or not. BS 5534 sets out the detailed requirements.
Where underlay laps are sealed using proprietary adhesives and tapes, they should be installed in accordance with the manufacturer’s recommendation, but usually would not be less than 100 mm.
The underlay should be installed to ensure adequate drainage between the underlay and tiling battens to the eaves gutter. When unsupported, this means the underlay should be laid with sufficient drape (not exceeding 15 mm) between the rafters or supports to ensure any water on the underlay discharges away from batten nail penetrations and down to the eaves gutter; when fully supported on insulation or timber sarking boards, counter battens should be installed to create a drainage gap between the underlay and tiling battens
The underlay should possess sufficient durability, weather tightness and tensile strength to resist the design wind uplift loads they may be subject to.

BMI Product Solutions for Low-Pitch Roofs

Modern roofing technology has evolved significantly, enabling the use of traditional materials in previously challenging low-pitch scenarios. BMI Redland’s product range specifically addresses these requirements, and the following tiles, alongside a full Redland system, come with the SpecMaster 15-year system guarantee:

Cambrian Slate

Made with recycled Welsh Slate, Cambrian delivers great performance while maintaining aesthetics. BBA-certified, lightweight and interlocking, this single lap tile allows for easy installation. Suitable at pitches as low as 15° at a 75mm headlap and 10m max rafter length, this tile can be beneficial for low-pitch roofing projects where double-lapped natural slate cannot normally be used.

Mini Stonewold

Flat, single-lapped interlocking tiles with a minimum pitch of 17.5° at 100mm headlap and with a maximum rafter length of 10m. This tile is suitable for new builds, extensions and home renovations. BMI-specified roof, this tile offers peace of mind for varying locations across the UK.

Redland 49

A versatile concrete tile suitable for both new-build and refurbishment, this iconic tile remains popular, especially in Greater London. Easy to install, available in a variety of colours. and with a minimum pitch of 17.5° at 100mm headlap with no maximum rafter length for through-coloured smooth finish tiles, this historic tile offers speed and economy. Granular finished tiles require a slightly higher pitch of 22.5° at 100mm headlap or 30° at 75mm headlap with no maximum rafter length.

Complete low-pitching systems:

Installing a low-pitched roof demands careful attention to detail, along with regular inspections and maintenance to effectively prevent water ingress. Redland tiles help simplify the installation process while ensuring long-lasting performance. With the SpecMaster Guarantee, BMI can select and specify the most appropriate Redland tiles, fittings and components for your roof, giving you full confidence, knowing that the BMI Redland Technical Team is available to support you throughout your entire project journey.

To learn more about our offerings, please visit our Redland page. If you have any questions, don’t hesitate to reach out to the BMI Redland Technical Specifications Team.

Frequently Asked Questions

What is the absolute minimum pitch possible with BMI tile products?

The minimum pitch varies by product. The Mini Stonewold tile can be installed at a 17.5° rafter pitch with a 100mm headlap at a 10m maximum rafter length, whereas the DuoPlain requires a minimum pitch of 25°. Each product has been extensively tested at these pitches in a wind tunnel simulating driving rain conditions to ensure weather resistance and long-term performance.

How do British Standards affect low-pitch installations?

BS 5534:2014 + A2: 2018 and BS 5250 provide comprehensive requirements for pitched roof installations. Key considerations include:

Minimum pitch and headlap specifications
Maximum rafter length, especially for flat tiles and slates
Wind load calculations and tile fixing requirements
Wind load calculations and underlay selection
Roof ventilation provisions

What guarantees are available with BMI systems?

BMI offers comprehensive guarantees:

- 15-year design liability and weather-tightness roof system guarantee (through our SpecMaster service)

Where can I get more information?

Sources and References:

- Building Regulations 2010 (as amended)

- BS 5534:2014 + A2: 2018

- BS 8000-6: 2023 in conjunction with BS 8000-0

- BS 5250: 2021

- BMI Technical Documentation

For detailed technical support and specification advice, contact BMI Technical Services or visit

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Low-Pitched Roofs: Measuring and Calculating