We are often asked if weep holes are necessary in rendered walls. So to clear up the confusion we have provided some guidance on how and when weep holes should be used in rendered walls.
Render and water penetration
Resistance to rain penetration for external cavity walls can be enhanced by the addition of render. However, whilst Polymeric render manufacturers claim their products offer enhanced protection compared to traditional sand cement renders, no render products are completely impervious to wind-blown rain. In fact such a product would hinder the ability of the building to dissipate internal moisture. As a result it is important to consider when weep holes should be used to allow water to escape from the cavity.
Are weep holes required in rendered masonry walls?
Whilst render potentially offers additional protection some water penetration will still occur. As a result it is important in rendered masonry walls that cavity trays are installed with stop-ends over openings and lower level abutments where the external wall becomes the internal wall. However, because of the increased protection offered by render British Standards state, “weep holes need not be provided if walls have a rendered external finish." This relaxation is based on the assumption that the small amount of water that may penetrate the outer leaf and collect in the cavity tray will dissipate by natural ventilation. This exception only applies when there is no unrendered brickwork above the rendered section.
Render and timber frames – do you need weep holes?
Timber frame construction is more susceptible to damage from water penetration, as a result the TRADA Timber Frame Construction manual calls for cavity trays, stop ends and weep holes in every situation. This is not relaxed when render is applied; in fact TRADA prefers the use of the term “full open perpends” as they do not accept that the smaller preformed pencil type weep holes provide adequate ventilation.
Where correctly designed and specified render is used on the outer leaf of traditional masonry cavity walls then cavity trays with stop ends are required but weep holes can be omitted.
Where the outer masonry cladding is rendered for timber frame construction then cavity trays with stop ends and full perpend weep holes are required.
By Frzana Ashraf