Overview
The Orlit Type II house is a precast reinforced concrete (PRC) system-built house from the 1940s and 1950s, manufactured by Orlit Ltd. Around 17,000 Orlit houses were built, a figure that covers both the Type I and Type II. They were part of the post-war drive to provide affordable, quickly built homes for a growing population.
After the Second World War there was a desperate need for housing, and government and the private sector worked together on system-built houses that could go up quickly and economically. The Orlit system relied on PRC components to speed the building process. The houses are predominantly found in suburban areas, often in clusters that form whole neighbourhoods of similar style, with common locations including parts of Scotland, the Midlands and regions of England where post-war demand was particularly high.
Typical characteristics & construction
The house sits on pad foundations with precast concrete stub columns cast into them. The frame is 5½-inch by 5½-inch PRC columns with integral beam stubs and bolted steel flange connectors, carrying 7½-inch PRC primary and secondary beams with concrete cover.
The external walls are a composite: 2-inch precast base bonding units, 2-inch precast slabs, a cavity, and 2½-inch lightweight precast slabs, with timber framing lined with plasterboard inside and galvanised mild steel ties. The separating wall is a PRC frame infilled with foamed slag aggregate concrete blocks, and partitions are lightweight concrete slabs.
The ground floor is concrete. The first floor is timber boarding on timber beams carried by the PRC secondary beams. Ceilings are plasterboard. The roof is formed of PRC beams spanning between internal columns, inverted PRC trough-shaped units with concrete fill, timber wall plates, timber trusses and purlins, finished in tiles on pitched roofs or asphalt or bituminous felt on flat roofs.
| Element | Typical construction | Inspection focus |
|---|---|---|
| Substructure | Pad foundations with precast stub columns cast in | Signs of movement |
| Frame | 5½-inch PRC columns with beam stubs and bolted steel flange connectors; 7½-inch PRC primary and secondary beams | Cracking, thin or damaged cover, carbonation, chloride-induced corrosion, cracking at stitches |
| External walls | Precast base bonding units and slabs, cavity, lightweight precast slabs, plasterboard on timber framing, galvanised MS ties | Cracks and deterioration, especially at gable apexes and tile hanging |
| Separating wall and partitions | PRC frame infilled with foamed slag aggregate blocks; lightweight concrete slab partitions | Cracking or movement |
| Floors | Concrete ground floor; timber boarding on timber beams on PRC secondary beams | Unevenness or damage; cracking to secondary beams |
| Ceilings | Plasterboard | Water damage |
| Roof | PRC beams, inverted PRC trough units with concrete fill, timber wall plates, trusses and purlins; tiles or asphalt/bituminous felt | Wear, leaks and water damage, particularly to flat roofs |
Potential hazards & inspection priorities
Primary concern: cover, carbonation and chloride in the PRC frame
The main PRC columns and beams have only up to 30 mm of concrete cover, which leaves them susceptible to carbonation and chloride ingress. Carbonation depths reach up to 20 mm, and significant chloride content has been found in some beams. Both lead to corrosion of the steel reinforcement and compromise the structure.
Inspection should look for cracking, particularly around the PRC columns and beams, and for anywhere the cover appears thin or damaged. A cover meter measures the depth of cover and helps identify areas at risk of carbonation or chloride ingress. Where significant deterioration is found, further investigation by a structural engineer follows.
Primary concern: cracking at the stitches and secondary beams
Cracking is a common issue in properties of this type, particularly in the ordinary Portland cement (OPC) and high alumina cement (HAC) stitches. The HAC concrete in the stitches is often of low quality, which adds to the weakness. The secondary beams are also prone to cracking. Beyond these, other PRC components deteriorate over time through environmental exposure and the inherent weaknesses of the materials, with results ranging from minor cosmetic damage to significant structural concern.
Secondary concerns: roofs and walls
Flat roofs covered with asphalt or bituminous felt are prone to leaks and water damage, which lead to further structural problems if not dealt with promptly. The external walls show wear and tear, particularly around the gable apexes and tile hanging. Inside, the separating walls and partitions should be checked for cracking or movement, the floors for unevenness or damage, and the ceilings for water damage.
Long-term consideration: remedial work
Where significant defects are found, the scope of remedial work needs to be understood. It includes repairing or replacing damaged PRC components, dealing with carbonation and chloride ingress, and strengthening the overall structure. A structural engineer should be consulted to develop a comprehensive repair plan.
Is this house right for you?
Orlit Type II houses are a significant part of the UK's post-war housing, but they come with inherent risks and structural issues. The decision should rest on a clear picture of the frame's condition and the remedial work needed.
- Commission a comprehensive building survey from a qualified surveyor
- Have the PRC columns and beams assessed for cover, carbonation and chloride content
- Engage a structural engineer where deterioration is found, and obtain a repair plan
- Budget for repairs and maintenance, including carbonation and chloride-related work
- Consider the long-term viability of the house where significant remedial work is needed
Thorough inspection, realistic budgeting and expert advice are what make a well-informed decision on an Orlit Type II house.
Mortgage & resale considerations
Under the Housing Defects Act 1984, Orlit Type II houses are designated as defective. The designation stems from the structural issues inherent in their PRC construction, particularly the susceptibility to carbonation and chloride-induced corrosion, and as a result these houses may require significant remedial work.
The cost of dealing with carbonation, chloride ingress and the other common defects should be factored in when weighing the property's overall value and its future value.
What a survey can — and cannot — tell you
A building survey provides an informed assessment of the visible condition and construction of an Orlit Type II house. Inspections are non-intrusive and limited to areas that are accessible at the time of inspection.
- Identify visible cracking to PRC columns, beams and other components
- Identify areas where the concrete cover appears thin or damaged
- Comment on flat and pitched roof coverings, gable apexes and tile hanging
- Review the condition of separating walls, partitions, floors and ceilings
- Highlight defects that may require repair, monitoring, or further investigation
However, there are limitations to what a survey can determine without intrusive investigation.
- Measure carbonation depth or chloride content without testing
- Confirm the condition of reinforcement concealed within the PRC frame
- Assess the quality of concrete in the HAC and OPC stitches
- 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.
