Non-traditional methods of construction gained momentum after the second world war. Houses needed to be built quickly and cheaply in order to address a specific and urgent housing shortage.
As traditional building materials and labour were also in short supply, a move to non-traditional ways of building meant that factories and work forces set up to support the war effort could diversify into the production of homes.
The result was a shift away from traditional solid wall or brick and block construction to new ways of construction, which incorporated four main methods: timber frame, metal frame, pre-cast concrete or concrete cast insitu.
The outcome – in total, over 1.5 million homes were built in these non-traditional ways.
You can read more about the history of UK non-traditional construction here.
As with most other industries, the non-traditional construction industry didn’t stand still but developed too. As a result, it was in the 1960s that a new form of building evolved under the name of Rationalised Traditional Construction. We commonly know this in its abbreviated form … RAT-Trads.
Rationalised Traditional Construction – Crosswalls
Rationalised Traditional Construction (RAT-Trad) was developed as a form of construction in the 1960s but became common in the 1970’s and hence, is normally associated with this era.
As the name “Rationalised Traditional Construction” suggests, it’s a form of construction which incorporates both traditional and non-traditional methods.
Specifically, RAT-Trad Crosswall construction is different in so far as the end gables and separating party walls were built out of traditional block and brick construction whilst the front and rear elevations were typically formed from a non-traditional timber frame which was then boarded / sheathed using a variety of materials often including plywood or asbestos paneling.
This unusual construction presents its own refurbishment challenge – the gable and party walls are structurally sound but the front and rear elevations are flimsy timber frame infill panels that move excessively under lateral wind loads and also suffer from significant air leakage.
The roof is usually of traditional timber construction.
The major advantage of Rationalised Traditional Construction was speed of build. Only the gables and party walls needed to be built on-site. Layouts, dimensions and details of the infill panels could be standardised and produced off-site. Certain existing timber frame non-traditional systems could be incorporated too. This meant the panels only required assembly on-site and the only additional site requirements after installation were the addition of glazing and doors. This resulted in a significant saving in both time and cost.
We commonly know these properties today as Crosswalls.
Specific Types Of Crosswalls
The BRE list 70 specific types of RAT-Trad Crosswalls properties as follows:
|Rationalised Traditional Construction – Specific Types|
|4 H/7||New Decade|
|Crouch Transitional||Plus 3 Contracts|
|Drury System 3||Ratrad – Terrapin|
|Faculty||Rat-Trad – John McLean|
|Federated System 1||Richard Thomas & Baldwin|
|Hallam Mk I||SASB|
|Halliwell Rat Trad||Scan|
|Harvey & Lawrence||Shaneley|
|Hemelite Homes (Rationalised Building Systems)||Shanplan|
|Kier BDC Housing||Span Type K|
|Lesser Homes||Surebuilt (Pearce and Barker)|
|Loudon||System 22 (Eton)|
|Mactrad (John McLean)||Thomson|
|MHC (Midlands Housing Consortium)||Trim|
|MJ Three||Ty-Built (Tiley and Sons)|
|Mod L||Unit M Plan|
|Modform||Unit System 66|
|Morris & Jacobs||Weaver|
Problems Associated With RAT-Trad Crosswall Houses
We’re talking about non-traditional, fast-build properties which were constructed around half a century ago and so the problems are, as you’d expect, wide ranging.
Masonry walls constructed back in the 70’s would have been built to conform with regulations at that time and will need to be refurbished to current standards. Seals, joints and damp-proof course all need to be checked as do the wall ties. There may or may not be insulation in the cavity but, regardless, as we’re only talking about the gable wall being of traditional construction, any cavity insulation will be ineffectual.
The timber framed front and rear elevations present a very different set of problems. The overall condition needs to be checked – especially around openings and sole plate level. And then, of course, there’s the existing cladding or sheathing which could be a variety of different materials – each presenting it’s own challenges.
With regards to any existing insulation, this again is likely to be lacking significantly.
Refurbishment Of Crosswalls
The good news is that these RAT-Trad Crosswalls CAN be efficiently refurbished to bring them up to current standards. The obvious option is through the use of an External Wall Insulation system. Apart from providing the most efficient way of adding insulation, an EWI system has the extra benefit of transforming aesthetics of the property and indeed, entire estates or communities by modernising through the use of colour, effects and texture.
However, a standard EWI system may not provide a complete solution – let me explain.
A standard EWI system typically consists of Mineral Fibre or Enhanced EPS insulation board at a thickness specified to achieve the desired U-value. These boards are then mechanically fixed back into the existing substrate. This is by way of a proprietary nail or screw type insulation fixing and typically at a rate of around 5 fixings per square metre as a minimum. Attaching an EWI system in this way, therefore means that the existing substrate has to be fit to receive and hold the entire system in place. This is not normally a problem when you’re dealing with the traditional block and brick gable end walls, It can however be a significant issue when you’re fixing into the timber framed front and rear faces and any dated cladding they may currently hold.
Structherm Refurbishment Of Crosswall Properties
For this reason Structherm have developed a unique and proven refurbishment system for these unusual Crosswall properties. It’s basically a “hybrid” system which uses a standard EWI solution alongside a Structural EWI (SEWI) cage system.
This means that a standard EWI system can be specified on the gable end walls. There is no benefit in specifying a structural system to this façade – it will add an additional, unnecessary cost with minimal benefit to the refurbishment of a traditional, masonry substrate.
The Structural EWI system can then be installed to the front and rear timber frame faces. Whereas a standard EWI system needs to fix directly to the existing substrate, the Structural EWI system is designed to span any existing cladding and fix into the actual timber frame itself. It therefore doesn’t matter what the existing substrate is or it’s condition – it can be left in situ or removed, it really doesn’t matter. The added benefit of the Structural EWI system is that it’s going to significantly reduce deflection and enhance the rigidity in these areas which are often weak and flexible.
As with a standard EWI system, the insulation and thickness of a SEWI system is specified to achieve the target U-value. And again, it can be finished with the same variety of colours and options … brick slips, brick effect, render, dash etc. This means that the two systems can sit seamlessly side by side.
At this stage, we should add that the specification process should always start with a structural survey. Whilst the condition of the existing timber frame is usually OK, it needs to be first checked. There may be times that a Structural EWI system may be specified on the gable walls (particularly depending on the condition of existing wall ties). Every project is different, so please Contact Us and we’ll help you every step of the way.
Structherm Crosswall Refurbishment Case Studies
Historically, the Structherm Hybrid system has been specified and installed on numerous Crosswall projects throughout the UK.