Cavity Tray and Weep System

A cavity works because water crossing the outer leaf can keep falling until it reaches somewhere it can leave. Anything that stops the fall turns the cavity into a shelf: a lintel over an opening, a floor edge, a roof arriving against the wall, or a change from one construction to another. The base of the cavity is the one interruption that belongs elsewhere: there, water falling inside the cavity and moisture rising from the ground arrive at the same element, and the damp proof barrier continuity system owns it. A tray is what gives the interrupted water a route back out.

The parts have almost no individual value and become reliable only as a set. A tray without stop ends is a chute that discharges into the wall at each end. A tray without weeps is a shelf holding water above the interruption. Weeps with no tray behind them drain nothing at all while looking entirely correct from the pavement.

The nearest neighbouring entry is the roof-to-wall abutment and flashing system, and a reader can separate them by looking for the roof. A cavity tray is defined by geometry rather than by material and appears wherever a cavity is interrupted, including in long stretches of wall with no roof anywhere near them. An abutment is the particular case where a roof causes the interruption, and it needs cover flashings on the outer face as well as a tray within.

The set behaves as a chain, and a missing link relocates the problem rather than reducing it. Without stop ends, a tray discharges into the wall beyond the ends of the opening, so damp appears to one side of a window rather than above it and is diagnosed as something else entirely. Without weeps, the tray holds what it collects until the water finds the top of the upstand or a joint in the tray, which puts it on the inner leaf at the one point specifically arranged to protect it.

The upstand only works if nothing bridges over the top of it. Insulation packed hard against both leaves, mortar droppings resting on the tray, or a wall tie sloping the wrong way above the tray line all give water a path from the outer leaf to the inner leaf that passes over the whole arrangement. The tray is then in sound condition and entirely irrelevant, which is why an inspection that has found the tray has not finished asking questions.

Continuity between separate trays is where the requirement and the programme collide. Trays over openings, at floor edges and at abutments are frequently installed by different trades at different stages, and each is correct within its own run. Where two runs meet, one has to lap over the other in the direction the water travels, and a butt joint between two well-formed trays is a break in the middle of an otherwise continuous arrangement.