Callum wrote:

Set square...the party walls are about 3m apart. There is a plaster

OK, I just did a quick test beam in superbeam for you: 3m span, C16
timber of 50x150mm configured as a load sharing system. Assuming a
uniform floor load of 0.8kN/m (i.e. the floor and no other dead load
structural elements like walls etc)

Surprisingly that does actually meet the required spec. It shows a max
deflection of about 7mm (the limit would be 9mm for a 3m beam according
to building regs). The problem here would be potential damage to the
ceilings. So I tried another test with 50x140mm timber. This just
scrapes through with a 8.7mm deflection! However that would allow you to
space the beams 1 cm higher than the ceiling level and hopefully they
would keep clear of it.

Personally I think I would either switch to C24 timber of 50x140mm (7mm
deflection) or alternatively stay with C16 but go for 75x140mm beams
(under 6mm deflection) to allow more margin for error.

(Note you may have difficulty buying 140mm high beams since this is not
a standard depth - however 150 could be thicknessed down if required)

and lathe wall directly below the joist supporting the studs. It's a

This may make life easier if it is currently load bearing since by the
look of your diagram is splits the span in half.

very small house (2 up, 2 down with 3 floors!) so not sure wether the
roof would require much support. Although the rafters look particulary
flimsy!

As long as you don't go hacking out purlins or any other stays that
support the rafters your ought to be no worse off. You will need to
insulate and clad the underside of the rafters, which will add a little
weight though.

What depth are the rafters?
Is there under tile felt (i.e. sarking)?
Are there any soffit / ridge vents?


--
Cheers,

John.

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