How Thick is a Flat Roof in the UK?

Flat roofs are commonly found on extensions, garages, dormers, commercial buildings and porches in the UK. Determining the right thickness is crucial for durability, preventing leaks, insulation, and adhering to building regulations. The thickness depends on the roofing material used, climate and weather conditions, pitch, and intended purpose.

This comprehensive guide examines the key factors in determining flat roof thickness in the UK. We’ll overview building regulations, analyze the most common roofing materials and required thicknesses, consider climate impacts, and answer frequently asked questions about maintaining optimal flat roof thickness.

Building Regulations for Flat Roofs

Building regulations set the standards for design and construction in the UK, covering critical factors like safety, energy efficiency, accessibility, sound insulation and fire safety. Relevant sections related to flat roofs include:

Part A: Structure – impacts strength requirements
Part C: Site preparation and resistance to contaminants and moisture – influences weatherproofing
Part L: Conservation of fuel and power – governs insulation

Minimum Thickness Requirements

The main regulation influencing flat roof thickness is Part C, which mandates a minimum construction thickness for flat roofs of not less than 30mm. This refers to the waterproof layer itself.

However, merely meeting this bare minimum thickness is not advisable, as it will likely lead to issues with leaks, ponding water, damage and mold.

Exceptions and Variations

Exceptions to the 30mm minimum standard include:

Non-habitable garages or detached buildings below 30 sq m
Refurbishment projects where the existing structure inhibits meeting standards
Installation of green roofs

In these cases, lower thicknesses may be allowable, subject to approval from building control inspectors.

Types of Flat Roof Materials and Thickness

Asphalt

Asphalt is a bituminous roofing material composed of strong waterproofing agents. For flat roofs, asphalt thickness should be 9-13mm over weatherproof decking.

Advantages:

Durable
Long-lasting (over 30 years)
Can withstand ponding water

Disadvantages:

Damaged by high heat and cold temperatures
Heavy (adds substantial roof load)

EPDM

EPDM (ethylene propylene diene terpolymer) is a popular synthetic single-ply flat roof membrane with reliable durability and flexibility. Recommended thickness is 1.14–1.52mm, with additional layers for areas likely to experience damage.

Pros

Extremely durable and weatherproof
Lightweight and flexible
Easy to install and repair

Cons:

Vulnerable to tears or punctures
Can experience shrinkage and contraction over time

Built-Up Roofing (BUR)

BUR systems (also called tar-and-gravel) provide exceptional protection and a long service life. They involve layers of bitumen membranes reinforced with materials like felt, topped with an aggregate surface.

For BUR flat roofs:

Bitumen waterproofing layer thickness should be ≥ 4mm
Total system thickness is typically 38-51mm

Benefits

Very effective waterproofing
Long lifespan – can last over 30 years
Withstand ponding water and snow loads

Potential Drawbacks

Additional roof load from multiple layers
Installation is labor-intensive
Vulnerable to solar heat damage

Climate Impact on Flat Roof Thickness

The UK’s temperate maritime climate means seasons exhibit variations in temperatures, rainfall, snow and wind. This affects the optimal flat roof thickness.

Cold Climate Considerations

In winter months, flat roofs must withstand heavy snow loads, ice dams, freeze-thaw cycles plus cold temperatures and winds. To cope with this:

Increase insulation thickness to retain warmth and prevent ice buildup
Use materials resistant to low temperatures like EPDM and asphalt
Install protective layers over waterproofing to shield from damage

Recommended minimum total thickness for cold climates: ≥100mm

Hot Climate Factors

UK summers can reach fairly high temperatures. To handle heat and sunlight exposure:

Use reflective coatings and surfaces
Ensure adequate vapor barriers and ventilation
Select materials capable of withstanding heat and UV light like EPDM

Suggested minimum for hot climates: ≥75mm

Frequently Asked Questions

What is the standard flat roof thickness in the UK?

The technical standard for flat roof waterproofing thickness is 30mm. However, additional protective and insulating layers mean total thickness typically ranges from 50mm up to 200mm.

Can I use insulation to compensate for thinner waterproofing?

Increasing insulation thickness cannot offset an inadequate waterproof barrier. Insulation protects the roof structure rather than weatherproofs it. Adequate waterproofing thickness is essential.

Are there eco-friendly flat roof options?

Yes, green roofs with soil and planted vegetation provide environmental benefits. They require specialist design and installation but are permitted to have reduced thickness. Other eco options include solar reflective paint, recycled materials and responsibly sourced suppliers.

How does roof pitch impact thickness?

Low-pitched flat roofs require greater thickness – at least 50mm. Steeper pitches enable improved drainage and weatherproofing, allowing possibility of lower thickness but still adhering to 30mm minimum standards.

What maintenance preserves thickness?

Inspect flat roofs annually and perform repairs quickly to prevent further deterioration or leaks which can damage waterproofing integrity. Control vegetation growth and clear drains and outlets regularly. Top up aggregate on BUR roofs when thinning.

Is planning permission needed to increase thickness?

If thickness changes exceed 200mm, planning permission may be required. Additionally, excess weight could require structural reinforcements. When planning flat roof projects, consult with your local authority building control department.

Roofing Chingford advises on the optimal thickness for flat roofs in the UK, considering insulation, durability, and building regulations.

Thanks for reading our post, feel free to check out our other services: