I'm thinking of adding a porch. It would be joining an existing rendered cavity wall and part is stone/slate cladding. Are cavity trays needed for rendered walls or are they just required were the extermal wall is face brick? There is also some foam insulation in the cavity.
For readers looking for related advice please see our cavities DIY how to projects pages which covers stuff like damp issues and fixes, vertical Dpc, insulation of cavities, cavity trays and lintels and pumped cavity walls.