Foundation Waterproofing – What Are the Most Common Mistakes?

A properly executed foundation waterproofing system ensures long-term protection.

It is best to invest in quality waterproofing at the construction stage. The cost of repairing a damaged or poorly applied waterproofing system is much higher than the cost of using high-quality materials and specialized labor. This is not an area where cutting costs is advisable.

By the time you notice damp walls, lifting parquet and tiles, or mold growth, it is often too late. In many cases, only the symptoms can be treated, not the root cause (since water can migrate from different areas of the structure).

It is difficult to predict future risks for a building’s foundation. Even the construction of nearby residential developments can disrupt groundwater levels, potentially leading to increased water pressure on the foundation.

Common Mistakes in Foundation Waterproofing

Buildings Without Basements

Mistakes:

• Foundation wall placed directly on the footing
  • Mistake: Not installing fillets (fascia strips) at the connection between the footing and the foundation wall.
  • Solution: Fillets must be applied both inside and outside.
• Poorly prepared substrate
  • Mistake: Not properly leveling the surface before applying waterproofing.
• Sharp edges not rounded
  • Mistake: Leaving sharp corners and junctions at wall-to-wall or footing-to-wall connections.

Recommendations & Tips:

Fillets inside the foundation should be applied at footing-to-wall connections and wall-to-wall joints to ensure seamless vertical surfaces.

Use waterproofing slurries instead of roofing felt (bituminous membranes).

• Moisture barriers should be made with mineral-based sealing slurries, as bituminous membranes cannot create a seamless barrier.

Apply external waterproofing only after the structural walls are completed.

• The waterproofing must extend at least 30 cm up the external walls (Sanier recommends 50 cm).
• If the waterproofing is applied too low, an additional perimeter waterproofing band must be installed to connect the foundation waterproofing with the wall waterproofing.

Buildings with Basements

Mistakes:

• Poorly prepared substrate
  • Mistake: Not properly leveling the surface before applying waterproofing.
• Sharp edges not rounded
  • Mistake: Leaving sharp corners and junctions at wall-to-wall or footing-to-wall connections.

Recommendations & Tips:

Install waterproofing between the lean concrete layer and the base slab.

• Avoid using roofing felt (bituminous membranes) or construction foil for waterproofing!
• If leaks occur, water can spread underneath the felt or foil, causing long-term damage.
• Construction foil is not a waterproofing material and should never be used for this purpose.

If the basement extends under the entire building, only external waterproofing (perimeter waterproofing) is necessary.

• Mistake: Applying internal vertical waterproofing on basement walls—this is unnecessary when a full external waterproofing system is in place.

Need More Information?

For detailed guidance on waterproofing processes, check out our waterproofing guide:

Part 1 – Click Here
Part 2 – Click Here

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