Water does not usually get into a wall through the field of the cladding. It gets in at an edge — where one layer stops and another begins, where a pipe passes through, where the roof meets the wall. Four of those edges decide most of it.
The roof-to-wall
The roof sheds onto the wall, so the barrier comes down over the flashing’s upstand rather than stopping beside it. Wall and roof move differently, and a lap that opens a few millimetres is a way in.
The opening head
The head flashing falls to the outside and turns up at its back edge, with the barrier above lapping over that upturn. Set flat, it holds water. Laid the wrong way, it takes water behind the barrier.
The penetration
Every pipe, vent and fixture is a hole in the barrier. The patch goes on in the order of the layers, upper over lower, and it seals to the barrier — by the time water is at the cladding it is already behind it.
The cavity base
The cavity drains only if it is open at the bottom. Weeps blocked with mortar, offcuts left in the cavity, or cladding run past the base flashing turn the drainage plane into a reservoir.
One trade owning all four edges is the reason we install the whole assembly rather than the cladding over somebody else’s work.
If you are pricing an envelope, these are the four details to ask about before the number. A quote that does not describe them is a quote for panels.