By the end of this lesson you will have learned about:
- Introduction to non-conventional construction
- Types of non-traditional construction, including:
- Pre-cast concrete frame
- Steel frame houses
- Timber framed houses
- In-situ poured concrete
- Pre-cast concrete panel
- Large pre-cast concrete panel
- Self-build housing
- Additional “as built” examples of non-traditional homes
As in Lesson 3.2, this lesson is based on the Unison material.
In the previous lesson, we summarised the traditional approach to house construction in the UK. In this next section, we look briefly at some non-traditional methods of housing construction, including a discussion of their advantages and various problems.
It is important for retrofitters to have some understanding of how these methods differ from the traditional approach if dealing with houses of non-traditional build.
Non-traditional approaches to house construction all essentially involve an attempt to shift some aspects of the construction process away from the site, to a factory. Hence, they are sometimes referred to as “industrialised” building techniques. Some elements of the construction are “pre-fabricated”, or they are complete “systems” which simply require assembly on site.
These industrialised methods considerably reduced the construction period, and reduced labour costs. However, because these were new approaches, a number of unforeseen problems have arisen. In this section, you will learn about some of these problems, and their implications for tenants / private owners and for housing organisations.
While non-traditional homes account for a relatively small proportion of dwellings in the UK, they are, nevertheless, numerically significant: it is estimated that almost one million such homes have been built since the Second World War. Most are to be found in urban areas.
Non-traditional techniques have resulted in a wide range of specific problems. This has significant implications for the repair, maintenance and retrofit of such homes.
2. Types of Non-traditional Construction
There are many different types of non-traditional construction methods. It is important that the particular type is identified, because each may have quite different implications for repairs and maintenance. However, from the outside, this is not always a simple matter!
Non-traditional homes may vary in two main ways:
- the design of the structure of the house;
- the methods and materials used in construction.
Non-traditional construction techniques may adopt one of two main approaches to structural design:
(a) Skeleton structures
This involves the construction of a structural framework, which is then clad in a number of possible ways. The frame is the load-bearing element in the design. The walls themselves are not load-bearing.
The internal frames may be of concrete, steel, or timber. The frame may be clad in a variety of materials, such as brick, blockwork, or concrete. Some of these types may, therefore, appear to be of traditional construction.
(b) Exoskeleton structures
In this approach, the external walls are load-bearing, as well as forming the skin of the home. These types may be rather easier to identify, since some appear, obviously, to be pre-fabricated homes.
Construction methods and materials
Since non-traditional approaches are “factory based”, it is important to recognise that each manufacturer developed an individual system. This means that, although the different types may be similar, each different manufacturer produced a unique system with a different trade name.
It is impossible to identify all possible methods in this section since there are so many! We have, therefore, concentrated on six main types of non-traditional construction methods, each using one of the two approaches to structural design.
The charts below indicate some of the more common trade names associated with each of the six methods.
(a) Predominantly skeleton structural systems
(b) Exoskeleton structural systems
In the sections which follow, we shall be examining each of the six types of non-traditional construction methods. This will include some examples of particular systems, with specific trade names. We shall also identify some common problems of each type, and indicate how these might best be resolved.
If your own home, or one of your projects is of non-traditional construction, decide if it belongs to any of the following categories and sum up its features. Also sum up issues that are typical of its construction, and whether any are present in your home / project.
3. Pre-cast Concrete Frame
This first group includes a very wide range of different house types. All have a structural system of pre-cast concrete frames, containing steel reinforcement to strengthen the concrete. This provides the load-bearing “skeleton”, onto which a variety of different types of cladding was attached.
A plan view, through a concrete column, with an external concrete cladding panel, and internal blockwork lining, is shown below.
Look for the steel reinforcement in the concrete column. Corrosion of this steel results in these types of homes having some of the worst structural problems. This is due to three main problems with the concrete, known as the “three Cs”.
3.1 The three Cs
This is a natural process, in which the concrete reacts with carbon dioxide in the air. This reduces the alkalinity of the concrete, so it can no longer provide the chemical protection to the steel reinforcement inside. This permits the steel to corrode.
