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Inverted roof insulation in the post-Grenfell era

Nathan Good

When it comes to fire safety, soundbites don’t always make for a sound specification

In association with
Ravago’s ultra-thin high-performance extruded polystyrene (XPS) thermal insulation boards being delivered to a project in Hanover Square, London.
Ravago’s ultra-thin high-performance extruded polystyrene (XPS) thermal insulation boards being delivered to a project in Hanover Square, London.

While designing a structure and devising a specification, architects routinely balance multiple competing objectives: cost, regulations, usability, artistic vision and so on.

Balancing trade-offs between these different elements is all part of an architect’s skill, knowing that an overemphasis on any one area can result in a sub-optimal project that leaves everyone involved with a sour taste in their mouths.
The tragic events at Grenfell Tower in 2017 quite rightly led to fire safety considerations being given a greater weighting and evidence presented during the subsequent public inquiry has left many shocked and appalled at the flagrant disregard for potential hazards in the pursuit of profit.

Regulatory changes, construction product industry initiatives and an emphasis on safety as part of architectural CPD are bringing about a much-needed and desired cultural shift.

This desire to avoid a repeat of Grenfell is what has led to clarion calls for an absolute ban on combustible materials for external elements of a structure. While it would be easy to acquiesce to these demands, it is incumbent on architects to remember that sound bites don’t always make for a sound specification. 

Fire safety considerations when specifying inverted roof insulation
Requirement B4 of Approved Document B outlines how external walls and roofs should resist the spread of fire, with Section 12 specifically addressing roof performance.

Only roof systems achieving the Broof(t4) classification are suited for safe use in any roof location. Broof(t4) requires that there be no penetration of a roof system within 60 minutes and that there is a flame spread of less than 0.38m across the region of burning while conducting the preliminary test - it is the highest performing classification for a roof system. 

The Ravago inverted roof system was tested by Warrington Fire in May 2019 to demonstrate it achieves Broof(t4), and therefore per Approved Document B, is considered safe for use in all areas of a roof.

Ravago's Ravatherm XPS X Ultra 300 SL is designed for inverted flat roofs.
Ravago's Ravatherm XPS X Ultra 300 SL is designed for inverted flat roofs.

What about non-combustible insulation?
Some alternative insulation materials are now being promoted for use in inverted roof applications, principally because they are classified as non-combustible. These include cellular glass.

In addition to frequently being considerably more expensive than traditional insulation options, this material faces questions about its long-term performance characteristics. Post-installation the boards are exposed to rainwater that can soak into the insulation material and potentially impact performance and lifespan because of the freeze-thaw cycle. 

Why the inverted flat roof remains relevant
Considering the high cost of land, particularly in urban areas where structures with flat roofs are more prevalent, we should look to the roof as an additional functional space. It is also worth considering how the insulation, waterflow-reducing layer and ballast protect the underlying waterproofing membrane from both mechanical and UV damage.

This approach also facilitates much easier access for maintenance, allowing for the 20 or 30-year warranty to be upheld. It is estimated that, taken together, these benefits can reduce total roof-life costs by almost a quarter.

Concern about the fire safety of buildings will justifiably remain high on the agenda - the spectre of the tragedy at Grenfell is an ominous reminder of what can go wrong when due care is not taken when specifying construction materials. 

For more information and technical support, visit


Nathan Good is technical sales manager at Ravago Building Solutions UK.


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