Strengthen Loft Joists for storage purposes

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Hi. I wanted to check out a recommended solution for strengthening my loft joists before boarding.

I have a Victorian terrace; 2 external walls with 2 supporting walls running parallel between them (staircase is in the centre of the house); span from each external wall to each supporting inner wall is about 3 metres, with 11 joists running on each side of the house (so 22 in all); the loft is purlin construction; the existing joists are 4" by 2".

Someone has suggested that screwing additional 4" by 2" joists directly 'on top' of the existing ones (effectively making the existing 4x2s 8x2s) will reduce the flexibility and provide sufficient strength for the purpose of storage.

My only concern is whether this approach adds weight to the existing joist - especially as the new joist wont sit on top of the external wall due to the pitch of the roof getting in the way. I've been reassured that this is not the case and the new joists can, in fact, fall a little short of either supporting wall; its the reduction in flexibility that is important.

This kind of makes sense (and I'm sure I saw something to this extent on a similar forum - possibly this one), but wondered if anyone in the know had any thoughts or had done similar.

Thanks.
 
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Leofric

Bending moment at the middle of the span and shear at the bearing are the things to consider ,Tony on here could explain it better if he sees your post.
If the new joists don't finish too far from the bearing on the wallplate they should be ok. They won't quite be equivalent to 8 x 2's but should be ok for that span.You can splay cut the tops of the ends of the joists to sit them as far as possible under the roof.
 
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Leofric

From the old Approved Document A ,SC3 floor joists at 600mm centres - 195 x 47mm will span 3.39m, and 170 x 50mm will span 3.08m ,so providing you can fix the new joists adequately on top of your existing joists (
Maybe others can recommend the best way of doing this )
the span is ok.
 
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