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What is Slab Edge Dampness?

We often find this defect at our pre-purchase inspections, but what is it?

Slab edge dampness refers to the presence of moisture or water infiltration along the edges of a concrete slab. This can occur when the slab is in contact with the ground or when there is a lack of proper moisture barrier or insulation.

Slab edge dampness can lead to a range of issues, including mold growth, deterioration of concrete, and damage to interior finishes. Treating slab edge dampness involves identifying the source of moisture and implementing appropriate remedial measures. Here, I'll explain slab edge dampness and provide some treatment methods.

Slab edge dampness typically occurs due to capillary action, where moisture is drawn upward through the concrete by the porous nature of the material. It can also result from poor drainage, high water table levels, improper grading, or inadequate waterproofing measures during construction. Additionally, condensation may contribute to slab edge dampness if there is a temperature differential between the interior and exterior of the building.

To treat slab edge dampness, the first step is to identify the source of moisture. This may involve a visual inspection of the affected area, moisture testing, or even employing specialized tools such as moisture meters or thermal imaging cameras to locate the exact point of water infiltration.

Once the source of moisture is determined, several treatment options can be considered:

1. Improve drainage

Ensure that the site has proper grading and slope away from the foundation to direct water away from the slab. Installing French drains or other drainage systems can also help prevent water buildup around the foundation.

2. Install a moisture barrier

If the slab is in direct contact with the ground, a moisture barrier should be installed between the slab and the soil. This can be a sheet of polyethylene or a specialized waterproofing membrane that prevents moisture from permeating the concrete.

3. Seal cracks and joints

Inspect the slab for any cracks or gaps and seal them using appropriate sealants or caulking materials. This will help prevent water from seeping through these openings.

4. Improve insulation

In colder climates, inadequate insulation can contribute to condensation and moisture buildup. Enhancing the insulation around the slab can help reduce temperature differentials and minimise the risk of condensation.

5. Install a vapour barrier

In some cases, it may be necessary to install a vapor barrier on the interior side of the slab. This barrier prevents moisture vapor from migrating through the concrete and reaching the interior space. It is typically installed under the finished flooring.

6. Improve ventilation

Proper ventilation is crucial to reduce humidity levels and mitigate condensation. Ensure that the building has adequate ventilation systems, such as exhaust fans or mechanical ventilation, to promote air circulation and moisture control.

7. Professional remediation

In severe cases of slab edge dampness or if the source of moisture is challenging to identify, it may be necessary to consult with a professional waterproofing contractor or structural engineer. They can provide specialized expertise and recommend appropriate remediation strategies tailored to your specific situation.

It's worth noting that the treatment methods for slab edge dampness may vary depending on the specific circumstances, climate, and construction details. Consulting with professionals who specialise in waterproofing and structural engineering is often the best approach to ensure effective treatment.

Slab edge dampness can be a significant issue that requires prompt attention to prevent further damage and ensure a healthy indoor environment. By engaging our expert inspectors to carry out your prepurchase inspection we will help locate the problem and in some cases identify the source of moisture, allowing you to implement appropriate treatment methods such as improving drainage, installing moisture barriers, sealing cracks, and improving insulation and ventilation.


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