Historical Context and Manufacturing Background

The Hawthorn Leslie house, also known as HLB or Plasteel, is a notable example of system-built properties in the UK. Manufactured between 1964 and 1969, these homes were designed by P Keate and F Mould and produced by Hawthorn Leslie (Buildings). A total of 3,600 units were constructed during this period, making them a significant part of the mid-20th-century British housing landscape.

These properties were primarily built as bungalows and two-storey detached, semi-detached, and terraced houses. They were designed to address the post-war housing shortage and were part of a broader movement towards prefabricated and system-built homes. The use of innovative construction techniques and materials aimed to speed up the building process and reduce costs.

Typical Locations and Identification Characteristics

Hawthorn Leslie houses can be found in various parts of the UK, often in suburban areas and new housing estates developed during the 1960s. These homes are easily identifiable by their distinctive architectural features:

• Roof: Shallow pitch gable roofs covered with interlocking concrete tiles or flat roofs covered with bituminous felt.
• External Walls: Storey-height cladding panels coated with various coloured aggregate finishes.
• Timber Cover Strips: Large section timber cover strips mask corner and separating wall junctions.
• Gable Apex: Clad with timber shiplap boarding.

These characteristics make the Hawthorn Leslie house stand out from other system-built properties of the same era.

Construction Details

Understanding the construction details of Hawthorn Leslie houses is crucial for potential buyers and investors. These homes were built using a combination of traditional and innovative materials and techniques.

Substructure

The substructure consists of concrete pads below stanchions, with a perimeter kerb formed in oversite concrete. A damp-proof course (DPC) is also included to prevent moisture ingress.

Frame

The main frame comprises nine RS hollow box stanchions (one single storey) and one RSA floor support beam. The frame layout is designed to provide structural stability and support for the dwelling.

Protective Coating

The frame is coated with brown Primocon paint to protect against corrosion and weathering.

External Walls

The external walls are made of storey-height, dwelling-width galvanised MS channel panels. These panels are clad with asbestos cement sheets and coated with various coloured aggregate finishes. They are lined with plasterboard and filled with polyurethane insulation for thermal efficiency. The panels are bolted through stanchions and overlapped at the first-floor level. Timber cover strips are used at corners and separating walls.

Gable Apex

The gable apex is clad with timber shiplap boarding, adding to the aesthetic appeal of the property.

Separating Wall

The separating wall is a block cavity wall, with 9″ blocks laid flat in the roof space to provide sound and thermal insulation between adjoining properties.

Partitions

Internal partitions are made of honeycomb plasterboard, which is lightweight and easy to install.

Floors

The ground floor consists of chipboard on timber joists, while the first floor also uses chipboard on timber joists. This construction method provides a stable and durable flooring system.

Ceilings

Ceilings are made of plasterboard, which is a common material used in residential construction for its fire resistance and ease of finishing.

Roof

The roof structure comprises timber trusses, covered with bituminous felt and interlocking concrete tiles. This combination provides a robust and weather-resistant roofing system.

Typical Hazards and Risks

While Hawthorn Leslie houses were innovative for their time, they are not without their hazards and risks. Potential buyers and investors should be aware of these issues to make informed decisions.

Corrosion

One of the most common issues with Hawthorn Leslie houses is corrosion. This can occur at the bases of RS hollow box stanchions and plain metal panel frames. Superficial corrosion can weaken the structural integrity of the property over time.

Loose or Missing Holding Down Bolts

Holding down bolts are crucial for securing the structure to its foundation. Loose or missing bolts can compromise the stability of the property, especially during adverse weather conditions.

Separation of Aggregate Coating

The aggregate coating on cladding panels can separate over time, leading to aesthetic and structural issues. This separation can also allow moisture to penetrate the panels, causing further damage.

Warping of Asbestos Cement Cladding Sheets

Asbestos cement cladding sheets can warp due to weathering and age. This warping can create gaps and allow moisture ingress, leading to potential health hazards associated with asbestos exposure.

Rain Penetration

Rain penetration at panel-window frame joints is another common issue. Poorly sealed joints can allow water to enter the property, causing dampness and mould growth.

Roof Space Separating Walls

Poorly constructed roof space separating walls can lead to sound transmission between adjoining properties and reduce thermal efficiency.

Deterioration of Bituminous Felt Roof Cover

The bituminous felt roof cover can deteriorate over time, leading to leaks and water damage. Regular maintenance and timely replacement are essential to prevent these issues.

Racking of Roof Trusses

In detached dwellings, the roof trusses can experience racking, which can compromise the structural integrity of the roof. This issue requires immediate attention to prevent further damage.

Asbestos Cement Sheet Claddings and Fire Protection Casings

Asbestos cement sheet claddings and fire protection casings pose significant health risks if disturbed. Proper handling and removal by certified professionals are essential to ensure safety.

Defective Premises Act and Classification

The Defective Premises Act 1972 aims to ensure that properties are built to a reasonable standard and are safe for occupation. While Hawthorn Leslie houses were innovative for their time, some of the issues mentioned above may classify them as defective under this act.

Properties with significant structural issues, such as corrosion, loose holding down bolts, and warping of asbestos cement sheets, may be considered defective. Buyers and investors should conduct thorough inspections and seek professional advice to determine the extent of any defects and the necessary remedial actions.

