Accidental Bridges Across a Cavity

The masonry cavity wall record is explicit that water crossing the outer leaf is expected rather than exceptional. The outer leaf sheds most of what arrives, and the void with its drainage deals with the remainder. Whether that remainder leaves again depends on the void staying open, the interception being continuous and the outlets being clear.

That is why a bridge is not a small imperfection. It does not reduce the performance of the void; at the point where it occurs it removes the void's function entirely, because water is handed straight across the separation the wall depends on.

The record describes the tie as having to do two contradictory things at once, transferring lateral force across the void while offering water no route across it. That is why the way a tie sits in the wall matters as much as what it is made of. One laid falling inward, or carrying a bead of mortar on its back, converts a structural connection into a bridge.

The consequence is precisely located. Water is delivered to the inner leaf at exactly the points where the leaves are most firmly connected, which are regularly spaced and repeat over the whole elevation. The same record notes that wall ties are widely treated as a minor fitting, when they are the only structural connection between the leaves and also the commonest accidental bridge across them.

Mortar squeezed from bed joints as the leaf rises has to go somewhere, and the records say where. It falls and collects on ties, on interception and at the base, forming bridges and blocking the outlets the whole arrangement depends on. The cavity wall record lists mortar droppings resting on ties or collected at the base of the void among the most ordinary causes of damp on an internal face.

There is a second effect at the interception line. Droppings resting above an interception give water a path from the outer leaf to the inner leaf that passes over the whole arrangement, so the interception is in sound condition and entirely irrelevant. The record's conclusion is that an inspection which has found the interception has not finished asking questions.

Where the void is partly occupied, the retained gap is worth only as much as the care taken to keep boards tight to the inner leaf and their faces clear. That is the cavity wall record's own wording, and it identifies the failure as a gap behind the boards rather than a defect in them.

The other half of the same problem is insulation packed hard against both leaves, which the interception record lists alongside mortar and a badly sloping tie as something that gives water a path over the top of an upstand. Neither arrangement is better in the abstract; each moves the burden onto a different component, and the burden lands where the installation was least careful.

The record describes closers as sitting where three demands meet: stopping air moving through the void, shortening the thermal path around the insulation, and not reconnecting the leaves for moisture. A closer that answers the first two by packing the gap with something conductive or absorbent has met them at the expense of the third.

This is why the record says reveals and the wall base are so often where internal staining appears first. It is not that those positions are more exposed. It is that they are where a component has been asked to satisfy several requirements in a confined space, and where the requirement most easily dropped is the one whose failure takes longest to appear.

The retrofit record is the clearest statement in the family of what fill does to a bridge that already exists. A tie sloping the wrong way, or a bed of droppings resting on ties or collected at the base, was a local bridge in an open void that water could still drain around. Surrounded by fill, that same bridge is continuous with the material either side of it and nothing interrupts the path.

The record draws the conclusion directly: cleanliness that was almost cosmetic in an empty void becomes central to how the wall behaves. It also pairs the outer leaf's porosity with the fill's water resistance as a single property rather than two, noting that a leaf which wets through under driving rain depends entirely on the fill refusing to pass that water on, and on the fill not settling later to leave a gap down which water can run.

That is why the same material behaves differently on a sheltered elevation and on the one facing the weather, and why the record treats the state of the ties and the cleanliness of the void as survey findings rather than assumptions.

Nearly every mechanism on this list is created while the wall is open and invisible afterwards. The record says so plainly: mortar left on ties, interception lapped the wrong way, boards not held tight, outlets blocked, none of it can be seen once the outer leaf is up, which is why inspection while the void is open carries unusual weight.

For an existing wall the equivalent is a survey rather than an inspection, and the records are careful about what one survey proves. A survey based on a few holes in one elevation is not evidence about the whole building, because orientation, shelter, repair history and what the original bricklayer left behind all differ. Any opening-up in an existing wall should be planned with a professional rather than improvised.

Damp on an internal face can come from either problem, and the remedies are not the same. If the void was interrupted by something and nothing was provided to intercept and discharge at that line, the answer lies in an arrangement that was never built, which is the subject of the companion guide on alterations that create a drainage requirement.

If the void is uninterrupted and something is carrying water across it, the answer lies in a connection that should not exist. One is a design and scope question; the other is a workmanship and survey question. Both can be present in the same wall, and establishing which is operating is the first step a professional investigation takes rather than the conclusion it reaches.