hi i would like to insulate the attic sealing on my Bisf house I believe to be A type it has wc and storage outside opposite with my neighbour side.
my plan is to insulate it with about 200mm thick insulation 50mm Kingspan 100 Rockwool 50 Kingspan like a sandwich
the thig is that the original joists between the steel beams are 120mm wide 35mm thick and log 1005mm, well i would like to replace them with c16 timber 200mm wide 45mm thick the quality is highly better than the original.
Now I don’t know if I add weight on the structure coz they are a bit thicker and wider than the originals one, am being advise to talk to structural engineer but guess wont advise me for free.
Is not a lot of information on here about joist or structure and the weight is capable to withstand.

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Yes, you absolutely understand me completely.
I wasn’t very clear in my question,
It shows that you know your stuff when it comes to Bisf construction and I thank you very much for your time to help me out! It is wonderful that a community like this exist to share information coz is almost nothing out there about Bisf detailed building. Thank you.
Like you suspected I will remove the sealing fibre board and replace the noggins with c16 200mm wide noggins to rise the loft floor so that the insulation is nicely tuck-in not compressed.
Like you see in the picture when we bought the house it was quickly tidy up to be sellable,
qualty plumer job




They screwed battens on top the fibre board then plasterboard like I wouldn’t notice …
battens on top the fibre board
battens on top the fibre board
Even the first-floor noggins are week.
noggins first floor
nogginns thickness
And gave me something else to warry about. The house used to have asbestos roof till the council replaced with ceramic tile roof, I wander if they had a specialist taking in consideration weight load
o-do I would think asbestos sheet are heavier than ceramic tiles.
roof ceramic tile

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Hi Danielb
I’m just trying to clarify exactly what you intend to do.

I take it that you intend to remove and replace the free moving timber noggins located in between the joists so as to effectively raise the loft floor.
Are you also planning on removing all the ceiling boards too in order to accomplish this as they are often screwed or nailed into the noggins, or do you intend to leave them in situ and run them alongside the existing noggins to purely act as a floor support?

I ask because you mentioned above that you were intending to replace them and I just wanted to clarify this.

Either way, the loft joists would obviously benefit from being strengthened and I would have no concerns regarding the additional weight, including the addition of loft boards.

The roofspace was designed to carry the weight of a certain amount of storage but no specific figure of weight has ever been stated.

The existing timber noggins in the loft floor are pretty lightweight which you have highlighted with your measurements, however the steel joists, being standard throughout the build, can hold the same weight as the first floor joists but the timber cross members are clearly significantly weaker.

I think it’s a wise move to strengthen the floor as this will help to prevent sagging of the ceiling boards which I have seen in several properties where the loft has been overloaded.

The loftspace itself is not considered suitable for habitation, in part due to weight issues but mainly due to the low pitch.

If you do store a large amount of goods in your loft, it is always best to try and spread the load as evenly as possible, so as to disperse the weight evenly down through all the vertical stanchions.
The stanhions themselves have a much great load bearing capacity vertically than they do horizontally, which I’m sure you already know but with the strengthened c16 timber supports, I don’t think you’ll have any problems at all.
The biggest problem associated with weight, happens when unscrupulous roofers install clay or concrete roof tiles onto the rolled steel roof trusses. This frame was never designed to carry such a large weight of several tonnes and often, the trusses sag and distort which requires the roof to be replaced with a lightweight roofing system.

In these cases, the building itself withstood the additional load as it was the rolled steel roof trusses that failed and steel the supporting floor joists.

As a footnote, it’s always best to seek professional advice though but sadly, many surveyors have little or no knowledge of the load values of the BISF house, unless they specialise in this area which is very rare.

Keep us informed with your progress and good luck.


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