- Manufacturer: R. Tarran
- Designer: R.Tarran
- Number Built: 600
- *Not to be confused with the Tarran-Dorran House
Designated Defective England & Wales Unless Repaired Under Approved Scheme
Dorran PRC House
During the 1950’s, the Hull construction firm of R.G. Tarran, builder of the Tarran Prefab, adapted the former dyeworks of Pullars of Perth to produce the Dorran House, a single storey bungalow built from precast panels.
- Built as Bungalows and 2-storey semi-detached houses.
- Having Medium pitched gable roofs covered with concrete tiles.
- External walls constructed of storey height precast reinforced concrete panels (PRC).
- Visible PRC ring beam at first floor level located above ground floor wall panels.
Brief Notes for Surveyors
- Visible cracking noted on PRC panels. Visible cracking of ring beam.
- High levels of carbonation and low levels of cast-in chloride have been found in all PRC panels including ring beam.
Below is an extract from an excellent article written by Architect, Mark Chalmers on the history and production of Dorran Houses in Scotland. The full article can be found at the Urban Realm Website HERE
Dorran houses are like Structural Insulated Panel Systems or SIPS houses, but pretty much without the “I”.
The Dorran system consists of storey height precast panels which are 16 inches wide, which seems an odd width for a structural module, but we now live in a world which has the luxury of setting out in metric, usually on multiples and fractions of 3000mm.
Whether Dorran licensed the system from Tarran, or the company was a direct subsidiary, isn’t obvious – but during the 1950’s, they sold to the public sector with houses for the Forestry Commission in outlying parts of the Highlands, small developments of social housing in rural counties like Argyll and Caithness, and even domestic accommodation at RAF Saxa Vord on Shetland.
Dorran Construction later offered a range of bungalows to private buyers; the houses had couthy names like The Glen Shean, The Glen Varloch, The Glen Frugart and The Glen Clova. By the 1960’s they fabricated around 10 houses each week at their precast factory in Perth, and a total of 2500 had been erected by 1962.
According to the Building Research Establishment report, two storey Dorrans have a characteristic protruding band at first floor level, like a string course, formed by a precast ring beam. Timber floor panels were strapped to the ring beam with joist straps, and the precast panels were tied together using steel bolts. Some Dorran bungalows appear to have a concrete ring beam at ground level, too.
For vapour control, the concrete panels were lined with 2ply bituminous felt as a dampproof membrane.
When the wall was erected, the joints were filled with gunned bitumen, then pointed up in cement mortar.
Interestingly, a newspaper article from the 1960’s notes that one of their outstanding features was the level of insulation in the walls and ceilings. A half inch thick internal layer of expanded polystyrene sat between the concrete panel and its plasterboard lining … half an inch, which is better than nothing.
By the 1970’s there was disquiet about nontraditional methods of construction, and two of the Dorran houses’ failings were shared by several other systems. The panels’ lack of proper insulation led to condensation problems, and serious corrosion of the steel ties between the panels which were a byproduct of it.
The BRE identified the problems during the early 1980’s, which eventually led the designs to be designated under the Housing Defects Act 1984, which effectively condemned them and made them unmortgageable.
However, the 1984 Act was consolidated into the 1985 Housing Act, which provided grant assistance for people who had unwittingly bought former council houses constructed using problematic systems.
Perhaps if the Dorran system had been constructed using stainless steel fixings, and with effective insulation, the houses might have succeeded. As it is, the remedial works entailed in fixing them range from adding a masonry skin outside the precast panels, to propping the roof then demolishing the panels and building a brand new external wall. Instead, many chose to demolish the existing house and build a timber kit on the site instead.
While it’s true that we probably won’t make exactly the same mistakes again in future, we may make slightly different mistakes in pursuit of the same goals.
A few years ago, I worked on a project with some of the first zero energy houses in Scotland, built using a SIP system comprising JJI joists skinned with OSB board, and a cavity filled with injected cellulose fibre.
There were no insulation or corrosion issues that I’m aware of – but the high degree of airtightness meant that when tenants switched off the whole house ventilation system (because they were concerned about running costs) they suffered from condensation problems in kitchens and bathrooms.
