extension first floor joists!

[damoe]

hi, my builder's just about to start my double story extension and on the plans it calls for a 3.5 m box beam lintel (150mm x 225mm) from outside skin of my original house to extension inside skin this is to support the joists from front to rear of extension, my builder tells me its more common practice to cut the joists into the existing wall which will eliminate the lintel poking down into the downstairs ceiling, which i would prefer so firstly would this be an acceptable alternative to the lintel and would i have to inform the inspector of the change (if its now the usual way of laying the joists) Plans are 5 years old.
any advice much appreciated!

Damoe

 

[Static]

Urm, not 100% sure what you are on about.. you description is pretty vague..

At best guess your talking about changing the span direction of the first floor joists from side-midspan beam-side to front to back spanning.. thus eliminating the need for a mid span beam?
Well this changes the loading on the foundations and you may find either the existing footings are inadequate to support the new load and thus need underpinning etc.. or the new footings arent big enough..

All of this should be run past your Structural Engineer who takes design responsibility for it.. either that or start checking your builders insurance

 

[DevilDamo]

Why don't the joists span side to side (3.5m)? You could get away with (min.) 47 x 170mm at 400mm c/c on that span.

If the box lintel is going on to help with the spans, then you're implying your floor joists will be more than 225mm deep and on that basis, you're talking about 47 x 220mm at 400mm c/c, which can span approx. 4.6m.

And... why can't the top of the lintel run at the same level as the top of the joists... because it sounds like your drawings are implying this lintel is going below the joists, thus creating a downstand? Obviously by having the lintel at the same level, the joists would have to be notched while putting the lintel below, the joists would just sit on top.