Air Brick Cavity Trays

[Leemondo129]

Hi All,

I am in the process of building a single storey extension to the rear of my property, I am doing as much of it myself and learning as I go. I am up to DPC level with the brickwork now and have a question that I cannot find an answer to and have no one to call over the weekend. It is a standard cavity wall with a standard DPC, there is no stepped cavity tray around the perimeter just 100mm DPC on the inner and outer leafs, with a lean concrete mix filling the cavity to 225mm below DPC.

I have three air bricks along the back of the extension that I have extended from my existing house, they are connected through the cavity wall via telescopic vents. These air bricks are below the DPC, so the question I ask is do these air bricks need a cavity tray above? Surely any water that might get through the outer skin and track on the telescopic vent would be dealt with by running out of the weep vents at the 225mm below DPC level where the cavity is filled with a slope towards the outer skin?

I could put cavity trays above two bricks easily but the third is under a doorway so won't work.

Thanks in Advance

Lee Smith