I am building over a flat roof garage which has a cavity wall except for the
front. Wall to be brick outer and block inner.
I need to put a cavity lintel over the front and this will be 3450mm long.
There is a choice of standard or heavy duty.

Which one do I need? The HD one is £200+ dearer than the standard. It's no
good asking the manufacturer as they just say have a structural eng. decide.
So can anyone tell me a typical application for each grade of lintel?

Thanks

mark