I was under the impression that standard practice is to install a preformed cavity tray over the top of cavity lintels to ensure any moisture in the cavity is directed to the outer leaf rather than either accumulating on the lintel or getting trapped behind it. But having just called around all my local builders suppliers it seems none of them actually stock them, which leaves me wondering how common the practice actually is? Surely, if this were as commonplace as I had been lead to believe, everyone would stock them, right?