BISF House Internal Roof Structure.

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.

truss One
Diagram A

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.

roof diagram
Diagram C

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.

apex bisf roof structure

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.

bisf roof space
Tubular Steel Frame and Grey non=permeable Membrane

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.


  1. While the roof isn’t next on my list of projects to do, I came across this coated steel product from Tata the other day called Colorcoat Urban and thought it could look fantastic as a replacement for the asbestos sheeting on the roof of a BISF house. I was thinking it would give modern twist to the original profiled asbestos, especially as it comes in several colours. I haven’t seen anything about it on a BISF house though but can’t see why not… the other thing of course is that Tata is the ultimate heir to the British Iron and Steel Federation of course!

  2. Wow Ed, considering your pretty new to DIY it sounds like you are doing a really good and thorough job.
    You know I’ve been trying for three years to get my other half to insulate ours :0)

    The cupboards that you mention are the same in our house and they are just empty spaces. My neighbour did put some sliding doors above hers whilst another one ripped out the cupboards and fitted a couple of normal wardrobes into the space created.
    It worked really well but i do know they had a boiler that meant the hot water tank could be taken out of the airing cupboard.

    Keep pushing on Ed, sounds like it’s going to be great when you finish it.


    1. Thanks Trish, my dad is a builder so I know a little of the theory but not much practice Smile It is a dirty, claustrophobic job insulating the loft so I don’t blame him ;)

      I hadn’t thought of sliding doors, that may be a better idea than the hinged doors I was thinking of. The wardrobes are quite nicely made so it would be a pity to rip them out I think. I’m wondering how they would look with the paint stripped off and varnished.

  3. One other thing I did notice up there is that at some point they have put a new water tank on a small platform above the built in cupboards. The platform is obviously not original and the tank is plastic.

    Anyway, it got me thinking about what the presently unused space on top of the built in wardrobes was for. I’m guessing maybe the cold water tank was originally in there? There’s a gap of about 30cm between the top of the wardrobe and the ceiling that’s just boarded up.

    I was thinking of opening it up and putting a row of cupboard doors up there, once I’m happy my DIY skills are up to making some that match the ones below reasonably well.

  4. In the past few days I’ve been boarding the area of the loft around the hatch. It’s been my first significant DIY project, so I’ll post some photos soon, as the ceiling of a BISF house is a little different from usual so I had to go about it in a slightly different way.

    I found that the timber noggins are smaller than the joists in a traditional house, measuring 75x30mm and so they are not deep enough to take 100mm loft insulation without compressing it. I got around this by screwing 47x22mm timber on top of the noggins to bring them up to roughly 100mm, then filling the gaps with the 100mm roll insulation.

    On top of this I put Knauf Space Boards for extra insulation and then chipboard loft boards. I’ve boarded from the gable end to the middle truss lengthways and between the middle three joists from front to back.

    For the rest of the loft I have just put down 100mm insulation and then 200mm insulation, without heightening the noggins or boarding.

    1. Sounds like your doing a proper job Ed but I do feel sorry for your back, theres not a lot of head height up there! I hope your coping well.

      Good idea to add to the noggins as they do tend to be a little thinner than expected. I’ve noticed in the past, several houses where the bedroom ceiling boards have warped due to insufficient support but only where fibreboard and not plaster board has been used.

      You have certainly covered a good area, how long has the job taken so far?
      I’m sure you already have thought this but dont forget to insulate the loft hatch itself as you would be surprised to see how much heat can escape through it and many people tend to forget it.

      I have a fair few items stored in my loft on loft chip boards and all is fine although every year I tell myself it’s all rubbish and it should be binned or car booted by boy is there a lot. :0)

      I still haven’t had access yet to one of my lofts to take a photo of the plastic covering I mentioned earlier but I will try to get it done in a day or so.

      Hope it all goes well and keep us informed.


      1. Yes, I did get some stomach cramps and leg aches from crouching up there actually! As I’ve only boarded the middle third under the ridge that bit wasn’t too bad, but getting the roll insulation under the eaves was pretty claustrophobic. It’s also quite difficult to know whether you have left enough of a gap for ventilation under there, because you can’t see.

        I had the week off work so I spent about four half days but that did include driving to Swindon and back for the Space Board and also it took me longer as I haven’t done anything like it before.

        My thinking was that if I did it properly I’d be more likely to actually use the stuff I put up there than if I’d just put it on top of the old birds nests and junk that was up there!

