Overview
The Underdown house was built between 1926 and 1927, manufactured by F Underdown Ltd and designed by D C Underdown. It belongs to a period when system-built houses were gaining ground because they were cost-effective and quick to put up, and it was designed to provide affordable housing for a growing urban population.
In total 4,700 were built, a figure that includes the houses built under the Winget name. Underdown houses are found in suburban areas across the UK, often as part of larger interwar housing estates, and are a common sight in many towns and cities. They are two-storey semi-detached or terraced houses, built using a combination of precast reinforced concrete (PRC) slabs and in-situ concrete.
Typical characteristics & construction
The house stands on concrete strip footings and underbuilding with a damp-proof course (DPC), a foundation method typical of the period. The external walls are rendered 12-inch x 3-foot x 3-inch PRC slabs forming a 9-inch cavity wall. The slabs are shaped to act as permanent formwork, so that reinforced concrete (RC) columns could be cast within them, and wire wall ties give the walls additional stability.
The separating wall is a PRC panel cavity wall, providing structural support and sound insulation. Internal partitions are brick or block. The ground floor is timber boarding on timber joists on a timber sole plate. The first floor is timber boarding on timber joists carried on an RC ring beam. Ceilings are plasterboard and the roof is timber rafters covered with tiles.
Several variants exist. Some houses have a concrete raft foundation in place of strip footings, which gives a more stable base and reduces the risk of settlement. Some have clinker aggregate PRC cavity external walls, which add strength and durability to the walls. Some have no wire wall ties, and in some, brick and mild steel (MS) strip are used in place of the RC ring beams, which affects the strength and stability of the house. Others have rectangular RC columns in place of the more common circular columns, a concrete ground floor in place of timber, unreinforced concrete columns adjacent to window openings, or secondary in-situ concrete columns within the cavity at the building corners, tied together with bent reinforcement.
| Element | Typical construction | Inspection focus |
|---|---|---|
| Substructure | Concrete strip footings and underbuilding, DPC (concrete raft in some variants) | Signs of settlement |
| External walls | Rendered 12-inch x 3-foot x 3-inch PRC slabs forming a 9-inch cavity wall, wire wall ties | Cracked render, cracking along vertical joints, horizontal cracking and spalling |
| Columns and ring beams | RC columns cast within the slabs; RC ring beam at first floor | Corrosion of reinforcement, damage |
| Separating wall and partitions | PRC panel cavity wall; brick or block partitions | Cracking, damage, signs of structural issues |
| Floors | Timber boarding on timber joists, on a timber sole plate at ground floor and the RC ring beam at first floor | Movement, rot or decay to joists and boarding |
| Ceilings | Plasterboard | General condition |
| Roof | Medium-pitch hipped; timber rafters and tiles | Damage or deterioration, water ingress |
Potential hazards & inspection priorities
Primary concern: corrosion of reinforcement
Corrosion of the reinforcement in the RC columns is a significant concern. It weakens the structural integrity of the property and leads to further cracking and spalling. The columns should be examined closely for corrosion or damage, as this is the area that bears most directly on the structure.
The RC ring beam has a low chloride content, which contributes to corrosion of the reinforcement and is a critical issue for the long-term stability of the house.
Primary concern: cracking and spalling of the PRC wall slabs
Horizontal cracking and spalling in the PRC wall slabs are common. They result from corrosion of the reinforcement within the concrete, which expands and cracks it. The external walls should be inspected for cracking, spalling and other damage, with particular attention to horizontal cracks that indicate corrosion.
Secondary concern: cracking of render and vertical joints
Cracking of the render, and along the vertical joints between the PRC wall slabs, is one of the most common issues with these houses. Thermal movement, settlement and moisture ingress all contribute. The vertical joints should be checked carefully at inspection.
Long-term considerations: variants, floors and roof
Variants without wire wall ties, with brick and MS strip in place of RC ring beams, or with unreinforced concrete columns next to window openings affect the strength and stability of the house and need careful inspection. The ground and first floors should be checked for movement and for rot or decay in the joists and boarding, the brick or block partitions for cracking or damage, and the timber rafters and tiles for damage, deterioration and water ingress.
Is this house right for you?
The Underdown house is a significant part of the UK's interwar housing, but it has recognised structural problems that have to be assessed and remedied. The variant, and the condition of the reinforced concrete, decide how serious they are in any one house.
- Commission a full building survey
- Establish the condition of the RC columns and the reinforcement within them
- Check the PRC wall slabs for horizontal cracking and spalling, and the vertical joints for cracking
- Identify which variant the house is, including whether wall ties and ring beams are present
Being aware of the common issues, and taking the right steps to assess and remedy defects, is what keeps an Underdown house safe and habitable for years to come.
Mortgage & resale considerations
The Underdown house is designated as defective under the Housing Defects Act 1984. This means the type has been identified as having significant structural defects arising from its design or construction. Buyers should be aware of the designation and take appropriate steps to assess and remedy any defects.
What a survey can — and cannot — tell you
A building survey provides an informed assessment of the visible condition and construction of an Underdown house. Inspections are non-intrusive and limited to areas that are accessible at the time of inspection.
- Identify cracked render, cracking along vertical joints and horizontal cracking in the PRC slabs
- Identify visible spalling and other signs of reinforcement corrosion
- Comment on the roof, floors and partitions, including movement, rot and water ingress
- Note visible features that point to a particular variant
- 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 condition of reinforcement within the RC columns and ring beam
- Establish whether wall ties are present, or whether columns are reinforced, behind render
- Measure chloride content or other properties of the concrete without testing
- 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.
