Wimpey No Fines Concrete
Manufacturer: George Wimpey & Co. Ltd
Period built: 1940s–1970s
Number built: 300,000
Alternative names: Butterfly, Butterfly No-Fines, Formwall, Gateshead Butterfly, Gateshead No-Fines, No-Fines Wimpey, Wimpey W6M
George Wimpey and Co. developed a system of housing using no-fines concrete and traditional construction methods. The concrete without sand has unusual properties in that it can be cast within a mesh or standard formwork and exerts less pressure while mobile than normal concrete. Its porous nature provides some insulating properties although not adequate for today’s requirements.
In excess of 300,000 low-rise dwellings from the mid-1940s onwards were built in configurations ranging from bungalows, houses and low-rise blocks of flats of up to 5 storeys.
Before 1951 the external walls were commonly 12” (305mm) thick, whereas between 1951 and 1964 this reduced to 10” (250mm).
After 1964 the external walls were commonly 8” (200mm) thick.
As a general method, a band of reinforcement bar was incorporated in dense insitu concrete at eaves level. Support above ground floor door and window openings is provided by precast lintels with projecting reinforcement, although before 1951 only reinforcement bar was used. The load bearing internal walls were sometimes masonry or timber rather than no-fines. The no-fines sometimes formed the inner leaf of a masonry-faced cavity construction. Rendering was applied in two or more coats to the external face depending on the exposure conditions. Internally the walls were dry-lined or hard plaster. The party walls were sometimes rendered to reduce sound transmission.
The purpose of this form of construction was to reduce cost and leave the wall aerated and thus aid insulation. The wails are un-reinforced and rely on their mass for strength.
The concrete was formed in-situ usually from a batching plant on site and the formwork used to frame out the wall removed after the concrete had set. It is normally the case that “No Fines” construction in domestic dwellings does not have a cavity.
In the case of this house there are vents in the walls, but we believe that these will relate to the previous pantry area and boiler location. During the course of our inspection we did not locate any definite evidence which would suggest that if a cavity exists, cavity wall tie failure was occurring.
In this house the external walls and central party walls are constructed from ‘no fines concrete’, with the external face of the walls rendered to increase weather protection.
1) It can be difficult to replace the render. The render achieves an excellent key with the concrete. Because the concrete has no sand one can often break away sections of the same when removing existing render. The same applies to plaste internally (if not dry lined).
2) There is usually no cavity, therefore the walls can be affected by leaking gutters etc., particularly if the render is not weather—proof.
3) Insulation properties of this form of construction did not live up to expectations. Dry lining and extra render tended to solve this to a certain extent.
4) The method of construction was unconventional. Some people can be prejudiced against the same. However this form of construction is not classed as defective under the Housing Defects Act 1984.
5) Alterations and repairs to walls sometimes prove more difficult than conventional brickwork.