Lintel for wall opening 600mm wide 400mm tall

[PlumbBob]

Hi, for cavity wall opening should a concrete or steel lintel be used? Or can either be used? The external wall is rendered so concrete can be covered up.

 

[23vc]

Either, but a catnic type cavity lintel makes the cavity tray easier

 

[PlumbBob]

Thanks so with concrete, 2 individual - one for each wall (inner and outer) or 1 for both walls?

With the single concrete one for both walls, effectively bridging, would this need a cavity tray too? (Not shaped as per a cavity)

 

[cdbe]

2 single.