The internal structure of a British Iron & Steel Federation house is unique in many ways. Most of the rigid steel support frame is hidden inside the wall cavities of the building and are rarely seen or exposed other than during full structural surveys or extensive renovation works.
Diagrams do offer a valuable insight into the overall construction methods used in then creation of these houses but they tend to fall short in exposing the finer detail.
BISF House Internal Roof Structure
The loft space is one of the few areas where we are given an unobstructed, clear view of the exposed roof structure and the type of fixings most commonly used. Unlike most traditional houses which have wooden roof trusses (diagram A). The BISF structure is built from angled and tubular steel that has been coated in nothing more than a protective layer of paint.
One of the most obvious differences in a BISF roof structure is the omission of a single Ridge Board that spans the Apex of a traditional wooden construction (see diagram C). The BISF House has two supporting metal supports (purlins) running parallel along each side of the apex.
We have to remember that the Bisf house was built offsite and in kit form, with speed of construction at the heart of its design. Everything including the roof sections were delivered on sight pre-assembled. The job of these steel purlins was simply to join the sectional panels together and offer structural support to the roof. Simple nuts and bolts were then used to join the components together thus providing a very strong support for the lightweight asbestos based roofing sheets. The roof structure itself could indeed hold the weight of traditional roofing tiles, however the combined weight of such tiles along with all other materials used in the entire build would have placed far too much excess weight onto the steel stanchions (legs) of the house. Combined with the need for speed of installation using unskilled labour, asbestos based corrugated sheeting was the obvious choice.
The opposite image shows the unique steel roof structure looking directly into the end Apex of the roof with all other roof components removed digitally to simply the view. Note the low levels of corrosion covering the outer surface of the steel. This is a common occurrence due to changing humidity levels experienced inside the roof space. A further coating from a proprietary anti corrosion paint would of course increase overall longevity and is within the scope of a competent diy person. We can clearly see the simple connection bolts used and judging by the surface corrosion, it could be assumed that not all components received a protective coating.
Opposite we can see the same image showing the steel outer sheeting covered in a red oxide type of paint. These sheets appear to have resisted corrosion well. It should also be noted that these images relate to a roof space that has been recently fitted with a new lightweight steel tile roofing system.
Looking further up into the Apex of the roof the absence of a ridge board can be clearly seen. In order that the replacement roof matches the adjoining property smaller battens have been located on top of the rafters to raise the overall height.
Directly below the marker we can see the preformed tubular steel roof support structures, providing the main structural support for the entire roof.
The grey protective paint on the central support was applied during a new roof installation. In the background it is possible to see the cast iron chimney flue which runs through the house in three joined sections. This is surrounded by a thermal heat blanket of insulation fibre which in some cases has been found to contain asbestos. This quilting is encased in a wire mesh.
It is also possible to see the grey roof membrane on the left of the image.
During cold weather, condensation can be see collecting on the inner surface of the lining, as the warm air entering the roof space, meets the cooler fabric of the roofing sheet. This is often a sign that more roofing insulation may be required. Newer installations will often use an insulated or non-permeable membrane.