Vertical Up – Stands around Decks/Balconies

The height required to prevent water breaching over the termination of the waterproofing around the edge of decks or balconies is often not high enough to stop water entry from wind driven rainwater. Australian Standard AS4654 – 2012 Australian Standard AS4654 – 2012 ‘Waterproofing membranes systems for exterior use – Above ground level Part 2: Design and installation’ has in Table A1 of Appendix A a guide for the height required to prevent breaching with heights of 40 to 180 mm depending on the exposure to wind speeds. The reason for its inclusion in an Appendix is that it is a guide only rather than a mandatory requirement. The height required really depends on an assessment of the exposure to wind driven rain at a local level. The exposure to wind could be affected by other buildings around the deck as well as the general wind conditions in the area.

There is generally reluctance to have steps at the door threshold onto the deck. The damage to the carpet behind the doorway onto a deck that was in a high wind exposure location is shown in Figure 1 with the lack of required up-stand below the doorway sill shown in Figure 2.

This deck was located the other side of a roadway fronting onto a sea front with the deck facing to the south-west, so it was exposed to major wind driven rain from the west to the south. The Table in AS 4654 part 2 would require a minimum upstand of 100 mm and not with the sill of the door frame sitting flush with the deck surface. With this installation there is also an issue with the door frame having the drainage hole in the frame flush with the sill. Wind would most likely push water flowing down the fixed windowpane back into this drainage hole. Australian Standard AS2047 – 1999 ‘Windows in buildings – Selection and installation’ gives guidance for the design of windows for various exposure conditions. With the lack of up-stand for the drainage onto the sill it highly unlikely that the window installed in this location would pass the design requirement of the Standard.

There is more than just the design of the waterproofing system that has to be considered in preventing water leakage into buildings in a vulnerable exposure condition.

Figure 3 shows another example where the up-stand under the windowsill is less than the 40 mm minimum given in AS 4654.2. These up-stands are only about 15 mm in height and even though this deck was in a sheltered position water still entered the dwelling behind the window doorway. If there is a need to achieve a level floor between the deck and the inside of the building this could be achieved by installing a grate drain across the doorway as shown in Figure 4.

These drains are expensive to install and can be difficult to install on cantilever decks as they require to be recessed into the top of the cantilever at the maximum stress zone of the cantilever. In these situations it may be worthwhile looking at having a syphon drainage system made to move the water away from these grate drains as these syphon systems suck the water out below atmospheric pressure which results in much lower water levels in the drainage trough so the drainage grate does not need to be as deep.

With all decks and balconies, it is important that consideration is given to the exposure to wind driven rain is made so an appropriate up-stand can be designed to prevent water entry into the building behind the deck. AS 4654.2 Table A1 in the appendix of the Standard is a useful guide in determining the required amount of up-stand.

Barry Schafer