        I’m not really worried about the strength of the noggins as they are only short (about 1m long) but I did want to make a bit more space for the 100mm roll insulation rather than squashing it down to the 75mm depth of the noggins.

        75x30mm seems to be a non-standard timber size and I guess they chose it to fit in the I-section steel joists.

        I haven’t yet insulated the loft hatch because I haven’t decided how to do it. I was thinking of putting some of the spare Space Board on it, but I’ll have to use more than one piece as it is only 500mm wide (to get through loft hatches!). It has a ventilator in the middle, but I think I should remove that as there’s plenty of ventilation for the loft from the outside and it must let a lot of heat out and a lot of dust in (you can see the dust building up in it).

  5. I’ve been up in the loft again today and was a little concerned to find quite a bit of condensation on the red-painted inner surface of the steel cladding on the gable, that can be seen in the third photo. I also put my arm down into the wall cavity below and there was condensation there too.

    Ventilation is good as there are all sorts of gaps under the eaves and in the gable. I did block the biggest one up though as I noticed a magpie was going in there – that was where one of the individual ‘plugs’ that fill the gaps under the corrugations at the gutter line was missing. Where the asbestos sheet curves up for each corrugation ridge, the gap is filled by something bolted in from above, except one had gone. Getting to it is difficult though due to the low pitch of the roof – you have to lie on your stomach and it’s quite clostrophobic!

    The steel still seems in good condition after 62 years so I’m not overly worried, but it has got me thinking about putting in a vapour barrier in the rooms to try and prevent too much condensation.

    1. I wouldn't worry to much about the condensation at the moment Ed unless you can see significant signs of corrosion anywhere.

      Don't forget that whilst the loft is stripped of insulation and the hatch is open warmer moist air is rising into the loft space, hitting the cold surface of the roof lining and condensing.

      I had the same problem in one property where recessed down lights had been fitted into the bathroom ceiling. Due to this no insulation could be fitted above the bathroom due to fire safety.

      This caused considerable condensation on the underside of the roof sheets. Once the lights had been removed, insulation was laid the condensation reduced substantially

      I would re assess the levels of condensation once the space has been insulated. Because you mention that the steels appear to be in good condition, I don't think you have a problem.

      1. Thanks for the advice, I'm still missing some insulation as I've taken up the old stuff but don't have enough of the new stuff yet – it really does make a difference in this cold weather!

        I was worried the magpie was making a nest up there so I got down on my stomach to poke my head under the eaves but there was just an old abandoned nest of some other bird – I guess the magpie was looking to rob it! Anyway, I did temporarily plug it up to ensure it couldn't get back in.

  6. What a fantastic set of images Ed!

    It is indeed a rare opportunity to see an original BISF loft space in all its original glory and showing the entire roof space structure complete with roof panel fixings.

    I certainly don't envy the task you have undertaken as removing old insulation and discarded contents is no mean feat. Just ensure that you wear a good quality face mask and some old clothes or a disposable paper boiler suit for protection.

    Remember too that the gaps that you mention are also required for ventilation. There is no problem filling some of the larger gaps to avoid birds entering the cavity but don't block off all the gaps as a free flow of air is required to prevent the build up of condensation.

    When the roof was replaced on one of my properties the contractors used a type of removable plastic vent cover that still allowed for air flow. I will try to get a photograph for you as not being a roofing contractor I have no idea what they are called.

    Also remember not to push the insulation too close into this same gap, ie blocking the loft floor and roofing sheets totally as this will also prevent air airflow.

    Great work though and looking forward to seeing the results.

    Btw, your posts should be automatically approved but for some reason this post was delayed for approval which shouldn't have happened. I will look into that for you.


  7. I thought readers may be interested to see these photos I took in my roof. I have been putting up loft insulation, but first I've been taking down the previous owners junk and the old fibreglass insulation which had broken up completely and was mixed with birds nests and other debris.

    You can see the that the ceiling is supported by steel beams about 1 metre apart which are attached to the steel stanchions in the gable end and also to the steel trusses. Between the steel beams are timber noggins to which the fibreboard ceiling is nailed.

    My dad tells me that the two thin strips of wood attached to the top of the noggins parallel to the steel beams originally had the electric cables clipped to them.

    It's also possible to see how the asbestos cement sheets are attached – they are hooked onto the angle-iron perlins and the other end has a thread on it, which is bolted on the outside.

    The previous owners have stuffed old cloths into the gaps in the gable and eaves to try and keep birds out I presume (not very successfully), but I plan to replace them with expanding foam.