Calculating non-standard joist load

[scape279]

I have a Victorian mid terrace cottage and I would like to use the loft for more than just storage (still ladder access and no finishing off etc - only worried about the floor). Currently there are 2"x4" 'joists' (notice my loose use of the word joist) spaced 12 inches apart.

How do you calculate the maximum load a timber can take without using joist load tables? I've looked at tables and none of them cover the load of 2x4 at the kind of spans I'm looking at (over 2m but under 3m).

I'm hoping to add extra 2x4 so they become 6" apart, or even go nuts and make them 4" apart! But I need to know how nuts I need to go.

I'd rather not add taller joists eg 5, 6, 8 by 2 or whatever as this would raise the floor level and may as well get a pro in to to a full on conversion properly.

 

[big-all]

this ones easy
you need to comply with building regs so run all your ideas through the building regs over the phone as they are the ones you need to please to get passed

 

[RonnyRaygun]

Do you mean 12" centres or 12" between the joists?

If 12" centres at 3 metre spans use C24 timber and you will be fine for standard domestic floor loads if you double up the joists.
I'd screw or bolt them to the existing rather than spacing them centrally.

If you mean 12" between the joists (14" centres) you might be struggling on deflections. If your spans are around 2.8m or less you should still be OK.

If you have any steelwork in the spine wall BC might want that checked too for the extra load...