Flooring the Loft

Hi, Can anyone tell me if it is possible to floor the loft in a Bisf house and if it is the best way to do it. I have been looking on here but can see any post regarding this.



  1. Hi CF,

    I boarded part of the loft in a slightly different way from the one Marc described, but his may be easier. It seems that the number and size of the timber ceiling noggins can vary – mine are only 35-40cm apart (from memory) so there was no problem. They are fairly thin compared with the woodwork in a traditional roof, but as they are close together and only short it is OK. Also I’m unsure about whether the roof trusses are always in the same position in a BISF roof – I have one half way between the gable and the party wall and another right against the party wall, so it is not easy to access the other half of the loft (you have to clamber through the gaps in the truss).

    Anyway, what I did was to take up and get rid of all the original glassfibre insulation as it was broken up and very dirty (face mask and gloves required!). Then I screwed battens on top of the noggins (parallel, not at right angles) to raise them to roughly 100mm to avoid squashing the new insulation. Then I put down 100mm loft insulation between the noggins. As it’s a grid arrangement I cut the insulation into individual rectangles to fit in. On top I put down Knauf Spaceboard from Wikes for extra insulation and then tongue-and-groove loft boards at right angles to the noggins.

    I think for the recommended level of insulation you are advised to put down 100mm of loft roll plus two thicknesses of Spaceboard, but I have only used one so far because adding another will involve raising the loft ladder which I haven’t got round to yet.

    I boarded from the gable to the centre truss from side to side and about a third of the depth of the house from front to back.


  2. Here’s the images.
    I see we have cross posted lol. Yes the batons are a good idea. The weight shouldn’t be a problem providing you don’t have several tons up there as the steels will carry the weight.

  3. Hi CF
    Boarding a BISF loft space can be pretty straight forward provided you do a little preparation work first. The floor/ceiling joists in the loft space are typically narrower than those in a standard brick built property as well as being set at varying distances apart. There presence is really only there to support ceiling boards and plaster board. The structural support is provided by the steel frame that runs throughout each level.
    With such narrow wooden noggins/joists set at random between the steel beams it can be a tricky job trying to lay and fix standard tongue and groove loft panels which really need to be fixed at regular points to enable strength and support.

    This is where the preparation comes in and it really will save time and tears.

    Simply obtain sufficient lengths of CLS timber from your builders merchant or even B&Q etc. CLS is probably the cheapest wood that you can buy of pretty good quality. It comes in lengths of 2.4metres and of varying thickness. See link here http://www.wickes.co.uk/invt/107177
    The thickness will be determined by how much loft insulation you have down, as you will want to clear this and not compress the insulation down too much.
    Simply lay the lengths down at regular intervals and screw them down into the visible ceiling joists with a few screws below. (Don’t over tighten the screws as there is a little play ensuring that the boards are supported in the middle and at each edge where they interlock. Just ensure that you position the CLS so that each board edge sits in the centre enabling you to screw them down.

    ED, one of our long standing members posted some fantastic bare loft floor images in a previous post here:
    http://nonstandardhouse.com/featured/bisf-house-internal-roof-structure/ This may be the post DR John is referring to.

    I will re-post them here for you as it gives you a clear view of the floor layout which may help you.

  4. Thanks for the reply DJ

    Maily storage for empy boxes ect. Possibly a model railway. I was thinking of laying battons of wood across the steel frame ( not the A frame roof supports) then sucuring the boards to that. Not sure if this would add to much weight.

  5. I needed to floor part of the loft in an emergency storage required situation a couple of years ago.
    It was pretty easy really using tongue and groove boards that I screwed together to create a kind of floating floor due to the lack of securing points, ie the narrow joists and steel frame.

    I’m sure that I once read a post on here written by ED who is one of the senior members here writing about loft boards but I can’t find the post again.
    What did you have in mind for your loft and what were you thinking of storing up there? ie weight wise etc?