Deflections in Fire Stopping: Designing Fire Stopping Systems with Deflection in Mind

With the introduction of the Building Safety Act and its gateway process, passive fire protection systems in buildings over 18 metres must now be specified much earlier in the project lifecycle. As a result, it’s important to account for any anticipated building movement during early design, ensuring fire stopping solutions remain compliant and effective.

Understanding Structural Deflection and Its Impact on Fire Stopping

Deflection refers to the bending or displacement of structural elements under load. When services like pipes or cables are attached to these structures, they move with them. Fire stopping systems must accommodate this movement to prevent failure.

Identifying whether deflection will occur is a key decision point in the planning of passive fire protection. If movement is expected, designers must integrate flexibility into fire stopping solutions – particularly at partition joints and service penetrations – to maintain performance and regulatory compliance.

Where Deflection Affects Fire Protection Most

One of the most sensitive areas in a building is where flexible partition walls meet rigid elements like floors and ceilings. These interfaces are prone to movement, which is typically managed using a deflection head. However, any fire stopping used at these junctions must also absorb this movement to stay intact and compliant.

A fire stop at the ceiling or floor line must match the movement capabilities of the wall system it’s sealing. Products used here must conform to the performance criteria of EN 1366-4 and demonstrate their ability to withstand extension, compression, and forces.

Service Penetrations and Movement Considerations

In addition to partition joints, service penetrations also present challenges when it comes to deflection. Where multiple services penetrate a compartment wall or floor – such as pipes, ducts, or cables – spacing becomes a critical design consideration.

To comply with EN 1366-3, apertures must be spaced with deflection in mind. Typically, the gap between a service and the edge of the substrate should be at least four times the expected deflection. Without this buffer, structural movement could compromise both the integrity of the substrate and the fire stopping performance.

Three-sided penetrations, which contact the slab above, clearly require flexibility to accommodate deflection. However, even four-sided openings not directly touching the deflection head still require movement allowances – since the services they support are ultimately connected to moving structural components.

While there are mechanical solutions such as spring-loaded supports, these are rarely used due to cost and complexity. The most practical and scalable solution is to integrate flexibility directly into the fire stopping system.

Challenges with Testing Standards

While EN 1366-4 covers linear joint fire stopping and includes provisions for movement, there is no equivalent formal testing standard for movement in service penetration seals. Despite this gap, best practice and building regulations (e.g. Approved Document B) recommend that movement still be factored into the design to ensure lifetime performance.

Movement should be accommodated not only in the spacing between services but also within the fire stopping material itself. This is particularly important at the slab edge, where deflection can occur between the building’s structure and facade. While open state cavity barriers offer some inherent flexibility, closed state systems require deliberate accommodation for movement.

The Importance of Early Planning

Rigid fire stopping products cannot be retrofitted into designs requiring flexibility – but flexible systems can be used in rigid setups. That’s why it’s important to establish movement requirements early in the design phase.

By working closely with passive fire protection manufacturers from the outset, specifiers and fire engineers, like our team at Ark Fire Protection, can ensure that flexibility is built into fire stopping systems wherever needed. Early specification helps secure long-term compliance and supports safe, future-proofed building designs.

What Is Acceptable Beam Deflection?

There is no single “safe” number. The limits are about serviceability, keeping a building usable, stopping finishes cracking and protecting whatever is fixed to the beam, rather than about the beam collapsing. UK and European practice (BS EN 1990 and the Eurocodes, and BS 449 and BS 5950 before them) works to well-established limits like these:

• span / 360 for deflection under imposed (live) load, on beams carrying brittle finishes such as plaster.
• span / 250 for total deflection under permanent and imposed load combined.
• span / 500 where the finishes or cladding are especially sensitive.

How much a beam deflects comes down to the load, the span and the beam’s stiffness. For a simply supported beam under an even load, the maximum deflection in the middle is δ = 5wL⁴ / 384EI, where w is the load per unit length, L is the span, E is the modulus of elasticity and I is the second moment of area. The longer the span or the heavier the load, the more it moves, which is why long-span beams and heavily serviced floors need the most careful allowance for movement.

Those limits are our starting point. Once we know how much the structure is going to move, we design fire stopping that can take it, so the seal stays compliant for the life of the building, not just on the day it goes in.