Walls Rendered Walls? - DPC Distortion Feeds Failure

Are renders waterproof or not? Publications will give you contradicting information. Some say cavity wall weeps to evacuate penetrating water are not necessary, whilst others say the opposite. So should rendered walls have weeps or not?

With regards to aesthetics, omitting weeps means the rendered surface can remain clear of those outlets that can distract visually - especially if there are a lot of them peppering the rendered finish. But is omitting weeps completely the right thing to do? It assumes all rendering is waterproof and will remain so throughout its total surface area, including wherever it meets an alternative surface or finish. It also assumes the rendered surface will not suffer cracks, fissures or movement ever over its intended lifetime, and that of a building - it will!

Think of a water tank in an attic or a common WC cistern. They have overflow pipes so when water needs an escape route, it’s in place and can service the removal of the unwanted water rather than allowing it to cause damage. The rendered wall should be viewed in the same way.

If one constructs anticipating wind-driven rains will eventually penetrate a rendered surface, and controls are in place so whenever this happens the fabric of the building can return to dry status as quickly as possible and not suffer deterioration, the outcome in terms of render longevity and visual appearance will improve. Not to mention keeping dry those parts of the building that should be kept dry!

Most rendered cavity walls need not have numerous weeps. Just two discreet Beak-Weeps, located each side of a rendered opening, can provide evacuation routes for water that needs to be removed out of the structure.

If no outlets at all are provided, water accumulating within the wall (on the opening DPC and trapped between stop-ends) will visually manifest as damp banding / patching. Unless addressed, this instigates cracking and deterioration. In Winter, freezing temperatures can rapidly initiate spalling.

Interestingly, omitting the stopends (which is a not-uncommon practice when the DPC is draped from the inner skin - see photo) can create the same problem but in a different place. When the window or door frame is introduced into position a draped/hanging DPC will distort as the frame pulls the ends of the hanging DPC inwards. The resultant DPC shape funnels and feeds arrested water directly into the reveal masonry. Whilst the inner skin is not usually affected by this, the outer skin is. The photograph shows what happens - the rendered wall is cracking and failing predominantly either side of the opening because water from the distorted DPC is feeding the area behind the render coat - that is consequently failing.

Use of preformed cavitrays over window and door openings can provide compliant ready-shaped protection with functional stopends. Trays can also provide that defined drip-lip mentioned in the NHBC manual - not easy to achieve using a roll DPC. Preformed Cavitrays always project sufficiently either side of the opening to provide the requisite protection of the lintel/reveal closing and importantly permit a stop-end to be incorporated within a naturally occurring perp joint. The discreet Beak- Weeps from the Caviweep range by Cavity Trays Ltd provide the water evacuation routes required so the wall is returned to dry status as swiftly as possible.

All renders are different. Whether traditional or modern versions with a BBA certificate (that often refer to a term of only 25 or 30 years), always being prepared and accepting a rendered surface is not watertight makes precautionary but constructive sense.


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Wet bleeding into the wall and trapped behind the render initiates cracking, staining and de-lamination.

DPC hanging down between skins with no stop-end provision. When the frame is introduced the ends of the hanging DPC are pulled in and the resultant DPC shape channels water into the reveal masonry behind the rendering.


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