Penetrating rain evacuated via Caviweeps | 15 May 2006 |
Whenever it rains, water quickly penetrates the external skin and the cavity prevents the water from making contact with the internal skin which is kept dry. The principal is established and reliable. Whenever a DPC, cavitray or lintel crosses the cavity, it is now normal practice to incorporate weeps within the perp joints of the external leaf, to permit the collected water to discharge out of the wall. However, open perps proved satisfactory in previous decades, so why is it bodies such as British Standards, LABC, NHBC and Zurich Insurances now refer to such devices? What are the advantages and why should you bother?
The answer is construction methods have changed. In so doing accompanying practices have changed also. An example is the use of stop-ends with lintels. The arrested water within the cavity is now discharged out of the wall via weeps, rather than discharging into the cavity itself. (Anyone who has witnessed damp reveals will appreciate the logic of eliminating the discharge of water off the end of the lintel.) The stop-ends create a ‘closed circuit’ DPC arrangement, which means each individual lintel must evacuate water out of the structure at all times, regardless of wind and rain direction. One should remember the cavity wall can also really be a ‘sandwich’ wall, with the cavity space fully or partially filled with an insulating medium. Water discharge concentrated into the sandwich filling is not recommended!
Leaving just open-style perps however is inadequate. On exposed sites they cannot perform satisfactorily at all times, especially under directly-blowing wind pressure.
In comparison, the correct choice of preformed weep uses wind-pressure to advantage. The Type W weep from Cavity Trays of Yeovil is an example. The Type W is really a dual-function unit, as it acts as a combined cavity weep and cavity vent. The delta shaped front ensures superior airflow, taking advantage of positive and negative air pressures in and out of the wall. Water is actually encouraged to discharge despite direct wind pressure. Within each weep are baffles that provide protection against rain entering in storm conditions. By providing a defined, reliable discharge route, all arrested water can be evacuated out of the structure.
When weeps are used within cavitrays, which also have stop-ends and function on the ‘closed circuit’ stand-alone discharge principal, there is a further benefit. The ‘closed-circuit’ principal means any query within a run of trays can be identified, isolated and investigated. The problems associated with long lapped horizontal runs of conventional DPC in which water penetrating in one area might cause a problem in another are eliminated. With ‘closed-circuit’ lengths one effectively creates a series of adjoining DPC compartments, each of which has its own stand-alone discharge of water.
In practical terms, the dual-function weep provides a further convenience for the builder. He need purchase just the one type of product rather than two types, because it performs as a ventilator also. Anyone who has studied the NHBC and Zurich manuals, will appreciate the requirement for both weeps and vents is extensive, - so using a universal style is convenient and avoids having too many of one and not enough of the other!
Paradoxically, the use of weeps can provide considerable benefit or very little benefit, -- depending on the type selected. When selecting a weep for use on your projects, consider the requirements of the authoritative bodies and choose a type that complies with all the requisite industry guidelines. Will the outlet choke if particles accumulate because perhaps the weep shape is too reduced? Ensure the design has undergone a flow-test - there is little point in installing styles that will block easily through the build-up of mortar debris-wash. Select wisely and in so doing your exposure to problems on site will be reduced.
The dual-purpose Type Type W caviweep / Cavivent is available with a cap that keeps it clean when rendering or pointing-in.
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Cavity Trays Ltd
Administration Centre
Lufton Trading Estate
Yeovil, Somerset
BA22 8HU
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