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.  

Lateral Damp… Salt damp is not the only damp issue.

What is Lateral Damp?

Our homes are traditionally built to protect us from the elements and be watertight.   However damp issues are one of the most common maintenance complaints around a home.   Commonly we are able to advise our customers that their issue is not Salt damp but in fact Lateral damp.

Lateral damp or penetrating damp is the result of the passage of water through a wall in a horizontal direction.    In South Australia most properties are built with brick or stone.   This masonry is very porous in nature.   If these walls become exposed to excess water the masonry will absorb the water into the wall.  This process is similar to that of putting a sponge in a bowl of water.

Many times the water also carries soluble salts.  The water and salts eventually move through the wall and evaporate onto the surface side on the opposite side.    A moisture stain appears and salts are deposited on the surface.   Signs to look for include staining, blistering paint, drummy plaster, salty deposits and efflorescence.

Unlike rising damp, lateral damp can occur at any height on a wall and will occur even where a suitable damp proof course is in place.

There are many issues which can result in your home suffering from lateral damp damage. Before fixing the issue you will need to identify the source of the moisture.  Please note that the remedies below are suggestions only. We always recommend that you consult with a damp specialist for correct diagnosis first.

Some common examples we encounter include:

Garden Bed has been built directly against the house wall.   No issue outside but the customer notices that the paint is bubbling in the room on the inside.
This is common.  Many homes have garden beds placed against an outside wall.   This causes an issue as it covers (bridges) the existing damp proof course but it also allows the passage of moisture from the soil directly to the wall surface.  Constant watering of the plants in this garden bed only makes the issue worse.
REMEDY:   If possible remove the garden bed so that soil is not directly against your wall. 

Plaster has begun to bubble in hallway or bedroom.   Occurs quickly over a short space of time.   A wet area is on the other side of the wall.  
In this situation the problem is often due a leak from the wet area.  It can be as serious as a pipe leak, or tap washers dripping.  In other cases it can be due to minor cracks in the tiles and grout allowing water to pass through into the wall.

REMEDY: Get a plumber in to pressure test your pipes, check for leaking bridging pieces or perhaps additional waterproofing or sealant requirements for your bathroom.

Salt Attack, Efflorescence and moisture stain on retaining, backfilled or cellar walls.
This often occurs when the waterproofing membrane is either missing or has broken down behind the wall.  Without the membrane ground water and salts are able to pass easily into the porous masonry wall.
REMEDY: Ensure there is no moisture sources leaking into the backfill (subfloor draining issues, leaking downpipes etc.). The best solution in this instance is to remove the backfill and place a waterproof membrane between the backfill and the wall. However, in most cases this is not possible and the wall surface will need to be dealt with as a regular maintenance issue e.g. cleaning off salts, poulticing to remove salts and inside sacrificial plastering.

Sprinkler systems; excessive moisture penetration on one section of wall from constant water.
This causes an issue as large amounts of water constantly hit the same area of the building over long periods of time.    Especially during summer when we all water more often.

REMEDY: Whilst your system is running check the perimeter of each of the sprays and adjust if any are consistently spraying directly onto your walls.

Leaking downpipes, a leaking air-conditioning system, a leaking hot water system; again excessive moisture penetration on one section of wall from constant water.

REMEDY: Home maintenance to check the perimeter of your home and then consult with roofing, guttering or hot water service specialists. Fix the issue asap.   


Once the problem has been accurately diagnosed and rectified please allow a minimum of an additional 3 months before carrying out any repairs to the aesthetic damage.  For further information or assistance please contact us.