Effective cold roof ventilation - A guide to ventilating cold roofs

Cold roof ventilation is crucial for preventing issues like condensation, damp, mould growth and timber decay, all of which can compromise the integrity of the roof structure. Effective ventilation of a cold roof requires the right combination of roof ventilation solutions, whilst also ensuring that the roof space is fully waterproofed and protected from rainwater ingress. 
 

What is a cold roof? 

The most common type of roof construction in the UK, a cold roof has insulation at ceiling joist level. This leaves the roof void colder in relative terms, than the accommodation below. Cold roofs are not suitable for habitation and are usually left empty or used as storage space.  
 An alternative to a cold roof is a warm roof, where insulation is installed above the rafters and below the waterproof membrane. This type of roof build-up is found in properties where the roof space is to be used as a habitable area and is common in loft conversions.  

Why is ventilation important for a cold roof? 

Adequate cold roof ventilation is crucial for preventing moisture buildup and its potentially damaging consequences. If moist air from the living area rises into the roof space and condenses, it can lead to structural damage, particularly to timber elements, as well as damage to any items being stored in the space. Mould growth, damp and other problems can also develop.  
 
A variety of moisture sources can lead to condensation: 

• In new builds, building products contain excess moisture that can take several months or more than a year to dry out 
• Occupants generate water vapour through activities such as cooking, cleaning, bathing and even breathing 
• Temporary condensation can occur during sudden weather changes 

 The building fabric should create a watertight envelope to prevent moisture ingress from the outside, but internally generated moisture is also a significant concern. Ventilation plays a key role in removing this excess moisture, ensuring the roof space and structure remain dry and free from damp, mould growth or decay. 

What standards and regulations apply to cold roof ventilation? 

BS 5250:2021 is the code of practice for the management of moisture in buildings. It sets out the minimum requirements for cold roof ventilation and takes a whole-building approach to managing moisture and preventing condensation. 

 
Building regulations also provide guidance on roof ventilation and vary by region: 

• England: Approved Document C: Site preparation and resistance to contaminants and moisture 

• Wales: Approved Document C: Site preparation and resistance to contaminants and moisture 

• Scotland: Technical Handbook 2022 Domestic Buildings Section 3.15.3: Control of condensation in roofs 

• Northern Ireland: Technical Booklet C: Site preparation and resistance to moisture 

• Republic of Ireland: Technical Guidance Document F, Section 2: Condensation in roofs’. 
  
  

In addition, the following British Standards provide guidance on the construction of pitched roofs, as well as dry-fixing methods for roof tiles and accessories, such as ventilation units: 

• BS 5534:2014+A2:2018: Slating and tiling for pitched roofs and vertical cladding. Code of practice 
• BS 8612:2018: Dry-fixed ridge, hip and verge systems for slating and tiling. Specification 

 

How to effectively ventilate a cold roof 

The right method for ventilating a cold roof will depend on the roof pitch, the type of underlay used and whether the ceiling is "well-sealed". BS 5250 recognises that theoretical design and real-world usage often don’t agree, so incorporating extra ventilation as a fail-safe can help to account for any discrepancies. 
 

Crossflow ventilation should be provided at the eaves and/or ridge level. This should ensure that effective through-ventilation of the whole roof is achieved. It also assists with pulling moisture-laden air out of the building and limiting the risks from condensation. 

 

While every roof is different, sufficient ventilation to achieve compliance with BS 5250 should typically include: 

• 25mm along the length of the eaves for pitches of 10–15° 
• 10mm along the length of the eaves for pitches of more than 15° 
• Additional continuous 5mm ventilation at high level for roofs where the pitch exceeds 35° or for roofs of any pitch with a span of more than 10m for lean-to or mono-pitch roofs. 

In most circumstances, the ventilation requirements outlined in BS 5250 can be achieved by using dry-fix and ventilation products such as eaves ventilation and ventilated dry ridge and hip systems. 
 

Cold roof ventilation from Marley 

Marley offers a wide range of cold roof ventilation products, providing versatile options for various building needs. Products include:  

• Ridge vents 

• Eaves vents 

• Tile vents 
  
  

These products are designed for easy installation and are compatible with our entire range of roofing materials. As part of the Marley Roof System, our cold roof ventilation products can help to ensure a watertight and well-ventilated indoor environment. 

Contact our technical team to discuss how our products can help to ensure effective cold roof ventilation for your next project.