Calcium Chloride was commonly added to the concrete to accelerate it’s setting. However, it has since been discovered that too much chloride reduces the chemical protection which the concrete offers to the reinforcement. Hence, it may result in corrosion of the reinforcement.
In some cases, the amount of concrete cover provided to the reinforcement was inadequate. The concrete was insufficiently thick. This results in insufficient protection to the steel reinforcement.
As a result of the three Cs, there may be very severe corrosion of the steel reinforcement. As it corrodes, it expands to crack the surrounding concrete. The entire structural frame may become unsafe. This process is illustrated in the diagram which follows:
3.2 The Housing Defects Act 1984
The problems identified above were first discovered during investigations following a fire in 1980 in a pre-cast concrete Airey house. This led to a nationwide investigation of all pre-cast buildings. After the discovery of further defects, financial institutions (such as building societies) were reluctant to lend money for the purchase or resale of such properties. They effectively became unmortgageable.
The government, anxious not to see the ‘Right to Buy’ legislation undermined, responded with the Housing Defects Act of 1984. The aim of the Act was to re-establish the mortgageability of a property, and make repair grants available to homeowners who bought their houses from local authorities unaware of the potential defect. The Act enables the Secretary of State for the Environment to designate defective classes of dwellings.
The Act only applies to properties built before 1960, presumably to exclude high-rise dwellings, most of which were constructed after this date. In most cases, the repair involves replacing the external concrete wall with traditional load-bearing brick and blockwork. Homeowners of designated properties are eligible for grant aid even if their dwelling is not itself defective.
The legislation has caused much controversy, because grants are only available to those who have opted to buy their houses under the Right to Buy legislation.
Under the Housing Defects Act a number of forms of non-traditional dwellings are “designated defective” and are eligible for assistance. The designated defective dwellings include:
Identifying designated defective dwellings
In 1984, the Department of the Environment and the Welsh Office issued a circular under the 1984 Housing Defects Act which set out the designated defective dwellings, and indicated their main characteristics. You should consult it to identify designated defective dwellings in your area. The circular is:
Housing Defects (Prefabricated Reinforced Concrete Dwellings) (England and Wales) Designations 1984: DoE and Welsh Office 1984.
We shall now examine in more detail a pre-cast concrete frame system, the Airey House, one of those designated under the 1984 Act.
3.4 The Airey House
This was one of the first post-War system-built concrete houses. It was constructed as follows, and as illustrated below:
(i) 4 inch and two and a quarter inch concrete columns are set at 18 inch centres, with one and a quarter inch diameter hollow mild steel locating tubes up the centre.
(ii) The external cladding is half inch exposed aggregate shiplapped concrete panels, tied to the columns by copper wire.
(iii) Internally, the cladding is plasterboard or fibre-board with one inch insulation in the cavity.
(iv) Roofs are pitched and tiled.
(v) Gable ends are board finished, or hung with tiles.
Most Airey houses were semi-detached. The shiplapped boarding gives them a distinctive appearance.
Problems with Airey houses
- Carbonation of the concrete columns allowing the mild steel tube to rust.
- Spalling of the concrete has occurred mainly at the foot of columns, although there has also been damage at mid-height and above.
- The very slender columns mean that the structural integrity is very easily impaired.
Of all the system-built concrete houses, Airey houses have been considered the most urgently in need of repair or upgrading. Many repair methods have been devised, including timber framing, replacing columns in steel or timber and masonry “enveloping” (brick and block system, Leeds). The Leeds system makes the existing columns redundant, and gives the house the appearance of traditional construction.
Source: Northern Consortium of Housing Authorities (1988)
4. Steel Frame Houses
This category forms the second largest range of house types. Skeleton steel frames are clad in various skins, ranging from steel sheets (BISF) to more traditional brick (Trusteel) and rendered finishes (Dorlonco), often making their true form of construction difficult to identify.
