The construction industry often cites the 40mm rule when discussing deflection heads because it appears in Approved Document B (ADB). However, this should not be interpreted as a requirement, but rather a structural design assumption for compartment walls.
In this article, we aim to explore the 40mm rule so that design teams can form their own judgment about its relevance and on how to proceed with compliant building design.
What is a deflection head?
A deflection head, also known as a head of wall, is located at the very top of a non-load-bearing wall where it meets the ceiling or the underside of a floor slab. 
Essentially, it allows structural elements of the building, such as concrete floors or steel beams, to move without transferring loads onto the interior partition walls.
What does Approved Document B say about deflection heads?
Paragraph 8.24 of Approved Document B Volume 2 states that compartment walls must tolerate floor movement when exposed to fire using one of these approaches:
- Provide a head detail between the wall and floor that can deform while still keeping its integrity.
- Design the wall to withstand the extra vertical load transferred from the floor above.
If a compartment wall sits within the central half of a floor between vertical supports, floor deflection may be taken as 40 mm, unless an assessment supports a smaller figure. For locations outside this area, the deflection may be reduced linearly to zero at the supports.
Is 40mm a mandatory deflection head requirement?
No, the 40mm figure is not a requirement for deflection heads and firestopping products. However, because of its mention in Approved Document B, this figure has been misinterpreted to be a mandatory movement allowance, requiring:
- all head-of-wall systems to accommodate 40mm;
- every head-of-wall detail to be tested to ±40 mm; or
- have a prescribed deflection gap.
What does the 40mm figure actually mean?
In actual fact, it represents a structural design assumption for designing compartment walls.
ADB recognises that, during a fire, the floor above can deflect as the structure heats up. If the wall is fixed to the floor, that movement could cause the wall to fail; therefore, the wall must either incorporate a deflection head detail that accommodates this, or be designed to resist the load from the deflecting floor. 
40mm represents an assumed downward floor deflection* in the middle half of the structural span between supports, where maximum structural deflection occurs, and it reduces to 0mm at the supports because structural elements deflect least at their bearings.
*The downward bending or sagging of a floor when weight is applied.
Where does the 40mm figure come from?
While it is unknown where the 40mm measurement originates, there have been suggestions that it may have been influenced by the 2005 BRE Report titled: The Integrity of Compartmentation in Buildings During a Fire.
This report highlighted the range of results for differing constructions. For instance:
- Page 7: Lightly reinforced composite slabs with unprotected beams reached a maximum deflection near span/10, with slight recovery on cooling. Precast units deflected up to span/60, leaving residual displacement over span/150. Flat-slab floors showed residual deflection of around span/100.
- Page 8: When intermediate beams are unprotected, the maximum deflection likely to occur within the fire compartment is approximately span/20. For protected beams, the maximum likely deflection is approximately span/100. Therefore, could a nominal 4-meter span at “span/100” explain the adoption of a 40 mm assumption?
How should the required deflection be determined?
Currently, there is no standardised fire-resistance test method that requires a wall-to-floor junction to be subjected to deflection and fire exposure.
From a Quelfire service penetration sealing perspective, it’s crucial to understand that the firestop seal is intended to maintain the integrity of the fire compartmentation.
So, the bigger question here is about the supporting constructions themselves:
- Is the floor tested to achieve the required fire rating, and how much does it deflect?
- Is the wall tested to achieve the required fire rating, and how much does it deflect?
- Is there a tested deflection head detail for the wall?
- Has the project-specific deflection been calculated or assessed?
What are the options for accommodating deflection?
Like all elements of construction, deflection relies on early engagement between teams to design with it in mind. There are really three options.
One, to design the compartment wall to withstand the additional vertical load from the floor above.
Two, if the wall manufacturer can provide a suitable deflection head detail, the Builders’ Work in Connection (BWICs) openings for the services must be located below that detail.
Three, utilise the Quelfire solution, which will be explained in the next section.
You’d then need to calculate or conduct an assessment of the expected deflection at each location.
Can a deflection head accommodate less than 40mm?
Yes, if the structural engineer can demonstrate that the expected deflection is, for example, 10mm, 18mm, or 25mm, those values can be used instead.
In this case, you’d consider a Quelfire head-of-wall detail, such as QB-FW100-D-HOW-03 [image above is for illustration purposes only], that accommodates the assessed movement within the scope of the detail and allows up to 25mm deflection.
What should design teams take away from the 40mm deflection head guidance?
Approved Document B is a guidance document and therefore requires due diligence when used for projects.
The purpose of Paragraph 8.24 is not to set a required movement capability for fire stopping products, but rather to make sure that the compartment wall continues to maintain its required fire rating when exposed to structural movement in the event of a fire.
Deflection needs careful consideration of many factors, such as structural movement and wall performance, but also to determine whether there is an appropriate deflection head detail – these are all matters for the project design team.
If you have any queries about deflection heads, please don’t hesitate to contact the Technical Team.
