The Origins and History of the Dorran House

The Dorran house, a notable example of post-war British architecture, was manufactured by R Tarran and designed by the same individual. These properties were constructed between 1947 and 1951, a period marked by a significant housing shortage in the UK. The Dorran house was part of a broader initiative to quickly and efficiently provide homes for the population. Approximately 600 of these houses were built, primarily in the form of bungalows and two-storey semi-detached houses.

These homes are also known by alternative names such as Clore Grange, Daleholme, Slingsby, and Tarran-Dorran. The Dorran house is a type of system-built property, which means it was constructed using prefabricated components. This method was popular in the post-war era due to its cost-effectiveness and speed of construction.

Identifying Characteristics of the Dorran House

Architectural Features

Dorran houses are easily identifiable by their medium pitch gable roofs covered with concrete tiles. The external walls are made of storey-height precast reinforced concrete (PRC) panels. At the first-floor level, a PRC ring beam oversails the ground floor wall panels, providing structural integrity and a distinctive architectural feature.

Common Variants

While the standard Dorran house features a gable roof, a flat-roofed version was developed in the 1960s in association with Trollope and Colls. Additionally, some variants include a brick underbuilding, which can affect the overall appearance and structural characteristics of the property.

Construction Details

Substructure

The substructure of a Dorran house typically includes a precast concrete (PC) underbuilding and a concrete slab. A damp-proof course (DPC) is also incorporated to prevent moisture ingress.

External Walls

The external walls are constructed using PRC perimeter beams and storey-height PRC panels. These panels are connected by coach bolts and lined with plasterboard. Insulation is placed between the frame studs to enhance thermal performance. At the first-floor level, a channel section PRC ring beam filled with mortar and reinforced with a single horizontal bar provides additional structural support. Steel tie bars connect the front and rear walls at the eaves level, and a timber wall plate is used to support the roof structure.

Internal Walls and Partitions

The separating wall between properties is typically a lightweight PC block cavity wall. Internal partitions are made of timber studs lined with plasterboard.

Floors

The ground floor consists of timber boarding on timber joists supported by the PRC perimeter beam. The first floor features tongue-and-groove (T&G) or laminated boarding fixed by hooked straps to horizontal reinforcement within the PRC ring beam.

Ceilings and Roof

Ceilings are generally made of plasterboard. The roof structure comprises timber joists and purlins, with concrete tiles providing the external covering.

Typical Defects and Hazards

Cracking of PRC Panels and Ring Beam

One of the most common defects in Dorran houses is the cracking of PRC panels and the ring beam. This can be caused by a variety of factors, including thermal movement, structural loading, and material degradation over time.

Carbonation and Chloride Levels

High levels of carbonation and low levels of cast-in chloride in the PRC panels and ring beam are also typical issues. Carbonation is a chemical reaction that occurs when carbon dioxide from the air penetrates the concrete, leading to a reduction in the alkalinity of the concrete and potential corrosion of the embedded steel reinforcement.

Structural Integrity

The structural integrity of Dorran houses can be compromised by the degradation of the PRC components. Over time, the concrete can lose its strength, and the steel reinforcement can corrode, leading to a loss of structural stability.

Designation as Defective

Dorran houses are designated as defective under the Defective Premises Act. This designation means that these properties are considered to have inherent structural defects that can affect their safety and habitability. As a result, they may be difficult to mortgage and insure, and potential buyers should be aware of the risks associated with purchasing such a property.

Inspection Tips for Surveyors

External Inspection

When inspecting a Dorran house, start with a thorough external inspection. Look for visible signs of cracking in the PRC panels and ring beam. Pay particular attention to the corners and junctions where different components meet, as these areas are more prone to cracking.

Internal Inspection

Inside the property, check for signs of structural movement, such as cracks in the plasterboard lining and uneven floors. Inspect the condition of the timber joists and boarding, and look for any signs of moisture ingress or rot.

Roof Inspection

Examine the roof structure for any signs of sagging or damage. Check the condition of the concrete tiles and ensure that the timber joists and purlins are in good condition.

Carbonation Testing

Consider conducting a carbonation test on the PRC components to assess the extent of carbonation and the potential risk of steel reinforcement corrosion. This test involves taking a small sample of the concrete and applying a phenolphthalein solution to determine the depth of carbonation.

Chloride Testing

A chloride test can also be useful to determine the levels of cast-in chloride in the PRC components. High chloride levels can accelerate the corrosion of the steel reinforcement and compromise the structural integrity of the property.

Practical Considerations for Buyers and Investors

Mortgage and Insurance

Due to their designation as defective, Dorran houses can be challenging to mortgage and insure. Lenders and insurers may require a detailed structural survey and may impose additional conditions or higher premiums. It is essential to consult with a mortgage advisor and insurance broker who have experience with non-standard construction properties.

Renovation and Repair

If you are considering purchasing a Dorran house, be prepared for potential renovation and repair costs. Addressing the structural defects can be expensive, and it is crucial to obtain quotes from reputable contractors who have experience with PRC repairs.

Long-Term Investment

While Dorran houses can present challenges, they can also offer opportunities for long-term investment. With the right repairs and maintenance, these properties can provide affordable housing options and potential rental income. However, it is essential to conduct thorough due diligence and seek professional advice before making a purchase.

Conclusion

The Dorran house is a unique example of post-war British architecture, offering both opportunities and challenges for home buyers and investors. Understanding the construction details, typical defects, and practical considerations is essential for making an informed decision. 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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