Historical Context and Background

The Underdown house, a notable example of early 20th-century British architecture, was manufactured by F Underdown Ltd and designed by D C Underdown. These properties were constructed between 1926 and 1927, with a total of 4,700 units built, including those under the Winget name. These homes are typically found in suburban areas across the UK, often in semi-detached or terraced configurations.

The Underdown house is a significant part of the UK’s architectural heritage, representing a period when system-built properties were gaining popularity due to their cost-effectiveness and rapid construction times. However, like many properties of its era, the Underdown house comes with its own set of characteristics, potential hazards, and considerations that prospective buyers and investors should be aware of.

Identification Characteristics

Underdown houses are easily identifiable by their two-storey, semi-detached, or terraced designs. They feature medium pitch hipped roofs covered with tiles and external walls that are rendered throughout. These homes were built using a combination of pre-cast reinforced concrete (PRC) slabs and in-situ concrete, which was a common construction method during that period.

Typical Locations

These properties are predominantly found in suburban areas, often forming part of larger housing estates developed in the interwar period. They were designed to provide affordable housing for the growing urban population and are now a common sight in many UK towns and cities.

Manufacturers and Designers

The primary manufacturer of the Underdown house was F Underdown Ltd, with the design work led by D C Underdown. This collaboration resulted in a distinctive architectural style that has stood the test of time, albeit with some structural challenges that need to be addressed.

Construction Details

Understanding the construction details of the Underdown house is crucial for anyone considering purchasing or investing in one of these properties. Here, we will delve into the specifics of the construction methods and materials used.

Substructure

The substructure of the Underdown house consists of concrete strip footings and under-building, with a damp-proof course (DPC) to prevent moisture ingress. This foundation method was typical for the period and provides a stable base for the property.

External Walls

The external walls are rendered and constructed using 12″ x 3′ x 3″ PRC slabs, forming a 9″ cavity wall. These slabs are shaped to act as permanent formwork, allowing for the casting of reinforced concrete (RC) columns. Wire wall ties are used to provide additional stability.

Separating Wall

The separating wall between properties is a PRC panel cavity wall, which provides both structural support and sound insulation.

Partitions

Internal partitions are typically made of brick or block, providing robust and durable internal walls.

Ground Floor

The ground floor consists of timber boarding on timber joists, supported by a timber sole plate. This construction method provides a solid and stable floor structure.

First Floor

The first floor is similar to the ground floor, with timber boarding on timber joists, but it is supported by an RC ring beam for added strength.

Ceilings

Ceilings are constructed using plasterboard, which is a common material for its ease of installation and smooth finish.

Roof

The roof structure consists of timber rafters covered with tiles, providing a traditional and aesthetically pleasing appearance.

Typical Defects and Hazards

While the Underdown house has many positive attributes, it is important to be aware of the typical defects and hazards associated with this property type. These issues can impact the structural integrity and overall safety of the home.

Cracking of Render and Vertical Joints

One of the most common issues with Underdown houses is the cracking of the render and along vertical joints between the PRC wall slabs. This can be caused by a variety of factors, including thermal movement, settlement, and moisture ingress.

Horizontal Cracking and Spalling

Horizontal cracking and spalling in the PRC wall slabs are also common. This can be a result of the corrosion of the reinforcement within the concrete, which causes the concrete to expand and crack.

Corrosion of Reinforcement

The corrosion of reinforcement in the RC columns is a significant concern. This can weaken the structural integrity of the property and lead to further cracking and spalling.

Low Chloride Content in RC Ring Beam

The RC ring beam in Underdown houses typically has a low chloride content, which can contribute to the corrosion of the reinforcement. This is a critical issue that needs to be addressed to ensure the long-term stability of the property.

Designated Defective Under the Defective Premises Act

The Underdown house is designated as defective under the Defective Premises Act. This means that the property has been identified as having significant structural issues that need to be addressed to ensure the safety and habitability of the home. Buyers and investors should be aware of this designation and take appropriate steps to assess and remediate any defects.

Inspection Guide for Surveyors

When inspecting an Underdown house, surveyors should pay close attention to the following areas to identify any potential issues:

External Walls

Inspect the external walls for any signs of cracking, spalling, or other damage. Pay particular attention to the vertical joints between the PRC slabs and the horizontal cracks that may indicate corrosion of the reinforcement.

RC Columns

Examine the RC columns for any signs of corrosion or damage. This is a critical area that can impact the overall structural integrity of the property.

Roof Structure

Check the roof structure for any signs of damage or deterioration. Ensure that the timber rafters and tiles are in good condition and that there are no signs of water ingress.

Ground and First Floors

Inspect the ground and first floors for any signs of movement or damage. Check the timber joists and boarding for any signs of rot or decay.

Internal Partitions

Examine the internal partitions for any signs of cracking or damage. Ensure that the brick or block walls are in good condition and that there are no signs of structural issues.

Typical Construction Variants

There are several variants of the Underdown house that may have different construction details. These include:

Concrete Raft Foundation

Some Underdown houses may have a concrete raft foundation instead of concrete strip footings. This provides a more stable base for the property and can help to reduce the risk of settlement.

Clinker Aggregate PRC Cavity External Walls

In some cases, the PRC cavity external walls may be constructed using clinker aggregate. This can provide additional strength and durability to the walls.

No Wire Wall Ties

Some variants of the Underdown house may not have wire wall ties. This can impact the stability of the external walls and should be carefully inspected.

Brick and MS Strip in Place of RC Ring Beams

In some properties, brick and mild steel (MS) strips may be used in place of the RC ring beams. This can impact the overall strength and stability of the property.

Rectangular RC Columns

Some Underdown houses may have rectangular RC columns instead of the more common circular columns. This can impact the appearance and structural characteristics of the property.

Concrete Ground Floor

In some cases, the ground floor may be constructed using concrete instead of timber. This can provide additional strength and durability to the floor structure.

Unreinforced Concrete Columns Adjacent to Window Openings

Some properties may have unreinforced concrete columns adjacent to window openings. This can impact the overall strength and stability of the property and should be carefully inspected.

Secondary In-Situ Concrete Columns

In some variants, secondary in-situ concrete columns may be present within the cavity at building corners. These columns are tied together with bent reinforcement and provide additional strength and stability to the property.

Conclusion

The Underdown house is a significant part of the UK’s architectural heritage, offering a unique blend of historical charm and structural challenges. Understanding the construction details, typical defects, and potential hazards associated with these properties is crucial for anyone considering purchasing or investing in one.

By being aware of the common issues and taking appropriate steps to assess and remediate any defects, buyers and investors can ensure that their Underdown house remains a safe and habitable home for years to come.

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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