Slab Edge Dampness – It’s not Salt Damp! (but it is still a significant problem)

Slab Edge Dampness

Disclaimer:  Please note this is general advice only.  The issue of Slab Edge Dampness is not Salt Damp and therefore we are not experts on this issue. We recommend involving a building inspector or engineer to assist in identifying and rectifying this problem.

Symptoms of Slab Edge Dampness

Are you experiencing any of the following?  

  • A persistently damp concrete slab
  • A damp line appearing on the surface of your concrete slab
  • White, furry, salts appearing on the edge of your concrete
  • Render on your foundation blistering and bubbling and falling off
  • Bad odours (due to dampness in floor coverings such as thick carpets)
  • Rusting and Corrosion of Metals around the slab edge.These symptoms could be an indication that your property has an issue with Slab Edge Dampness.  Slab Edge Dampness can also be referred to as Efflorescence or Slab Edge Wetness.   All of which are a problem that is becoming more prevalent in properties throughout SA.

(And despite the common confusion – it is not Salt Damp, and therefore we are unfortunately unable to assist or treat by the usual means of treating Salt Damp).

So what causes it?

Slab edge dampness can be a result of a variety of factors such as:

  • poor quality concrete,
  • incorrect slab construction and drying,
  • poor drainage,
  • inadequate and lack of a waterproof membrane,
  • weather events,
  • post-construction landscaping or paving.Most of these factors allow the slab edge to become affected by contact with water and the salt contained within it. It is this that can cause the concrete to erode.

It can be difficult to determine what has been the exact cause of the problem and that is why we always recommend engaging an expert to undertake a thorough investigation.

Why Salts?

Salts are damaging when in contact with masonry or concrete.   And water-soluble salts can be found in many sources, most commonly groundwater.   When this groundwater is in direct contact with foundations, slabs, or retaining walls the moisture and salts pass through the masonry at a rate dependent on their porosity.  As moisture passes through the slab and evaporates, salts are left behind and crystallise below or on the surface. This creates a build-up of white powdery fluff known as efflorescence.

It can take years for any evidence of efflorescence from groundwater to appear, or it can take very little time at all.

Another potential source is that salts can exist in the mixing water that was used in the mortar, render or concrete.   Or the concrete was exposed directly to the elements and moisture was allowed to absorb before construction.

Effect of Salts?

These salts if left unchecked will begin to affect the overall appearance of the masonry/concrete. Firstly the salts themselves look unsightly.

Secondly, the efflorescence will harden and will slowly begin to erode away the affected surface.    This will crumble, pit and potentially expose the aggregate.

What can I do about this?

Yes, we keep banging on about this, but we recommend, firstly engaging a licenced and experienced consultant to assist you with identifying the cause of your issue and then offering possible solutions to help rectify or prevent further damage.

Some of the options they may suggest include:

  • Increasing drainage around the perimeter of your slab.
  • Correct surface water flow when landscaping
  • Digging down and installing a waterproof barrier between the edge of the slab and the ground below.
  • Sealing the external perimeter of the slab.
  • Fixing any guttering, storm-water or down-pipe issues.Ultimately, you will need to remove any sources of excess water. And create a barrier between any groundwater and the edges of your concrete slab.

Need more information then follow this link to another data sheet from the CCCA.