Non Traditional Construction (Part 2)

PRCs, Cornish Type 1

What’s a PRC?

PRC stands for ‘PreCast Reinforced Concrete’. The drive for more cost-effective ways of building house began before WWll. So a number of ‘system built’ homes were constructed in the Thirties. However, the war caused huge skilled labour shortages along with was a surplus of steel and aluminium production.  As a result, many new varieties of prefabricated concrete (in both pre-cast and in-situ forms), timber framed and steel framed systems emerged. Between 1945 and 1955, around half a million homes were system built. That was about 20% of the new housing stock! And a further three quarters of million between 1955 – 1970.

What’s the problem?

When the panels were cast originally, either off or on site, the metal reinforcements were embedded in the concrete. Concrete is a highly alkaline material and offers protection to the embedded steel.

However the carbon dioxide in the atmosphere,  carbolic acid in rainwater and chlorides in the original concrete can mix. Over time, this can affect the composition of the concrete. The concrete undergoes a chemical change called carbonisation. The metal reinforcements in the panels and supports then become less protected and the metal can corrode. When the metal corrodes it will expand and cause the surrounding concrete to crack and fail. This can be readily seen in concrete fence-posts in gardens which fail and expose the rusted metal reinforcements.

These failings were highlighted in the 1980’s, when such Council housing came onto the private market. This often meant that mortgage companies would refuse to lend on PRC properties. The government then stepped in and many of these systems were declared as “defective” under the 1985 Housing Act.

The government then set up a company to license “repairs” of these properties. This involved stripping out the old concrete and replacing it with brick and blockwork. This was often done whilst the occupants were still in-situ. This type of work was expensive, but it gave the house a PRC Certificate. This enabled mortgage lenders to consider the property as suitable for lending against. It should be noted that if an adjoining property has not been “repaired” this can also be grounds for a lending companies Valuer to refuse a mortgage so look at the neighbouring properties as well.

The Cornish Type 1

One of the most instantly recognisable PRCs is the Cornish Type 1. This has a distinctive mansard hipped roof, clad with vertically hung tiles on the upper floor.A mansard roof has sloping sides, each of which becomes steeper halfway down. The Type 1 looks a bit like a bungalow but withdormer windows around the first floor!

Thirty thousand Cornish 1 & 2 units were constructed in England over a 20 year production run. They were made by the Central Cornwall Concrete & Artificial Stone Co. Many are a distinctive part of the Bristol landscape and they can be seen from Chipping Sodbury to Long Ashton. The Type 1 was particularly common in Brentry, Thornbury,Patchway and also in Little Stoke.

Both Cornish systems were declared as “defective” under the 1985 Housing Act and many were subsequently ‘repaired’.  However, over 30 years after this legislation, we still see many units that are in their original condition, especially in Patchway.

What repairs are usually required?

Common issues are the joints where the soil vent pipe penetrates through the roof covering this is a design issue and normally remedied by better flashing around this joint and is normally noticeable by staining of the ceiling in the Bathroom.

These suffer badly from poor thermal performance of the first floor which can lead to mould growth. The original insulation in the Mansard type roof was notoriously thin at around 25mm in thickness, which is very minimal and if improvements have been made they are often only visible using a thermal imaging camera.

As with most PRC properties asbestos was also used in soffits and floor tiles and roofing of the outbuildings.