What is a cavity tray, and why does it matter?
A barrier built into a cavity wall to collect water that has crossed the outer leaf and direct it back outside — and missing or badly installed cavity trays are among the most common causes of persistent, baffling damp in modern buildings.
How a cavity wall works. Two leaves of masonry with a gap between. The outer leaf is expected to get wet — brickwork is porous and driving rain penetrates it. The cavity's purpose is to provide a break so that water reaching the inner face of the outer leaf runs down the cavity rather than crossing to the inner leaf.
Where the problem arises. Wherever something bridges or interrupts the cavity, water running down has somewhere to go other than downward. At those points a cavity tray is required: a sloping impervious barrier, running upward from outer leaf to inner leaf, collecting water and discharging it back through weep holes in the outer face.
Where trays are needed:
Above openings — windows and doors, over the lintel.
Where an abutting roof meets the wall, such as where a single-storey extension joins the house. Stepped cavity trays follow the roof line.
Above any structure penetrating the cavity — beams, joists, ducts, and services.
At changes in cavity construction, and above cavity fill.
Why failures are so hard to diagnose. Water enters at one point and appears somewhere else entirely — frequently a metre or more away, and at a different level. The damp patch is not where the fault is, which is why these problems are repeatedly and expensively misattributed to rising damp, condensation or a leaking pipe, and treated accordingly with no effect.
The common installation faults:
Weep holes omitted or blocked, so the tray collects water with nowhere to discharge it and eventually overflows onto the inner leaf.
Trays not stopped at the ends, so water runs off the end into the cavity.
Insufficient upstand, or the tray not built into the inner leaf.
Mortar droppings sitting on the tray, bridging it.
Damaged during construction.
Retrofitting is possible but disruptive, requiring removal of masonry — which is why getting it right during construction matters so much.