Inspection Guide for Surveyors

When inspecting a Hawthorn Leslie house, surveyors should pay close attention to the following areas:

External Inspection

• Roof: Check for signs of deterioration in the bituminous felt or interlocking concrete tiles. Look for any racking of roof trusses in detached dwellings.
• Walls: Inspect the cladding panels for separation of the aggregate coating and warping of asbestos cement sheets. Check for corrosion at the bases of RS hollow box stanchions and plain metal panel frames.
• Windows and Doors: Examine the panel-window frame joints for signs of rain penetration and poor sealing.
• Gable Apex: Ensure the timber shiplap boarding is in good condition and free from rot or damage.

Internal Inspection

• Floors: Check the chipboard on timber joists for any signs of sagging or damage.
• Partitions: Inspect the honeycomb plasterboard partitions for stability and any signs of damage.
• Ceilings: Look for any cracks or signs of water damage in the plasterboard ceilings.
• Separating Walls: Ensure the block cavity wall in the roof space is properly constructed and free from gaps or damage.

Structural Elements

• Stanchions and Beams: Check for any signs of corrosion or damage to the RS hollow box stanchions and RSA floor support beams. Ensure all holding down bolts are secure and in place.
• Protective Coating: Inspect the brown Primocon paint for any signs of wear or damage that could expose the underlying metal to corrosion.

Asbestos

• Cladding Sheets and Fire Protection Casings: Identify any asbestos cement sheets and fire protection casings. Ensure they are in good condition and have not been disturbed. If asbestos is present, recommend professional handling and removal.

Typical Defects and Remedial Actions

Understanding the typical defects in Hawthorn Leslie houses and the necessary remedial actions can help buyers and investors make informed decisions.

Corrosion

Defect: Corrosion at the bases of RS hollow box stanchions and plain metal panel frames.
Remedial Action: Remove any rust and apply a protective coating. Replace severely corroded elements to restore structural integrity.

Loose or Missing Holding Down Bolts

Defect: Loose or missing holding down bolts.
Remedial Action: Tighten or replace holding down bolts to ensure the structure is securely anchored to its foundation.

Separation of Aggregate Coating

Defect: Separation of the aggregate coating from cladding panels.
Remedial Action: Reapply the aggregate coating or replace the affected panels to restore the aesthetic and structural integrity of the property.

Warping of Asbestos Cement Cladding Sheets

Defect: Warping of asbestos cement cladding sheets.
Remedial Action: Replace warped asbestos cement sheets with modern, non-asbestos alternatives. Ensure proper handling and disposal of asbestos materials by certified professionals.

Rain Penetration

Defect: Rain penetration at panel-window frame joints.
Remedial Action: Reseal the joints to prevent water ingress. Replace any damaged window frames or panels.

Roof Space Separating Walls

Defect: Poorly constructed roof space separating walls.
Remedial Action: Reinforce or rebuild the separating walls to improve sound and thermal insulation between adjoining properties.

Deterioration of Bituminous Felt Roof Cover

Defect: Deterioration of the bituminous felt roof cover.
Remedial Action: Replace the bituminous felt roof cover to prevent leaks and water damage.

Racking of Roof Trusses

Defect: Racking of roof trusses in detached dwellings.
Remedial Action: Reinforce the roof trusses to restore structural stability and prevent further damage.

Asbestos Cement Sheet Claddings and Fire Protection Casings

Defect: Presence of asbestos cement sheet claddings and fire protection casings.
Remedial Action: Engage certified professionals to safely remove and replace asbestos materials with modern, non-asbestos alternatives.

Variants and Modifications

Over the years, various modifications and variants of the Hawthorn Leslie house have emerged. These changes were often made to improve the functionality and aesthetics of the properties.

Ground Floor Slab

In some variants, the ground floor slab is thickened around the perimeter to form a ground beam, providing additional structural support.

Intermediate Stanchions

Intermediate stanchions are omitted in narrow-fronted dwellings, simplifying the construction process and reducing costs.

Gable External Wall Stanchions

Gable external wall stanchions are omitted in some variants, relying on alternative structural supports.

Small Detached Bungalows

In small detached bungalows, stanchions are omitted, and alternative construction methods are used to provide structural stability.

External Wall Panels

External wall panels may be flush jointed with timber cover strips or overclad with timber or PVC shiplap boarding for improved aesthetics and weather resistance.

Additional Linings

Additional linings of plasterboard or honeycomb plasterboard may be added to improve thermal and sound insulation.

Concrete Ground Floor

Some variants feature a concrete ground floor instead of chipboard on timber joists, providing a more durable and stable flooring system.

Timber Stud Partitions

Timber stud partitions lined with plasterboard are used in some variants, offering a lightweight and easy-to-install alternative to honeycomb plasterboard partitions.

Roof

The roof may be flat or have a shallow pitch, covered with bituminous felt for weather resistance.

Conclusion

The Hawthorn Leslie house represents a significant chapter in the history of British housing. These system-built properties were innovative for their time, offering a quick and cost-effective solution to the post-war housing shortage. However, they are not without their challenges. Potential buyers and investors should be aware of the typical hazards and risks associated with these properties and conduct thorough inspections to identify any defects.

Understanding the construction details, typical defects, and necessary remedial actions can help buyers and investors make informed decisions and ensure the long-term safety and stability of their investment. 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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