Keld Rindom, managing director of Green Drain, examines how hybrid occupancy is reshaping drainage performance in commercial buildings and why designers must reassess traditional trap seal assumptions in both new-build and retrofit projects.
Hybrid working is reshaping not only how buildings are used, but how their systems must perform. Fluctuating occupancy levels mean entire floors, amenity areas and ancillary facilities may remain unused for extended periods before returning to peak activity. For architects and specifiers, this presents important questions about system resilience under intermittent use.
While much industry attention has focused on ventilation strategies, energy modelling and flexible layouts, below-ground drainage performance is often overlooked. Yet hybrid occupancy directly challenges long-standing assumptions embedded in conventional drainage design.
Traditional systems assume regular usage. Washbasins, showers, WCs and floor drains incorporate water traps designed to retain a volume of water that forms a seal against foul air. Under steady occupancy, routine use automatically replenishes these seals.
In hybrid buildings, that assumption no longer holds. Reduced and uneven usage means traps in low-traffic areas can sit dormant for days or weeks. In warm, mechanically ventilated environments, evaporation gradually depletes retained water. Once lost, the seal between the drainage system and the occupied space is broken.
From a compliance perspective, this is significant. The integrity of trap seals underpins conformance with hygiene and air quality requirements set out in the Building Regulations. Approved Document H (Drainage and Waste Disposal) requires that drainage systems prevent the ingress of foul air into buildings. However, hybrid occupancy introduces performance variables not traditionally accounted for at design stage.
The most immediate consequence of seal failure is odour ingress. In premium office developments, education settings and mixed-use schemes, internal environmental quality is closely linked to occupier satisfaction and asset value. Persistent odours can undermine perceived building quality and generate avoidable maintenance callouts.
The issue is not defective design, but changed operating conditions. Systems engineered for predictable daily use are now expected to perform reliably in buildings with highly variable occupancy patterns. This exposes a gap between historic design assumptions and contemporary use.
Historically, mitigation has relied on mechanical or automated interventions such as trap primers or programmed flushing systems. While effective in maintaining water seals, these solutions introduce additional components, increase water consumption and require ongoing maintenance. In projects where water efficiency targets and environmental certifications such as BREEAM are central to the brief, continual water input may conflict with sustainability objectives.
Given that the Building Regulations require drainage systems to prevent foul air ingress, designers may need to consider whether reliance on retained water alone remains sufficiently robust in intermittently occupied buildings.
As a result, there is growing interest in solutions that remove dependency on standing water. Waterless trap seal technologies incorporate flexible membranes or one-way valves within floor drains and outlets. These devices open to allow discharge and close when inactive, forming a physical barrier against foul air without requiring retained water.
The design implications are notable. By eliminating evaporation-related seal loss, performance becomes independent of occupancy frequency. Under Approved Document H, maintaining an effective barrier between drainage systems and occupied space is fundamental, reinforcing the need for resilient trap sealing strategies.
From a specification perspective, such systems can typically integrate into standard drainage assemblies or be retrofitted into existing pipework, supporting both new-build and refurbishment projects. As buildings are increasingly expected to scale up or down in response to tenant demand, passive solutions that do not rely on active water supply or programmed controls can reduce long-term operational risk and simplify maintenance regimes.
Hybrid working is unlikely to be temporary. As design teams future-proof buildings through adaptable layouts and flexible infrastructure, drainage performance must form part of that conversation. By acknowledging how intermittent occupancy affects trap seals, architects and specifiers can ensure regulatory compliance, protect internal environmental quality and support long-term asset resilience in hybrid-ready buildings.