Raising the lintel in chimney breast opening

[purplewires]

Hi,
Have a 1930 semi with fireplaces (once) in every room.
In a downstairs room, adjacent to the kitchen, I'm raising the opening of the chimney breast to circa picture rail height.
This is to accommodate a new WB stove.
Am also opening rear of breast into kitchen, from just-above worktop height to same picture rail height.

This would seem like a straightforward thing to do except, the new lintel height will mean that the "load triangle" (of bricks being supported) extends beyond the floor joists of the first floor. Presumably I have to now consider the dead and live loads of the 2 1st rooms (a bedroom and the bathroom)?

It looks like I'm going to have to get an SE in, but I'd rather argue the over-engineering approach with the BR inspector. Intuitively, I would think that a 100x65x1500pre-stressed lintel over each opening (1150mm wide) would suffice. The mortar is nice and solid, does that help with the composite action of the PSC lintel?

Does anyone have a ready-reckoner for calculating the loads?

Any constructive pointers would be much appreciated.

 

[Static]

Phone up catnic's technical/sale lines and get them to do the calc.. ok you may have to put in a steel lintel but the cost difference is minimal.

You may find it difficult to argue composite action on existing masonry.. again would be cheaper to up the lintel depth than even do the calc..