Introduction to the Bison Crosswall House

The Bison Crosswall house, a notable example of system-built properties, emerged in the 1960s as a response to the post-war housing shortage in the UK. These properties, designed and manufactured by Concrete Ltd, were part of a broader movement towards prefabricated construction methods aimed at expediting the building process and addressing the urgent need for affordable housing. This article delves into the intricate details of the Bison Crosswall house, providing home buyers and investors with essential insights into its construction, typical characteristics, potential hazards, and inspection guidelines.

Historical Context and Manufacturing Details

Period and Location

The Bison Crosswall house was predominantly constructed from the 1960s onwards, with a significant number of these properties built across various regions in the UK. These houses were primarily erected in urban and suburban areas, where the demand for quick and cost-effective housing solutions was most pressing.

Manufacturers and Designers

Concrete Ltd, both the manufacturer and designer of the Bison Crosswall house, played a pivotal role in the development and proliferation of this construction method. Their expertise in prefabricated concrete components allowed for the rapid assembly of these homes, making them a popular choice during the housing boom of the mid-20th century.

Typical Characteristics of Bison Crosswall Houses

Architectural Features

Bison Crosswall houses are typically two-storey semi-detached or terraced properties. They exhibit a range of roof styles, including shallow pitch gable roofs covered with concrete tiles, monopitch roofs with aluminium sheets, and flat roofs with bituminous felt. The external walls are often constructed entirely of brick or feature a combination of brick and horizontal timber boarding on the front and rear elevations.

Construction Details

Substructure

The substructure of Bison Crosswall houses consists of concrete strip footings and concrete under-building, with a damp-proof course (DPC) to prevent moisture ingress.

External Walls

The external walls are a mix of brick, cavity, and precast (PC) blocks. The front and rear walls typically feature brick-on-edge copings, while the flank walls incorporate a 2-inch cavity and 4-inch storey-height PC panels with cast-in loop reinforcement and vertical lacing bars. Infill concrete is used between the panels, and PC wall capping and insulating blockwork complete the construction.

Separating Walls

Separating walls are constructed using 7-inch storey-height PC panels with cast-in loop reinforcement and vertical lacing bars, along with infill concrete. Wall ties project from the ends of the walls on the front and rear elevations, and panels used for projecting cross walls are thickened to 10 inches at the external wall.

Partitions

Internal partitions are typically timber stud walls lined with plasterboard.

Floors

The first floor comprises screeded 9 1/2-inch Bison Widespan M hollow-cored PC floor slabs with 1/2-inch compressed polystyrene insulation. The ground floor is constructed of concrete.

Ceilings

Ceilings are generally finished with plasterboard.

Roof

The roof structure includes 9 1/2-inch Bison Widespan M PC floor slabs, insulation to falls, and bituminous felt.

Variants

Several variants of the Bison Crosswall house exist, including:

• Shallow pitch timber roofs covered with concrete tiles and PC gable apex panels.
• Monopitch roofs with reinforced concrete (RC) spandrels, timber trusses, and aluminium sheets.
• Cross walls made of plasterboard on timber framing, with or without insulation.
• External walls with foam insulation in the cavity.
• Front and rear walls featuring timber frame infill panels clad with horizontal timber boarding.

Typical Hazards and Risks

Structural Concerns

One of the primary concerns with Bison Crosswall houses is the potential for structural issues related to the durability of the reinforced concrete components. Over time, these elements can suffer from corrosion, particularly if the concrete has not been adequately protected from moisture ingress. This can lead to a weakening of the structural integrity of the property.

Thermal Performance

The thermal performance of Bison Crosswall houses can vary significantly depending on the specific construction details and any subsequent modifications or upgrades. Properties with inadequate insulation may suffer from poor energy efficiency, leading to higher heating costs and potential issues with damp and condensation.

Moisture Ingress

Moisture ingress is a common issue in older Bison Crosswall houses, particularly those with flat roofs or inadequate damp-proofing measures. This can lead to problems such as damp, mould, and timber decay, which can compromise the overall condition of the property.

