The Duo-Slab House

The Duo-Slab house, also known as the Airey house, forms part of the UK’s inter-war non-traditional housing stock. Designed by William Airey and manufactured by William Airey & Sons Ltd, approximately 4,650 units were constructed during the 1920s in response to the post-World War I housing shortage.

These properties are most commonly found as two-storey semi-detached or terraced houses, typically located in suburban settings. The system was also adapted for flats, although these are less frequently encountered.

A typical Duo-Slab (Airey) house with rendered external walls and tiled pitched roof construction characteristic of 1920s non-traditional housing.

Duo-Slab houses are generally two-storey semi-detached or terraced dwellings with medium pitch hipped or gable roofs covered with tiles. The external walls are rendered throughout, giving a uniform external appearance that can obscure the underlying structural system.

The main structural frame comprises clinker aggregate concrete columns. These are infilled with 3-inch clinker aggregate precast concrete blocks, followed by a 2-inch cavity and a further 3-inch layer of clinker aggregate blocks. Timber formers are incorporated within the wall construction.

Ground floors typically consist of timber boarding on timber battens laid over concrete, with concrete flooring commonly found in kitchen areas. First floors are formed with timber boarding laid on timber joists. Separating walls are constructed using clinker aggregate concrete columns with cavity infill blockwork. Internal partitions at ground floor level use clinker aggregate infill, while first-floor partitions are timber stud lined with plasterboard.

High carbonation within clinker concrete columns is a recognised concern in this construction type. Carbonation reduces the alkalinity of the concrete, increasing susceptibility to cracking and potential structural weakening.

The clinker aggregate concrete may also contain elevated sulfate levels. Sulfate reaction can result in expansion and progressive cracking, affecting both structural columns and infill elements if deterioration is advanced.

Horizontal and vertical cracking of the render, particularly at joints between concrete blocks, is commonly observed. These defects create moisture pathways that can accelerate internal deterioration and encourage mould growth.

Bowing of flank external walls has been recorded in some examples. This may arise from settlement, thermal movement or prolonged moisture exposure and warrants close structural assessment. The nominal bearing of lintels is another area of risk, where insufficient support can lead to cracking and localised masonry instability.

Although Duo-Slab houses are not classified as defective under the Defective Premises Act, this does not remove the need for careful condition assessment.

A Duo-Slab house may appeal to purchasers comfortable with non-traditional construction and the need for informed maintenance. The structural system differs significantly from traditional brick cavity housing and should be assessed on its individual condition rather than appearance alone.

• Obtain a detailed building survey focused on concrete condition
• Review evidence of past repairs to render and structural elements
• Assess any history of cracking, movement or damp-related issues

Where significant cracking, bowing or concrete deterioration is evident, further specialist investigation may be required before proceeding.

Lender appetite for non-traditional construction varies. Mortgage decisions are influenced by documented condition, evidence of structural stability and the clarity of professional reporting.

Unresolved carbonation, sulfate attack or significant movement can affect both lender confidence and future resale prospects. Conversely, well-maintained examples with clear survey documentation are often treated more favourably.

A building survey provides an informed assessment of the visible condition and construction of a Duo-Slab (Airey) house. Inspections are non-intrusive and limited to areas that are accessible at the time of inspection.

• Identify visible cracking to clinker concrete columns and associated render
• Assess signs of dampness, moisture ingress, and condensation risk
• Comment on roof coverings, drainage details, and external envelope condition
• Review the general condition of accessible floors, ceilings, and structural elements
• Highlight defects that may require repair, monitoring, or further investigation

However, there are limitations to what a survey can determine without intrusive investigation.

• Confirm the internal condition of concealed clinker concrete reinforcement
• Assess hidden structural elements behind finishes or linings
• Determine the full extent of carbonation or sulfate-related deterioration within concrete
• Inspect areas that are inaccessible or covered at the time of inspection

Where higher-risk indicators are identified, further specialist or intrusive investigation may be recommended.

For a thorough building survey in Greater London, an instant quotation can be obtained through the online survey calculator.