Condensation Reported as a Leak

In each of these assemblies there is a layer whose job is to keep internal air away from a cold surface, and that layer is rarely the one anybody looks at. In a built-up metal roof or wall it is the liner sheet. In a cold deck flat roof and a ventilated pitched roof it is the ceiling. Around a roof aperture it is the vapour control layer carried up the kerb. Where any of them is open, air rather than vapour moves through, which is a different order of moisture supply.

The cold deck record describes that difference precisely. With a continuous plane, what reaches the void is limited to what passes through the construction itself; open the plane with recessed fittings, an ill-fitting hatch or a cable run and warm moist air rises into the bays under its own buoyancy, carrying moisture to the coldest surface in the roof by a route that seepage never provides.

The built-up metal roof record explains why this is so easy to miss. The liner is asked to do unrelated jobs and only the visible one gets judged on site: as a ceiling it is assessed on line and finish, as the air and vapour control element it is assessed on whether its laps are sealed and its perimeter closed. A liner that looks faultless from below and is open at its side laps has failed at the duty that matters.

The wall version of the same build-up states the outcome as a diagnosis. Sealing the outer laps and neglecting the liner laps produces a defect that presents as a leak and is not one, because warm moist air passes the unsealed liner, reaches the cold outer sheet and condenses on it, and what runs down the inside of the outer sheet and appears at a lap is being read as rain that has come in.

Under a cold deck flat roof the record puts the symptom in its own list of misunderstandings: condensation appearing on a ceiling below a cold deck is often reported as a leak, and treating the membrane will not address it. Because the deck above is impermeable, everything that condenses in the void has to leave the way it came, as vapour in moving air, and a break in the ceiling, the void or the openings shows up on the underside of the deck.

The record adds the consequence for anybody arriving at the symptom: it will read as a roof leak long before it is understood as a ventilation failure. The pitched roof version describes the same route, with the coldest thing the air can find in the space being the underlay rather than a deck.

The rooflight kerb record contains the sharpest sentence in the family on this subject: a kerb that is structurally sound and thermally forgotten leaks nothing and still puts water on the ceiling. The construction ringing a roof aperture stands outside the field build-up and is therefore colder than everything around it.

The record then explains where the water goes. A glazed unit is usually the coldest surface in the room below, so air reaching it deposits moisture on the glass and the frame, and manufactured units generally deal with what forms on their own surfaces. What forms on a cold uninsulated kerb behind the lining is not their concern and has nowhere to go except the reveal. It also notes that where the vapour control layer was stopped at the deck rather than carried up the kerb, the aperture becomes a route by which internal air reaches cold construction from behind, where nobody is looking for it.

Two records note that the surface the moisture arrives at decides how it behaves. The light steel frame record observes that metal does not absorb or buffer moisture, so anything that condenses on the sections stays where it forms, as liquid. The composite panel roof record raises the same question about the inner end of a fastener crossing the panel thickness.

That is why these assemblies report the condition as running water rather than as a damp area. There is nothing in the construction to hold it, so it collects, runs along the profile and leaves at the first joint it reaches, which is a lap.

If the water never crossed the outer layer, nothing done to that layer changes the supply. The cold deck record states it directly about the membrane, and the metal roof record states it as a general warning that the two arrive by different routes and are corrected in different layers.

There is a further consequence. Sealing the outside more thoroughly can remove the only route by which moisture already inside the assembly was leaving, and several records describe that pattern in different forms. The practical point for an owner is that a quotation to reseal an outer sheet or renew a membrane is a proposal about the wrong layer if the diagnosis is condensation, and it will not be obvious afterwards that it did not work until the next cold spell.

The evidence is behavioural rather than visual, and most of it can only be supplied by somebody in the building. Whether the appearance follows rainfall and wind direction, or follows cold weather and internal activity, is the strongest single piece of information. Whether it can occur in dry weather is decisive in one direction.

Occupancy changes matter too. The records repeatedly tie condensation risk to the internal conditions, and the bridging record notes that a colder interior surface only produces condensation if the air meeting it carries enough moisture, so bridging, internal humidity and ventilation act together. A change of use, heating pattern, extract arrangement or number of occupants is relevant history rather than background.

Every record places this with a qualified professional and says why. Whether condensation is a risk in a given build-up depends on the internal conditions, the climate and the whole assembly together, and is assessed rather than inferred from the layer order. The published questions ask what condensation assessment was carried out and what internal conditions it was based on, which is the question to put about an existing building as well as a new one.

The bridging record adds a caution worth carrying into any remedy: treating a cold surface without addressing internal humidity and ventilation can move a condensation problem rather than resolve it. A remedy that changes one of the three and not the others may simply relocate the evidence.