Defective Premises Act

Bison Crosswall houses are not generally classified as defective under the Defective Premises Act. However, it is essential for potential buyers and investors to be aware of any specific issues or defects that may affect individual properties, particularly those related to the durability of the reinforced concrete components.

Inspection Guidelines for Bison Crosswall Houses

Pre-Inspection Preparation

Before conducting an inspection of a Bison Crosswall house, it is crucial to gather as much information as possible about the property’s history, including any previous repairs or modifications. Reviewing relevant reports and guidelines, such as the BRE Report BR 118 and BRE Digests on reinforced concrete durability, can provide valuable insights into potential issues to look out for.

External Inspection

Roof

Inspect the roof for signs of damage or deterioration, particularly if the property has a flat roof or monopitch roof with aluminium sheets. Look for issues such as cracked or missing tiles, damaged felt, or signs of water ingress.

Walls

Examine the external walls for signs of cracking, spalling, or other damage to the brickwork or PC panels. Pay particular attention to areas around windows and doors, as these can be prone to movement and settlement.

Damp-Proof Course

Check the condition of the damp-proof course (DPC) and look for signs of rising damp or moisture ingress at the base of the walls.

Internal Inspection

Floors

Inspect the floors for signs of movement, cracking, or unevenness. Pay particular attention to the condition of the screed and any insulation materials.

Walls and Partitions

Examine internal walls and partitions for signs of cracking, movement, or moisture ingress. Check for any signs of corrosion in the reinforced concrete components, particularly in areas where the concrete may be exposed.

Ceilings

Inspect the ceilings for signs of sagging, cracking, or water damage. Pay particular attention to areas around roof penetrations, such as chimneys or skylights.

Additional Considerations

Thermal Performance

Assess the thermal performance of the property by checking the condition and effectiveness of any insulation materials. Consider the potential for upgrading insulation to improve energy efficiency and reduce heating costs.

Moisture Management

Evaluate the property’s moisture management measures, including the condition of the roof, damp-proof course, and any ventilation systems. Look for signs of damp, mould, or condensation, and consider the potential for remedial works to address any issues.

Typical Defects in Bison Crosswall Houses

Concrete Deterioration

One of the most common defects in Bison Crosswall houses is the deterioration of the reinforced concrete components. This can manifest as cracking, spalling, or corrosion of the steel reinforcement, particularly in areas exposed to moisture. Regular maintenance and timely repairs are essential to address these issues and prevent further deterioration.

Thermal Bridging

Thermal bridging can occur in Bison Crosswall houses, particularly in areas where the insulation is inadequate or has been compromised. This can lead to cold spots, condensation, and increased heating costs. Upgrading insulation and addressing any gaps or weaknesses in the thermal envelope can help mitigate these issues.

Moisture Ingress and Damp

Moisture ingress and damp are common issues in older Bison Crosswall houses, particularly those with flat roofs or inadequate damp-proofing measures. This can lead to problems such as mould, timber decay, and damage to internal finishes. Regular inspections and maintenance of the roof, damp-proof course, and ventilation systems are essential to manage moisture levels and prevent damp-related issues.

Movement and Settlement

Movement and settlement can occur in Bison Crosswall houses, particularly in areas with poor ground conditions or inadequate foundations. This can lead to cracking in the walls, floors, and ceilings, as well as issues with doors and windows. Monitoring and addressing any signs of movement or settlement is crucial to maintaining the structural integrity of the property.

Conclusion

The Bison Crosswall house represents a significant chapter in the history of UK housing, offering a unique blend of prefabricated construction and traditional architectural features. While these properties can present certain challenges, particularly related to the durability of the reinforced concrete components and thermal performance, they also offer potential for improvement and modernisation.

For home buyers and investors, understanding the typical characteristics, potential hazards, and inspection guidelines for Bison Crosswall houses is essential to making informed decisions and safeguarding their investment. Regular maintenance, timely repairs, and potential upgrades can help address common issues and enhance the overall condition and value of these properties.

At Flettons, we’re committed to safeguarding your investment. When considering a property purchase, trust our seasoned expertise to reveal any hidden threats. For a thorough building survey, get your instant quote through our quote calculator or reach out directly at 0203 691 0451. Your home’s safety is our top priority.

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