Bisf joist positions?  

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Is there any reason anyone knows why the joists run side to side in a BISF house, I always thought the standard was front to back? I'm also looking for a picture showing the location of the steels if anyone has one?

Thanks!

Edited: 2 months  ago
 
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Do you mean the floor joists or the roof joists? I presume by joists you mean the steel beams and not the timber noggins between them.

In the roof the beams run from side to side as they are at right angles to the roof trusses and thereby support each other. In a traditional house the roof joists would be supported by an internal wall around mid-span ie there would be a structural supporting wall where the non-structural wall and built-in cupboards are between the two main bedrooms in a BISF house.

For the first floor the beams run from front to back because they are supported at right angles by a beam in the top of the wall between the living room/dining room/kitchen that is itself supported by two stanchions. Again, this wall would be structural in a traditional house.

Having the roof and floor joists going in opposite directions also probably gives extra strength to the house.

 
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Hi, thanks for the reply!

My question was regarding the floor joists as in the upstairs they run from side to side as if between the houses. the boards run from front to back eg bedroom to bedroom! I then noticed a steel though the middle that also runs front to back?

wish there was more info on these houses!

Hi Matt,

I think I understand your question better now!

OK, in a BISF house there are no floor joists in a traditional sense. The equivalents are steel I-section beams very roughly a metre apart which run from front to back. As floorboards and ceiling boards can't be nailed directly to steel beams, there are timber noggins between the steel beams to which the boards are nailed.

The noggins are notched at each end and slotted into the steel I-beams. So if you look at the floorboards and where they are nailed it looks as though they are nailed to joists running from side to side, but in reality the noggins are each only about 1m long and the real structure runs from front to back.

Someone posted a photo of the ceiling in the living room removed which showed it all clearly but I can't find it. However, the structure is essentially the same as in the loft, except up there the beams run from side to side and are a bit smaller. If you imagine this was the floor, the floorboards would be nailed on across the noggins and parallel to the steel beams:-

Ed

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Strangely enough I was going to ask a question about the steel floor beams myself, so I guess I will here rather than starting another thread.

I have got a heat recovery ventilation unit to install in the loft. Basically it is like an extractor fan, but instead of just blowing the air outside and wasting the heat, it uses the heat from the extracted air to warm the air it sucks in. It needs to take air out of a warm moist room (in my case the bathroom, as we're getting condensation problems) and supply it to a habitable room (in my case the living room).

Anyway, to do this I need to put in ducting between the loft and living room. I have flat 60x204mm PVC duct. I was going to put it in one of the built-in cupboards between the two main bedrooms where it will be out of the way. I will need to cut a hole through the floorboards and ceiling so I need to avoid the steel beams.

Does anyone know of their exact position or spacing? I can see one if I lift a loose floorboard upstairs and know their general position, but not well enough to be certain to avoid one when cutting the hole!

Ed

 
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I think I may have answered my own question and part of yours, but it would be good if anyone else can confirm this.

If I lift a floorboard on the landing I can find the location of a floor beam. Its edge closest to the party wall lines up with the end of the wall between the two main bedrooms (between the doors). It is 84 inches (I'll use inches as the original BISF measurements all appear to be inches) from the end wall of the house. Assuming the beams are equally spaced and there is another one between there and the end wall, they must be 42 inches apart.

If you look at Doug's photo of his flue removal, you can see a beam right next to the party wall edge of the boxed in flue, which is 84 inches from the landing beam I found. It is then another 84 inches to the party wall, which would seem to confirm them being at 42 inch centres.

Ed

Hi Ed, I haven't got a fully exposed property to take any photos that may have helped and I can't get to many of my images right now due to data issues. What I can show you is the location of one large steel on the landing that may help you in working out your measurements / cut.
The doors shown lead to the back bedroom on the left and the front bedroom on the right.

bisf house landing floor joist1

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Ed im not sure I understand your measurements, but if you would like me to confirm exact measurements of where the steel cross beams are let me know ASAP as I have these exposed currently underneath. Also do you still have the chimney flue? if not that would be the perfect place to run the pipe back down...

Hi Marc, thanks for the photo. Strangely enough that is the exact same floorboard I have lifted! The only difference is that the pipes and cables pass under the beams not over them (I take it those are three copper 8mm microbore central heating pipes, like we have).

I think knowing the position of this beam and that there is one above the centre of the living room window from Grangey's photo, it's possible to calculate the position of the others with good confidence.

Ed

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Thanks Grangey, that would be great! We still have the chimney flue, it is in use by the boiler. There is room for various pipework and cables in between the frame and the flue but not for something as large as a duct.

Basically I calculate that the beams are at approx 42 inch centres, but if you could measure their location in relation to something I'll have here too such as the party wall that would be great.

Ed

 
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No problem, I shall confirm tomorrow morning for you 😉

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Edited: 2 months  ago
 
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PS I ask as because as you can see in my picture, the steel frame around where my flu was is split in 2, I forget which side the flu went up but I would have thought the other side would be a pretty prime position potentially

 
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