Damp proof courses and flashing installation in masonry construction

Purpose

This advisory note provides guidance for builders, architects, façade engineers, certifiers and bricklayers on how Damp Proof Courses (DPCs) and flashings are to be installed in a building. It is relevant to buildings which are following the Deemed-to-Satisfy pathway for compliance.

Buildings which adopt a performance solution must comply with the site specific design. It is applicable to both Class 1 and Class 2 buildings.

Issue

Damp proof courses and flashing installation in masonry construction.

Referenced standards

Class 1 buildings following the Deemed-to-Satisfy pathway must comply with one of the following:

• Australian Standard AS 3700; or
• Australian Standard AS 4773.1 and AS 4773.2; or
• The ABCB Housing Provisions.

Additionally, windows in class 1 buildings must comply with Australian Standard AS 2047. AS 2047 states that flashing shall comply with AS 4773.2.

Class 2 buildings following the Deemed-to-Satisfy pathway must comply with AS 3700. AS 4773 is not applicable to Class 2 buildings but AS 2047 is applicable. Lastly, the Design and Building Practitioner’s Act requires façade designs to be declared by a Design Practitioner – façade.

In all cases, building proponents can elect to use a performance solution in lieu of following the applicable standards.

The difference between DPC and flashings

A DPC is a flexible sheet material designed to stop moisture from the ground migrating up through the bricks. 

A flashing is a flexible sheet material designed to discharge moisture from inside the cavity to the outside.

Both DPCs and flashings can use the same material and, in some cases, the same piece of flashing can serve as both the DPC and flashing.

Download Figure 1a

Download Figure 1b

There is some confusion on whether a piece of flashing needs to extend the full width of the masonry leaf and be turned down. 

Figure 1 illustrates the purpose of a DPC versus a flashing. A DPC is installed close to the ground (or any surface where moisture can wick up into the bricks) to prevent migration of moisture. The important requirement of DPC is that it must extend the full width of the brick and be visible from the outside. When installing a DPC, the material must be wider than the brick, the excess can either be neatly folded down or cut flush with the façade. In either case, the DPC must be visible even after render or paint is applied. 

The purpose of the cavity flashing is to direct internal moisture out. This can occur around openings (typically windows) and at the base of the wall at every level (applicable to multi-storey developments). Cavity flashings do not necessarily need to project from the façade or be visible from the outside.

Acceptable Deemed-to-Satisfy construction

Width of DPC

AS 3700 requires flashings acting as DPCs to at least 20mm wider than the thickness of the masonry member (cl 4.7.3). The Standard permits the excess to be either folded down or cut flush with the wall (cl 12.4.16). If cutting flush, the DPC must still be visible when viewed externally and it must be flush with the wall inclusive of any finishes (such as render).

Height of DPC above ground

The minimum height of the DPC above the ground is defined in AS 4773.2. Class 1 buildings adopting AS4773.2 must comply with cl 9.6.1 for veneer walls and cl 10.5.1(d) for cavity walls AS 4773.2 allows for the height to be reduced in certain conditions. AS3700 does not give specific requirements on the minimum height. It is recommended Class 1 and 2 buildings following AS 3700 take guidance from AS4773.2. For Class 2 buildings, the height of DPC is a matter for the façade design practitioner to determine and must be included in the declared designs.

The following configurations illustrates common scenarios and acceptable construction.

Common examples

Scenario 1

Cavity brickwork supported by strip footing. Bricks may extend below the natural ground level.

Separate flashings are used as DPC and cavity flashings.

Requirements

• Cavity to be filled with mortar and graded away from the building.
• The DPC to extend full width of both brick leaves and be visible from the outside.
• The cavity flashing directs water externally. The flashing does not need to extend the full width of the masonry unless it is also serving the function of a DPC.
• DPC height may be reduced as per AS 4773.2 cl 9.6.1 or cl 10.5.1(d) in the case of a Class 1 building.

Download Figure 2

Scenario 2

Brick veneer on slab on ground where a singular piece of flashing acts as both cavity flashing and DPC.

Requirements

• Flashing to extend the full width of the masonry, excess width can be turned down or cut flush.
• DPC height may be reduced as per AS 4773.2 cl 9.6.1 or cl 10.5.1(d) in the case of a Class 1 building.

Download Figure 3

The following diagram shows the key design details our inspectors check when auditing a standard brick veneer wall. 

Download Figure 4

1. The height of the DPC depends on ground conditions but is generally 150mm. DPC height may be reduced as per AS 4773.2 cl 9.6.1 or cl 10.5.1(d) in the case of a Class 1 building.
2. Cavity flashings fixed to inner frame at 600mm centres.
3. Combined DPC/cavity flashings to have 150mm laps.
4. Cavity to be drained by weepholes at 1200mm centres.
5. If the flashing is acting as both DPC and cavity flashing, it must extend the full width of the masonry and be visible at the face of the wall. Excess width can either be trimmed or turned down.
6. Cavity width to be at least 25mm and free of obstructions.
7. Adjacent ground should be sloped away from the building.

Acceptable Deemed-to-Satisfy construction at window openings

Flashings around window openings that are not acting as a DPC (such as the head flashing) do not need to extend the full width of the masonry member. However, the flashing must direct moisture away from the cavity. 

There is potential for the cavity flashing to finish vertically and direct water down into the cavity when the flashing and lintel lengths do not match. Refer to figure 5.

Download Figure 5

This situation can arise because the minimum bearing length for a lintel is 100mm but the minimum that a flashing must extend past the window opening is 150mm. As both AS4773.2 and AS2047 allows for the cavity flashing to finish vertically and rely on the horizontal leg of the lintel to discharge water, a flashing to lintel length mismatch could create the potential for water ingress.

Download Figure 6

To remedy this, it is recommended that the flashing be continued down further and have a horizontal turn out at the bottom to direct moisture away from the cavity. Other solutions such as moving the flashing higher whilst having a horizontal turn out along the bottom or increasing the length of the lintel are also acceptable.

Flashing detail at masonry parapets

Building Commission NSW is aware that the Australian Standard for external above waterproofing (AS4654.2) shows a cavity flashing which projects and is turned down on the external face.

Download Figure 7

Building Commission NSW’s position is that the detail on the right hand side of the diagram is not a normative requirement. Figure 7 is a diagram that is is referenced in clause 2.8.2.2 which states: 

"The top edges of the membrane shall be protected by the downturn of the cavity flashing."

This means that the cavity flashing detail on the left hand side of the parapet that is protecting the membrane upturn is a normative requirement, but the rest of the detail is informative. Building proponents should follow the referenced standards for masonry to determine the proper configuration of cavity flashing and not AS4654.2 if there’s no membrane termination.

Acknowledgement

Building Commission NSW gratefully acknowledges the valuable assistance and information provided by Think Brick Australia in the development of this advisory note.