In general the steel sheeting skins and asbestos sheet roofs are now coming to the end of their useful life. For the BISF house type this has resulted in a range of different remedial enveloping solutions, which again can disguise the house type’s true identity. This type of solution permits radical upgrading of wall insulation values.
We shall look, next, in more detail, at the BISF house.
4.1 BISF houses
The most common non-traditional steel framed house is the BISF (British Iron and Steel Federation). Some 30,000 properties were completed after 1945, most of them semi-detached. They are not designated properties under the Housing Defects Act and, therefore no grant assistance is available to owners. A typical BISF house is shown below.
The frame is made from steel stanchions or columns, with steel sheeting cladding the upper storey. The ground floor cladding consists of render on metal lath, which is tied to the steel columns. Internally, the frame is hidden by plasterboard fixed to timber battens, bolted across the columns. Steel trusses support the roof covering, which is asbestos cement sheeting. Construction details are shown below.
They are very popular with owner-occupiers and tenants because, despite some of the defects, they offer very roomy accommodation.
Source: Northern Consortium of Housing Authorities (1988).
4.2 The main problems with BISF houses
The Building Research Establishment (The BISF Steel Framed House, 1986) has identified a number of defects, some of which have structural implications.
(i) Cracking of the render, which can be caused by impact damage. In addition, minor structural movement has occurred in some properties. This can result in rusting of the metal lathing, followed by total failure of the rendering.
(ii) Rusting of the steel cladding to the upper storey has also been a problem. It is caused by failure of the weather stripping on the gable ends, and by condensation. In severe cases, this has led to rusting of the sheeting rails which support the cladding.
(iii) The asbestos cement sheeting which forms the roof can deteriorate, and the material then becomes brittle and cracks. The only option in this situation is to re-cover the roof with a modern alternative such as aluminium.
(iv) Some corrosion has been found in the stanchions, particularly those at the corners of the building.
(v) There are other defects such as poor thermal insulation, discoloration of the steel sheeting and rusting of the chimney cowls. However, the majority of BISF houses are structurally sound, and those with deterioration of the structural framework can be relatively simply repaired by cutting out and replacing the affected members.
5. Timber Framed Housing
Norwegian timber frame systems were first imported during the 1920s, and these remain an attractive addition to the housing stock. Shortage of timber and labour during the immediate post Second World War period precluded the use of this type of construction, but during the 1960s and 1970s, it became popular again. It again lost favour following a “World in Action” television programme of 1983, which identified some of the problems of the system, which we shall examine shortly.
Generally, timber framed houses consist of load-bearing ply-faced timber panels, nailed together. The frames are factory made. In the UK, timber framed construction takes two forms, both illustrated below.
(i) The platform frame, in which the panels are single storey height.
(ii) The balloon frame, when the panels are “house size” or two- storey”, and the first floor incorporates a ring beam.
Internal walls are timber partitions. Externally, the frame may be clad in a range of materials, most commonly brick for a traditional appearance.
The problems associated with timber frame houses are due, for the most part, to poor erection procedures, and poor quality control on site. Examples include:
- missing, or misplaced fire-stopping;
- panels or units not properly fixed, or nailed;
- missing damp-proof courses and flashings;
- tears in the plastic sheeting vapour barriers;
- missing or incomplete insulation;
- frame not properly fixed to floor slab;
- no allowance made for expansion and contraction;
- lack of ventilation in cavity between frame and cladding; These have resulted in:
- condensation on the timber frame and dampness in the cavity, both of which can lead to attack by dry or wet rot;
- internal condensation;
- internal cracking;
- potential fire risk.
Timber frame construction is still fairly uncommon in the social housing sector. It was much more popular with speculative builders, so it is mainly owner-occupiers who face these problems. In recent years, however, the construction industry has made a great effort to improve quality, with some increase in popularity of the method in both private and public sectors.
6. In-Situ Poured Concrete
This category of non traditional houses is unique, in that unlike all of the other groups, it does not rely on the assembly of factory components (a kit of parts). Instead of being system-built, it involves innovatory approaches to wall construction, using in situ poured concrete.
