Elongation of Waterproofing Membranes

29th May 2019


Waterproofing membranes are sold based on the merits of the elongation properties and their ability to bridge cracks.
The current Australian Standards classify membranes into Class I, II and III according to % Elongation:

Class I: 0-59%
Class II: 60-299%
Class III: 300% and above

Note: The % E is measured without any re-enforcing mat – which when added, will reduce the elongation.


The membrane can have restricted or non-restricted movement depending on its bonding to the substrate. The way the membrane is bonded also affects the extent of the flexibility and elasticity of the product.

If you use a bond breaker over a crack, it is much better if the crack were to open up later on. This is shown in Figure 1 (bond breaker) in comparison with Figure 2 (no bond breaker).

The stress on the membrane in Figure 2 is much more severe and likely to lead to failure. Even a small opening of the crack in the underlying concrete substrate will require extensive elongation.


Cracks formed after the application of the membrane have no membrane distance so they are like Figure 2. When this occurs, the membrane will extend but the result will be serious necking of the membrane across the newly formed gap and may lead to the membrane tearing when subject to small movements.
It is important to note that the membrane can only stretch to a limited length before its waterproofing properties fail, regardless of its elongation capability.


The elongation properties of a waterproofing membrane are best preserved in situations where the membrane is not directly bonded over the crack due to a bond breaker being in place.

When cracking occurs after a membrane has been installed, then membrane failure is inevitable regardless of its elastic properties.