What a Survey Must Establish Before Enclosing a Wall

Driving rain exposure varies with orientation, shelter and height, so the building does not have one exposure and its elevations do not share one answer. The cavity fill record makes the consequence explicit: neighbouring houses of apparently identical construction respond differently to the same work, and one elevation of a single house may sit outside what is reasonable while the others do not.

Exposure is the input that most of the other questions are weighed against. How much water the outer leaf passes, how much drying the wall still has available and how much a fill or a wrap is being asked to take over all depend on it.

A masonry outer leaf is accepted as permeable rather than assumed to exclude water. What matters is how much it passes under driving conditions, and that depends on its units, its mortar, its history of pointing and repair, and anything that has been applied to its face. A leaf that passes little water can be backed by almost anything; a leaf that wets through depends entirely on what is behind it.

Where a facade is to be bonded onto the wall, the record adds a further question: whether the background has finished moving, since bedding onto a background that is still shrinking asks the bond to hold across movement that has not finished.

A cavity cannot be cleared once the wall is standing. What is in it — mortar squeezed from bed joints, debris fallen during construction, sometimes earlier partial insulation — can only be worked around or used as a reason to rule the work out. The survey therefore establishes the width, the cleanliness and the contents of the void, and it does so on each elevation rather than at one convenient corner.

The consequence of getting this wrong changes with the work. In an open void, a bed of droppings resting on ties was a local bridge that water could still drain around. Surrounded by fill, the same bridge is continuous with the material either side of it and nothing interrupts the path.

Wall ties are the only structural connection between the leaves and also the commonest accidental bridge across them. They cannot be seen from either face, they age by corrosion within the joint rather than by weathering at the surface, and a wall can look entirely sound while the connection between its leaves has deteriorated.

So the survey question is not only whether ties are present but what condition they are in, how that was established rather than assumed, and whether they sit falling the right way. After a fill, those same ties are being asked to last for the rest of the wall's life in an environment that may hold moisture for longer.

Trays over lintels, at abutments and at the base were detailed to discharge into an open void. After a fill they discharge into a material, which is why their presence, their continuity and their stop ends are among the first things a survey looks for. A collection surface with no exit holds water permanently against the wall at that level, which the junction record describes as worse than not collecting it at all.

The same survey has to find everything else crossing the void: sub-floor vents, flue ways, extract ducts, meter boxes and the ends of trays. None of these is drawn on an ordinary existing house, so they are located by inspecting both faces and the roof space. Each is either a route by which fill escapes or an air path that must not be blocked.

Where floor timbers bear in masonry, they sit in a material that is periodically damp by design, and their durability depends on that masonry still drying and on air reaching them. Lining the room face cools the wall above and below while the timber stays buried in it, and the record treats whether those surroundings become wetter as one of the first questions a survey should raise.

It is a question that survives the choice of route. Wrapping the outside, filling the void and lining the inside each change the conditions around embedded timber in a different direction, and none of them leaves it as it was.

Salts left in masonry by earlier damp continue to attract moisture after the original cause is fixed, which can make a successful repair look like a failure. Existing internal damp, poor pointing and heavy exposure on the same elevation are, in the records' words, a reason to pause rather than a detail to work around.

Rainwater defects, ground levels and gutter faults belong in the same category. They are better corrected before a wall is enclosed, because afterwards they act on a wall that can no longer dry the way it did.

Everything the survey looked at is about to become invisible. The records are consistent on what follows: what was found in the void, where fill was and was not placed, how openings were treated, how drilling was made good, where the control layer runs and where fixings may safely be made are the only evidence anyone will have when a damp patch appears years later.

A survey based on a few holes in one elevation is not evidence about the whole building, and the record should say how many inspection points were used and where.