Loft joist suitability and possible strengthening due to span of the room below

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Hi All, I have a question about joist suitability and possible strengthening. I live in a dormer bungalow which has an attic conversion (according to the previous owners it was done at the time it was built). At the minute the attic is split into 2 large rooms and I want to turn it into 3 bedrooms and a bathroom which will require building control sign off. Problem I have is one of the bedrooms will span across 2 rooms below which are both 4.2M x 4.2M in length. The joists that are in place in the loft are 40mm x 170mm and 400mm apart. According to the building control regs I have read they are not suitable for a span of 4.2. So my question is are they not suitable? Can I just double them up to pass building regs or what are my options? Thanks.
 
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The bedrooms above are not relevant. It's the span between spring walls below.
Presumably the joists and spans are already undersized. You are looking to upgrade them as part of the refurbishment above. Unfortunately with that span you'll struggle to get any timber to work due to the self weight. You might be able to use engineered joists instead.
For anything complicated àn se can help.
 
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If there is already a loft conversion you don't need to touch the joists.
 
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says that there are existing bedrooms
Not quite.....

the attic is split into 2 large rooms and I want to turn it into 3 bedrooms and a bathroom
It could mean bedrooms or it could mean spaces that currently store suitcases and boxes. Dunno.

170mm joists does seem a little lean for the proposed loads. I'd be concerned about the increase in floor height relative to the stair landing, should you wish to increase the joist depth.
 
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If it's in a roof space, it's likely that the joists will not be carrying the normal domestic loading of 1.5kN/m2 over the full span; the area going down to the eaves will be unloaded, in which case they could well be OK.
 
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Cheers for the responses. I contacted building control they said they will have to do a survey and the work will need signed off so I better do it by the book. I don’t want to affect the floor level so preferably I would like to double up if it’s an opinion, maybe with engineered joists.
 
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Nope it's all black and white to me. It's a conversion or its not a conversion.

The point is, a whole bedroom floor that is currently OK does not need to be replaced just because the op wants to put a wall down the middle.
 
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