The complete house was cast on site, using specially designed “house-sized” formwork. The example we shall examine is the Wimpey No-Fines house.
6.1 Wimpey No-Fines
The walls of the house were cast on site, using large, re-useable, formwork moulds. ‘No-Fines’ refers to the type of concrete used: a mix of cement and coarse aggregate, without any sand (fines).
The walls are load-bearing and the concrete is full of air holes – giving a slightly better insulation value.
The claim to prefabrication lies in the manufacture of large re- useable shuttering – the “moulds” into which the concrete is poured.
This type of construction enabled the house to be quickly erected using non-skilled labour. This made it particularly suitable to meet the demand for large-scale social housing estates built during the 1950s and 1960s. Wimpey No-Fines was used to build multi-story flats as well as low-rise and houses. A typical semi- detached house and typical flats are shown in the images below.
The rendered finish of these types of houses hides the fact that they are constructed from No-Fines concrete.
Source: Northern Consortium of Housing Authorities (1988)
Source: Northern Consortium of Housing Authorities
A weakness of the design was the difficulty of compacting the concrete properly on-site, particularly below window openings.
It was also essential to render the outside of the structure to make it weathertight.
(i) Condensation due to fairly poor insulation values. Upgrading is usually by external insulation.
(ii) Cracking may appear around the doors and windows, leading to water penetration.
(iii) Cracking or spalling of the render can result in damp penetration.
Structural failure is not prevalent or intrinsic to this form of construction. Movement cracks do appear, however, at some window and door openings, permitting water penetration.
7. Pre-cast Concrete Panel
These houses are built with load-bearing pre-cast concrete panels. This means that the concrete storey height panels have to perform the dual role of structural support and external skin, and are completely exposed to the rigours of the elements.
One major area of confusion with this type of construction is the fact that identical house types are identified by different names by various local authorities. All the houses in this group are designated defective under the 1984 Housing Defects Act.
We shall now look at the example of Wates houses.
7.1 Wates houses
(i) The basic system comprises pre-cast load-bearing panels of one storey height. The panels have a tray profile – that is, with flanges around the edges.
(ii) The vertical edges of the panels are grooved. When butted together, and the groove filled with fine concrete and reinforcement, these form in situ columns supporting the panels. The tray hollows could be filled with lightweight concrete.
(iii) Horizontal joints are of mortar, with continuity reinforcement.
(iv) There is a reinforced concrete ring-beam string course at first floor level and eaves.
(v) Roofs are traditional pitched.
7.2 Wates houses: problems
(i) Cracking of joints between panels (mostly vertical joints) due to differential movement. This can lead to corrosion of the reinforcement.
(ii) Cracking and spalling of panels, due to thin panels – a design defect.
(iii) Rusting of the reinforcements, particularly over openings, due to exposure to the elements.
(iv) There may be ring-beam (string course) deterioration, particularly in the connections to panels.
(v) Corrosion of string course reinforcements may also occur.
As you saw, Wates houses are designated defective under the 1984 Act.
Source: Northern Consortium of Housing Authorities
8. Large Pre-cast Concrete Panel
This category of non-traditional housing was mostly used during the 1960s and 1970s, in medium and high-rise developments and in two storey semi-detached and terraced houses.
The factory-made large panels were delivered to the site ready for assembly. While quality of the pre-cast concrete components was certainly better than earlier systems, this housing still suffered to some degree from the three Cs: carbonation, high chloride levels, and lack of cover.
(i) Each wall panel would have door and window openings “cast in”, and may even have been glazed and painted in the factory.
(ii) Floor finishes were also cast in.
(iii) Panels needed no internal plastering; they were ready to decorate.
(iv) Externally, the concrete might have different textures, so each block might look different.
8.2 Bison wall frame
Between 1964 and 1975, approximately 40,000 Bison wall frame units were constructed, of both high and low-rise buildings, ranging from two to twenty storeys. Examples are shown below.
(i) This was a large panel system, with load-bearing pre-cast walls and pre-cast flooring, stairs and lift shafts.
