Perhaps I'm being daft, but shouldn't there be a concern about the steel in the lintel rusting and causing the lintel to deteriorate. After all, they're not really designed to be sat in wet mud are they?
Or would the op need to specify one with high tensile stainless rod inside? Can you even get such a thing?
Or would the op need to specify one with high tensile stainless rod inside? Can you even get such a thing?