A different kind of prefabrication, a new type of problem, caused partly by a novel approach to design and more so due to poor communication between the landlord and tenant about how the house worked. The other lesson of Dorran Construction Ltd. lies in how we may try to meet today’s unmet demand for housing by using offsite construction, which is the new, untainted name for system building, aka prefabrication.
In the early 1960’s, Dorran looked southwards and after 1964 during which it made a profit of £96,000, the firm opened a precast factory in Consett and another in London. By 1967 it had built 4000 houses across Scotland, but by signing up to fixed price contracts in England during an era of rampant inflation, Dorran committed its fatal mistake.
The company lost money on its English contracts, the factories in Consett and London closed, and the parent firm in Perth struggled to survive. By the time that Dorran Houses began displaying the first symptoms of structural corrosion, the firm was no longer producing them – and the postwar housing boom was over. As demand slackened, the industry pulled back from mass produced housing.
“House factories”, the dream of technocrats and Le Corbusier alike, are much trickier to achieve in practice than you might think. Dorran Houses have been condemned, both literally and metaphorically, as a failed experiment in mass production. After the 1984 Act, mortgage lenders viewed precast panel houses as “Nontraditional” and as far as many buyers were concerned, that euphemism was the kiss of death.
Now, we’re burdened with the legacy of the past when cheap housing was constructed at breakneck speed to replace overcrowded slums; consequently we ended up with a modern version of the same.
Extract Article written by Mark Chalmers 05/02/16
PRC Homes In Scotland
The majority of the PRC Homes that were constructed throughout Scotland were built under the public provision of social housing.
The majority of these properties were owned and administered by Local Authorities but the Scottish Special Housing Association (SSHA) attempted to increase the supply of these homes which soon became an alternative source for the proliferation of non-traditional PRC homes in Scotland. Scottish Homes inherited this housing stock when it was formed in 1989 to replace the SSHA. Scottish Homes itself was disbanded in 2005 and replaced with The Scottish Sanctuary Housing Association.
Since then, additional transfers of housing stock to other associations has taken place thus reducing the knowledge and expertise that was once held in relation to these and other unconventional property types.
This has caused issues within obtaining reliable information as to current stock numbers and condition states, including records of identification, improvement or repair.
There are no clear records or sources of information relating to the privately built stock of PRC homes. The majority of privately built Dorran properties in the highlands were generally built through a private company in Inverness that no longer exists and it is believed that all original records pertaining to these properties has been lost.
There are many examples of planning consents being granted for the demolition of Dorran Houses and their replacement by new dwellings due to the significant cost of repair and limited lifespan.
One example of such a case relates to the Banff and Buchan Area Committee where permission was granted for demolition of five Dorran
bungalows. The planning report stated that the Dorran Construction was classified as a “defective house type” by the Housing Act of 1985 (now repealed in Scotland) and that structural surveys suggested the
houses would only be structurally stable for a further seven to ten years at most. In conclusion it was deemed that “The Dorran properties no longer met building standards and in the wake of specialist investigations that they had a very limited future as tenanted accommodation”. Permission was thus granted for demolition of 5 Dorran bungalows to be replaced by 18 new dwellings of various types providing densification of the estate.
This proposal was supported by all of the existing tenants.
The fact that there are still surviving Dorran Homes today, is testament to the quality to which they were in fact built. There are specialist
companies that do provide refurbishment and repair solutions for these dwellings and other defective dwelling types. Landmark PRC for example, replace all external concrete panels with new insulated panels which whilst extending the life of the dwelling but unfortunately this process does not, without additional refurbishment, meet current Decent Homes Standards. Even when repaired, the Dorran house is considered unable to match the life span and performance of todays newly constructed dwellings. Therefore the most successful route to regeneration and achievement of Decent Homes Standards with respect to Dorran Bungalows in particular, is considered to be via their demolition and re-development.
Whilst this view appears to be the most favourable in respect to regeneration, the private Dorran homeowner may not be in a position to undertake such drastic measures. This would be particularly evident in cases where the property is or has been subject to significant investment via a mortgage or other funding route. In these cases, the homeowner should carefully consider the cost vs return ratio to undertake extensive repair works and perhaps more importantly, obtain a written assessment as to the perceived life extension that such works would provide, if any.