Passive Cavity Wall Masonry
Author/Issuer: Cavity Tray Standards Added:
The formation of cavity wall masonry affects its performance and weathering qualities
The external skin of a cavity wall is designed to saturate during heavy rain. It is not relied upon to prevent moisture penetration. Thus the external skin is termed passive masonry and rain striking its external surface will pass through it. This occurs predominately via the mortar joints and to a lesser extent through the masonry itself.
Such penetrating water has to be controlled, and failure to appreciate the possible extent of penetration and the accompanying volume can result in dampness reaching parts of the structure that are intended to be kept dry.
Mortar type and continuance play a large role in creating an external skin that will adequately tolerate rain penetration. The bedding of masonry must be without interruption – gaps, voids and hollows should not feature. The bond between masonry units is compromised if insufficient mortar is present. The ‘frogs up’ rule promotes a solidity of bond, but the mason must ensure the frog is completely full. It will otherwise function as a reservoir for water when saturation point is approached. Similarly perforated bricks can be problematic if the bedding is intermittent. Penetrating water encountering open perforations will use them as a conduit to travel downwardly through the masonry. The consequence is often apparent when the flow of water reaches a lower level within the masonry skin where a dpc or similar is present. The accumulation has to go somewhere.
Intermittently bonded areas and inconsistently bonded areas weaken the structure and its ability to provide an arrangement that is as resistant as possible to driving rain.
Such rain falling onto the external face of the external skin is drawn through the masonry by a pressure difference that exists between the outside surface and the inside surface. A ventilated cavity can reduce this difference in pressure (and thus the amount of rain permeating), but it will not eliminate it. Some styles of masonry are sufficiently permeable to rain that the volume passing through can be extensive. In excess of 2.25 litres per sq metre per hour has been witnessed during monitoring. Such volumes of water must be controlled and must have a designed evacuation route.
Capillary action is another factor that can feature in masonry walls. The most likely route is again via intermittent mortar beds and perps. However, capillary action will also manifest via dry-bedded dpc’s and lintels. Without thorough mortar bedding the relationship between masonry and dpc, or masonry and lintel, can prove hazardous to say the least. The British Standards and Building Regulations stipulate bedding in and on mortar, but the failure to observe this practice by some operatives is still witnessed on many sites.
Capillary action can manifest some time after a structure has been completed. Modern buildings expand and contract, and commonly fissures and hairline cracks will occur in the best of cavity walls. As well as the usual junctions around window and doorframes, cracks can appear anywhere the extent of the masonry and its proximity to an interruption or reduction in mass creates an uneven tolerance to movement. In a nutshell, when temperature and moisture variations prompt a building to expand and contract, it is usual for such movement to manifest in the weakest structural arrangement. Look where the volume of masonry between openings is slight. Paradoxically, where the mass is really extensive the absence of expansion joints will encourage the masonry to introduce its own! The masonry wall always needs to be able to breathe – and breathing is normally is accompanied with movement.
Cavity trays or dpc’s within a cavity wall must prevent penetrating water travelling beyond a defined point – normally the bedding course in which the tray or dpc is positioned. The exception is when a tray is acting as a cavity wall arresting barrier. The purpose of an arresting barrier is to invisibly arrest and thus reduce water wash (volume) from an area of cavity wall above a feature or structural arrangement. By so doing the handling of the large volume of water is shared in more than one place. Such an approach can eliminate overload potential that might exist where an extensive area of masonry (catchment area) is present above a given structural arrangement and/or the site is exposed. Arresting barriers can also be deployed effectively where stone or cast mullions are present and it is advantageous to lessen the extent of water wash within the cavity prior such features.
The above information has been extracted from the library records of Cavity Trays Ltd of Yeovil, which continues a damp defect analysis survey in the UK. The ongoing survey has now been operating 74 months. Cavity Trays Ltd is the longest established specialist in its field and has a proven unmatched track record. It is the only cavity tray manufacturer awarded European Technical Approval. © Cavity Trays Ltd 2008.