Loft conversion floor joists

1 Oct 2013
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United Kingdom
I am about to undertake the start of a loft conversion, I will have other trades in to do some work, but my first dilemma is how to run my new joists.
The existing joists run from front of house to back with a span of 6350mm, then sat on top I have 3 binders which run from gable end to gable end at 7035mm. Problem is there is no full supporting wall to help with the load under the existing joists, tho there is a supporting wall running parallel with the joists approx 4.3 in from one end. Normally the new larger joists would run parallel with the existing, but the span is too long for this, i believe, so what is the alternative, can anyone help!
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Yes, its all going through building control, I have called them today, to ask for a suggestion, but as I thought they dont want to give me information and have said to get a structural engineer, I was hoping I wouldnt need to go down that route, which is why I came on here!
Nine time out of ten you'll need an SE. Unless your loft already has an open void you'll need to remove some existing structure at least. Only someone suitably qualified will be able to determine what size member are used etc etc. Building Control are not an advisory service.
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You probably need to span with steels and then joist in the other direction. But as above you need some overall design input. Ultimately you will need somebody to do calculations for building control anyway.
Thank you both for your input, I will take your advise with the SE. More money!! But money well spent. ;)
Look into 'engineered i beams' - these can span some very impressive distances and you may be able to, like you can with an engineered roof truss, get a floor designed for you without having an SE involved. I.e the suppliers spec will be sufficient to satisfy the BCO.

It is just an idea, as I know they span vastly greater distances than any c16/24 solid timber and they're nice and light to fit yourself!

Saying that, 6m+ is a long way, so it could work out cheaper to do it steel - so don't close your mind off to the idea. However you will need SE calcs, and holes in your gable to pass them through, access and or a crane.


EDIT: A quick google and -

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