Clay Hollow Pot Floor Systems (Pot & Beam Pre-Cast Floors): A Technical Guide for Retrofit and Adaptive Reuse

Clay Hollow Pot Slabs in Retrofit Practice

From WWII warehouses to high-end restaurants, clay hollow pot slabs still shape

today’s retrofit challenges. MBBM Studio Architects breaks down how to upgrade,

certify, and coordinate these systems for compliant reuse.

Introduction

This guide outlines the characteristics and retrofit considerations of clay hollow pot floor systems—commonly referred to as Pot & Beam Pre-Cast Floors. While rarely specified in contemporary UK construction, these systems remain prevalent in mid-century and industrial buildings. We’ve encountered them recently at Waterloo Place, SW1Y, where MBBM Studio Architects is leading a high-end restaurant refurbishment, and in Essex, CM3, where a WWII-era warehouse is being converted into 60 flats with associated work units.

As a London-based architectural practice specialising in retrofit architecture, planning consultancy, and adaptive reuse, we frequently encounter legacy structural systems. This guide explains how clay hollow pot slabs perform, how to upgrade them, and how to coordinate around their limitations—especially in the context of planning applications, building regulations, and technical documentation.

System Overview and Build-Up

Clay hollow pot slabs are a form of ribbed floor construction that combine structural efficiency with reduced concrete volume. The system typically comprises:

• Clay pots: Hollow terracotta blocks (approx. 230mm x 270mm x 150mm) laid between beams. These act as permanent formwork and reduce concrete volume.

• Beams: Pre-stressed or cast in-situ reinforced concrete spanning between load-bearing supports.

• Screed: A concrete topping poured over the clay pots and beams to provide diaphragm action. Screed is typically unreinforced unless performance upgrades are required.

These systems are typically found in buildings constructed between the 1950s and 1980s, though some suppliers still offer variants for specialist applications.

Structural Performance

Clay hollow pot slabs typically span up to six metres, depending on beam depth and reinforcement. Longer spans may incorporate steel joists or post-tensioning.

Key points:

• Beams carry the primary loads; clay pots act as void formers.

• Screed provides diaphragm action and load distribution.

• Structural engineers should verify suitability for new use cases, especially in change-of-use projects.

Fire Performance

Many existing slabs do not meet current fire resistance standards. Original ratings may be undocumented or insufficient.

Upgrade options include:

• Specialist coatings (e.g. Aithon SFS) to achieve REI 60 or REI 120.

• Application to soffits and beams, coordinated with acoustic treatments.

• Fire strategy overlays in planning documentation to indicate upgraded zones and certification references.

Acoustic Behaviour

The ribbed geometry can disrupt sound transmission paths, but without resilient layers, airborne and impact sound transmission can be problematic.

Retrofit solutions:

• Floating screeds or acoustic mats for impact isolation.

• Suspended ceilings with insulation for airborne sound.

• Careful perimeter detailing to prevent flanking transmission.

Regulatory Compliance

Clay hollow pot slabs must be assessed against current Building Regulations when retained or upgraded:

• Part A (Structure): Confirm load-bearing capacity and robustness.

• Part B (Fire Safety): Upgrade fire resistance and document certification.

• Part E (Sound): Ensure acoustic performance meets residential/commercial standards.

• BSR Gateway Exemptions: May apply in low-rise or standard-risk retrofit projects. Justify with:

• Confirmation of slab typology as non-novel.

• Fire resistance certification.

• Structural calculations or precedent references.

Site Coordination and Survey Strategy

These slabs present specific coordination challenges during refurbishment:

Common issues:

• Restricted horizontal service runs due to clay pots.

• Risk of damaging reinforcement during core drilling.

• Dimensional variation affecting finishes and insulation.

• Limited access for inspection and verification.

Recommended survey steps:

• Visual inspection of pot layout and beam spacing.

• Trial openings to confirm dimensions and reinforcement.

• Structural scanning to locate reinforcement.

• Documentation with annotated drawings and photographs.

Reuse and Sustainability

Retaining clay hollow pot slabs offers sustainability benefits:

• Reduced demolition waste.

• Embodied carbon savings.

• Alignment with circular construction principles.

Material sourcing:

• Reclaimed pots from demolition sites.

• Limited availability of new pots from UK suppliers.

Challenges in Retrofit and Refurbishment

While structurally sound, these slabs pose challenges:

• Limited load capacity for new finishes or mechanical systems or installation of heavy steel staircases with support in form of steel beams. The position of steel beams (whether below, within or over existing slab)

• Restricted service integration—vertical drops must be planned early.

• Missing or unreliable original documentation.

• Dimensional variability requiring levelling or shimming.

• Fire and acoustic limitations requiring retrofit upgrades

Conclusion

Clay hollow pot slabs are rarely specified today, but they remain relevant in retrofit and refurbishment—especially in mid-century and industrial buildings. Understanding how they perform, how to upgrade them, and how to coordinate around their limitations is essential for successful adaptive reuse.

If you’re working with one of these systems and need help drafting planning responses, coordinating fire upgrades, or preparing technical documentation, MBBM Studio Architects can assist with clarity, precision, and strategic advocacy.