Condensation beneath fibre cement roofing: tracing the source of moisture

An example is a warehouse into which wet equipment was driven and all openings were then closed. If moisture remains inside, a tight covering alone will not remove it. It is therefore worth observing not only the roof but also how the building is used. The source of the water may be far from where the first drops appear.

Fibre cement sheets have properties that depend on the composition and structure of the product. Some systems are described as able to absorb part of the moisture and release it later. This is a process with limited capacity. If the moisture supply continues for a long time and drying is hindered, the material will not solve the problem on its own. One should therefore not promise the absence of condensation solely on the basis of choosing fibre cement. What is needed are the right conditions in the building, ventilation and the arrangement of layers. The favourable property of the material matters in a correct solution, but it does not replace calculations or checks of the moisture sources.

Rainwater can get in through damage, a badly laid overlap or a detail at the chimney. Condensation forms from water vapour. Both phenomena can occur at the same time, so it is not worth choosing one diagnosis after looking at a single stain. Fibre cement sheets must be checked together with the fixings and flashings. A record of the weather and the time when the moisture appears helps. Does the problem occur only during rain from a particular direction? Do drops also appear after a cool night with no rainfall? Observing the whole area is more valuable than spot-sealing visible joints before the actual source is established.

An opening in the wall does not always ensure sufficient flow. The inlet, outlet, air path and varying conditions of use must be taken into account. Asbestos-free fibre cement sheets on the roof of a livestock building or a wet workshop operate in a different environment from those over a dry shelter. The ventilation arrangement should correspond to the specific moisture sources. Openings should not be closed merely because cold can be felt through them in winter. Such a change can worsen the situation under the roof slope. If a change to the heating method or the building envelope is needed, it is worth assessing it together with the ventilation. Removing moisture and maintaining comfort are not independent tasks.

Moist air can enter the building envelope through leaks on the interior side. Damaged connections at the wall, ducts or hatch can be more important than the surface of the membrane itself. Fibre cement sheets are then only the outer part of the whole assembly. Air tightness, the vapour resistance of the layers and the possibility of drying must be assessed. Another tight covering should not be added without checking whether it will trap moisture inside. In difficult assemblies, a thermal and moisture analysis is needed. It helps to determine where vapour control should act and where a ventilated space is required.

It is worth recording the indoor temperature and humidity and comparing them with the weather and the work being carried out. A single measurement at noon may not reveal a problem that occurs towards morning. Fibre cement sheets are inspected from a safe position, without walking directly on the sheets. It should also be checked whether the inlets and outlets are blocked by insulation, stored items or contamination. Such a check often reveals a change that was not in the original design. The data collected makes it easier to talk to the designer. It allows a seasonal situation after wet work to be separated from a permanent fault in the building envelope or ventilation.

If a leak has been identified, the defect in the covering is removed and the damp layers are checked. If the problem is condensation, changes to ventilation, airtightness or usage may be needed. Fibre cement sheets do not need to be replaced automatically just because droplets have appeared on them. First, the condition of the material and the conditions under which it dries must be assessed. Sometimes the solution is to restore the patency of the existing air path. In other cases, the layers will need to be rebuilt. After the correction, it is worth continuing to observe under similar weather conditions. The absence of water on one dry day does not yet confirm that the source of moisture has been eliminated.

When selecting asbestos-free fibre cement, describe the humidity and the way the building is used. The covering, ventilation and roof layers should correspond to the same conditions.