Patricio Contreras, technical manager at Kerakoll UK Ltd, provides a summary of the problems that can arise and ways to prevent them in the occurrence of rising damp.
For flooring tile installations on structures directly above the ground, it’s essential to ascertain whether a DPM is present to act as a capillary barrier. This prevents cement-based adhesives from contacting with salts from the ground that would attack them chemically. This is most frequently encountered when working on existing substrates, from externally on ground-based structures, or internally on the ground floor or at basement level.
In the last case, there’s an additional problem – the risk of negative water pressure. This occurs where the water table is located above the level of the tiled floor. The chemical properties of most of the cement-based systems used to install ceramic tiles can’t effectively resist exposure to these salts, leading to a gradual failure of the system, as the effect of sulphates and other types of salt becomes irreversible over a long period of exposure. In addition to chemical attack, if the salts adhere to the surface of the porous elements they’ll crystallise, causing mechanical damage as they expand.
Consequently, when there’s no structural DPM, or if there’s evidence of salts on the surface of the concrete screed, it’s essential to install a capillary barrier as the first element of the tiling system. An effective solution is to lay an unbonded screed over an anti-capillary polymeric barrier, however, there’s often no space available to include this screed in the system or the costs, and application times make it impractical. In this case, the recommended procedure to prevent the problem is first to thoroughly clean the substrate and mechanically remove any salt crystals that may have formed on the surface, as these would prevent the adhesion of the subsequent system to the support. The next steps depend on whether the substrate is dry or wet.
On a dry substrate, we recommend applying a waterproofing agent such as Aquastop Nanoflex. This is a single component cementitious waterproofing agent with vapour permeability, i.e. it stops the passage of capillary water, and salts, but it allows water evaporation owing to climatic conditions to continue, avoiding the build-up of vapour pressure with its mechanical risk of the floorcovering becoming detached. The installation should be finished with an adhesive from the H40 range which has the best mechanical compatibility with Nanoflex, ensuring strong adhesion. Finally grouting with Fugabella Color is recommended as it performs suitably inside and out and allows water vapour to escape.
On the other hand, with an existing substrate with high humidity, we would use an osmotic waterproofing agent, capable of occluding the surface pores of the concrete substrate and resistant to contact with the salts present in the rising damp. In basements where the floor is lower than the water table giving a risk of hydrostatic pressure, the structural stability of the concrete slab should be assessed by a competent technician, to verify that if waterproofed, the structure can withstand the tensions generated by the water pressure and meet the requirements of BS 8102:2022.
As a final point, we’re frequently asked about breathable systems for areas at risk of flooding which would allow them to dry out quickly if flooded. In this case, we recommend a multi-layer anti-fracture polymeric membrane to be added to the previous system. This promotes the elimination of water vapour through its internal channels, and absorbs any transverse movements that may occur, ensuring the integrity of the tiled floors. The above is a brief summary of the problems that can arise and ways to prevent them, however if you’re unsure of how to apply this to your situation or need more advice, don’t hesitate to get in contact.
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