Cement Renders - what YOU NEED to know about structural movement and substrates  

Cement Renders - what YOU NEED to know about structural movement and substrates   

Cement renders and coatings have been to waterproof basement walls and floor for over a Century, however, they can be affected by structural movement and soft substrates, which can lead to cracking, detachment, and other forms of damage.  Dissolved salts in the ground water can also crystalise within the render and as the salt crystals grow the render perishes, in a similar way to ice crystals causing frost damage. Here is some information about these issues and how to address them:


1. Structural Movement:

Structural movement refers to the shifting or settling the building's foundation or structural components. This movement can cause stress on the cement render, leading to cracks and detachment from the substrate. Common causes of structural movement include soil settlement, temperature fluctuations,  vibration from traffic, differential movement between structural elements, for example between the wall and the floor.  and inadequate construction practices.


To address structural movement and minimize its impact on cement renders, the following measures can be taken:


- Proper Building Design: Ensure the foundation and structural components are designed and constructed to withstand anticipated movement. This may involve consulting with structural engineers and following building codes and standards.  Underpinning and / or replacement of the floor slab with reinforcing tied into the base of the foundation walls can help reduce structural movement.


- Flexible Render Systems: Consider using flexible render systems that accommodate some degree of movement without cracking or detachment. These systems often incorporate specialized additives or reinforcing materials to enhance flexibility and durability.


- Expansion Joints: Install expansion joints in the cement render at strategic locations to allow for controlled movement. These joints can help absorb the stress caused by structural movement and prevent cracks from forming.


2. Soft Substrates:

Soft substrates, such as weak or unstable surfaces, can challenge cement renders. These substrates may not provide a stable base for the render, leading to poor adhesion, cracking, or detachment. Soft substrates include old brickwork, soft stone, such as sandstone, lightweight cement block and very old building materials such as cobb ( a mixture of clay and straw).


To address soft substrates when applying cement renders, the following steps can be taken:


- Substrate Preparation: Ensure the soft substrate is correctly prepared before applying the render. This may involve reinforcing the substrate with additional materials, such as mesh or boards, to provide stability and improve adhesion.


- Bonding Agents: Apply a bonding agent or primer to the soft substrate before applying the render. This can help improve adhesion and create a strong bond between the render and the substrate.


- Alternative Render Systems: In some cases, it may be necessary to consider alternative render systems specifically designed for soft substrates. These systems may incorporate specialized adhesives or reinforcing materials to ensure proper adhesion and durability.  Another alternative is to use a drainage membrane as opposed to a render.  These materials allow water to enter the primary structure and to drain down safely, ultimately to a sump pit from where the water is pumped safely away.


In many cases repair and strengthening of the structure is uneconomical or impractical, especially in cases where external groundwater pressure is high leading to the need for an alternative approach.  Combining drainage systems, designed to remove most of the groundwater can help.  The Waterguard internal drainage system is designed for internal drainage, installed above the footer where it can never clog with dirt or silt.



It is important to consult with professionals experienced in rendering and plastering, especially when dealing with structural movement or soft substrates. They can assess your project's specific conditions, recommend appropriate solutions, and ensure that the cement render is applied correctly to minimize the risk of damage.

You can schedule a free 30 minute consultation with an expert right here


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