Waterproofing the stepped joint with brick veneer

By - , Build 84

BRANZ has offered an 0800 helpline for the building industry since 1993. If you phone 0800 80 80 85 (and press 1) you will invariably speak to Eddie Bruce, who fields more than 4,000 calls a year. These are some of the queries he deals with.

Figure 1: Waterproofing the stepped joint at the base of brick veneer

Q Under NZS 3604: 1999, is it necessary to waterproof the stepped joint at the top of a foundation wall supporting brick veneer?

A Moisture ingress problems reported to BRANZ have often been caused by a lack of waterproofing at the base of brick veneer cavities. Just contemplate what is needed to fix the problem — it means removing the lower courses of brick!

Taking a step back before addressing the waterproofing, first make sure all materials and workmanship comply with NZS 4210: 2001 Masonry construction. Also ensure that the surface supporting the veneer is 50 mm or more below the surface supporting the timber framing (with the bottom plate protected from the concrete by a DPC), and that the cavity is 40–75 mm wide (as shown in Figure 1).

Then look at what is covered by NZS 3604. The reference diagram is Figure 11.1 in NZS 3604, which shows brick veneer on a concrete or masonry foundation wall, or concrete slab edge foundation, with a 250 bondbeam supporting the timber framing. The bondbeam provides the required stepped joint for the veneer.

This diagram shows a waterproof membrane to the stepped joint, but the text refers to habitable spaces (clause 11.7.4.6) and this is where some confusion arises. Is the stepped joint adjacent to the floor shown or not? There is a comparable situation in Section 7 of NZS 3604, where Figure 7.10 shows a DPC to the stepped joint for a slab-on-ground situation. Obviously the intention is to provide waterproofing to the stepped joint in any situation.

Figure 1: Waterproofing the stepped joint at the base of brick veneer

The reason behind this is that water running down the inside of the veneer settles at the bottom of the cavity before exiting through the weep holes. In many cases there will not be sufficient water to create a flow, and so the bottom of the cavity could remain damp for considerable periods. If mortar droppings have not been removed during construction they act like a sponge soaking up even more moisture. The concern is that moisture could ‘wick’ through the edge of the slab, bypassing the damp-proof course under the bottom plate and leaving the outer top edge of the floor slab damp.

Therefore the answer is yes, the stepped joint should be waterproofed, preferably with its own waterproofing. BRANZ recommends the option given in Figure 1 opposite, where the waterproofing to the step (a glued-on butyl rubber flashing or bituminous coating) is separate to the DPC for the timber framing.

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Figure 1: Waterproofing the stepped joint at the base of brick veneer

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