(ii) The bathroom and toilet were cast as one unit incorporating walls, floor, ceiling, airing cupboard, ducts for plumbing and ventilation.
(iii) Each panel, weighing up to 7 tonnes, was lifted into place by a crane.
The diagram below shows the 21 individual panels which were assembled to produce a 2 bedroomed flat. The complete bathroom unit is panel number 4.
Source: Northern Consortium of Housing Authorities 1988
8.3 Common problems
(i) Rain penetration, through roof parapets, through vertical and horizontal panel joints, and around window openings, sometimes resulted in accelerated deterioration of the internal finishes, and of the structure itself.
(ii) Differential movement: this may occur between the cladding panels, and between the panels and the window frames. This can result in cracks appearing internally, at vertical junctions between panels, and also between ceiling and floor levels. There may also be leaks around windows, causing the frames to rot.
(iii) Condensation: a very common problem, especially in high rise blocks, due to wind exposure and lower temperatures at high levels. The system suffers from “cold bridging” at floor and ceiling levels, due to the poor thermal insulation properties of concrete.
(iv) Roof problems: these units were built with flat roofs. Many suffer from leaks and water penetration through the roof. Due to poor insulation, there may also be dampness due to condensation forming inside the roof. Further problems arise when felt roof-covering fails as a result of stress cracking, due to its inability to cope with differential movement.
(v) Sound insulation poor, due to gaps between panels and poor jointing. As a result, smells and smoke may also be transmitted between units.
(vi) Spalling concrete: causes bits of panels to break off. It may be due to lack of tolerance – that is, fit – between panels. A more common cause is the use of calcium chloride in the concrete mix at the factory. Spalling exposes the steel reinforcement, causing it to rust. This can lead to major structural problems.
(vii) Asbestos: commonly used as a lightweight insulating product, but as it deteriorates and fragments it becomes a major health hazard. It was used for fire-protection, fire stopping between units, and for some cladding panels.
9 Self Build Housing
There are various methods applicable to self build; the example listed here is a timber frame method invented by an architect, Walter Segal. The method is summarised below using text from “Out of the Woods: Environmental Timber Frame Design for Self Build”, published by The Centre for Alternative Technology, Machynlleth, Powys, Wales.
9.1 Post and beam frames
The structural principle of ‘post and beam’ framing is that loads are carried by small timbers – tiling, lath and studs – which in turn are carried by slightly larger pieces – rafters, floor joists, ceiling joists – to beams, which are supported at larger intervals by posts, which are carried by the foundations.
10. As built examples of non-traditional homes (from Hi Resources)
The pdf document below lists non-conventional dwelling construction methods and represents a useful summary.
Required reading: pdf of non trad examples
It was sourced from http://www.hiresources.co.uk/NT-AllType.asp (accessed 2013).
Note: these webpages are no longer available at this address, however it is cached here: http://archive.is/xb30f (& other cached pages may be accessed using appropriate free online software, for example see: http://archive.org/web/)
Further reading: http://en.wikipedia.org/wiki/User:Colin_68dots/PrefabSocial
The main types of non-traditional construction methods are:
- pre-cast concrete frame;
- steel frame;
- timber frame;
- in situ poured concrete;
- pre-cast concrete panel;
- large pre-cast concrete panel.
Most have given rise to a wide range of repair and maintenance problems. Some have been found to have inherent structural problems, with significant implications for repair and maintenance, and in some cases calling for radical rehabilitation solutions.
However, the non-traditional dwellings are still a significant housing resource, some providing roomy accommodation, and popular with tenants.
- The Hi Resources information was sourced from http://www.hiresources.co.uk/NT-AllType.asp (accessed 2013). Note: these webpages are no longer available at this address, however it is cached here: http://archive.is/xb30f (& other cached pages may be accessed using appropriate free online software, for example see: http://archive.org/web/)
- Examples of as built non traditional construction: pdf of non trad examples
- Further reading: http://en.wikipedia.org/wiki/User:Colin_68dots/PrefabSocial