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Why is it important to waterproof a screed?

Under, over, both, neither? These are the questions we get asked about waterproofing and screed.


It depends a lot on the situation and the design by the engineer or architect, so its worth understanding a bit about screed, how it responds to water and what waterproofing does for screed installations.


Firstly, screed is a porous material. If exposed to water, it will absorb the water and then release it when drying out. This is one of the reasons why screed is not a final wearing layer, but a substrate for a final product that provides the finished surface as well as protecting the screed underneath.


Repeated cycles saturation and drying of a screed is unwanted and can have a number of effects including:

  • Failure of the bonding slurry anchoring the screed to the underlying concrete slab resulting in a delamination of the screed (also known as "drummy screed")

  • The expansion and contraction of the screed in drying cycles can cause cracking in the screed

  • The exposure of the screed to weather can affect the surface quality of the screed resulting breakdown of the surface strength and finish

Additionally, particularly for thick screeds, moisture once trapped under a screed can take a long time to dry out and if wet locations where there is regular rain, it may never completely dry out.


For all these reasons it is very important to protect your screed from the elements.


Waterproofing underneath a screed


Sometimes the concrete slab is waterproofed and screed installed over the top. This has challenges for screed adhesion which you can read more about here which are generally solved using a sand seeded membrane. This layer of waterproofing can be used to protect the underlying slab from any water that penetrates into the screed for whatever reason, or can be intended to protect the screed from rising damp through the slab.


We often see this type of installation on a rooftop for example combined with an additional waterproofing membrane over the top of the screed as well to ensure that both the screed and the slab/ceiling have protection from the elements.


Waterproofing over a screed


The most common waterproofing installation is waterproofing over the top of the screed. For example in wet areas, balconies, rooftops and anywhere exposed to regular water. Water ingress to a screed should always be avoided and waterproofing over the top of the screed provides the first layer of protection for the screed and everything underneath it.


What happens if water gets in the screed underneath the membrane?


This is a situation that we see from time-to-time unfortunately. The most common scenario is if there has been rain on a screed (eg a rooftop) and a membrane has been applied over the top prior to all moisture evaporating from the screed. At the time it may have seemed like the screed was dry but if the rain has penetrated deep into the screed it may evaporate out for some time. This is why it is very important to protect screed from the elements prior to applying the final top layer product.


Depending on the type of waterproofing membrane that is applied over the screed, trapped moisture in the screed from rain can cause the membrane to bubble or emulsify often resulting in the failure of the membrane and sometimes damage to the screed from the trapped water attempting to get out.


There are many different types of waterproofing products and system designs for screed and waterproofing and its important that advice is taken from a qualified engineer, architect or waterproofing professional on waterproofing design.


This video below shows the effect of having rainwater trapped under a membrane that then forces itself to the surface in the heat and then emulsifies membrane's primer from underneath before spurting out under foot traffic.






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