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.  

Your Damp House Winter Checklist

Last winter in Adelaide it rained.  I mean it really rained.  The storms were quick and ferocious.   Now, our house has small leaks that we know about and thanks to lots of gum trees, gutters that overflow.  Its all pretty typical for an older house and a busy family with other priorities.  What do they say about builders that never get around to fixing their own home? Well, damp specialists never getting around to fixing their own damp house. 

But last winter something else happened that caused my husband and I to sit back and decide it was time to do something about it.     It was a particularly heavy rain event and we were having some friends over for dinner.  Their son came out of our toilet and complained that hot water had been falling on his head.   We both rushed to look and discovered that water was leaking from our roof through the ceiling light onto the floor.  We made the area safe at the time but decided that night it was time to waterproof our damp house.

 Luckily as damp specialists we knew where to start but we have prepared this checklist for you to check your home this winter and avoid any nasty surprises. 


  1. Clean out and check your gutters

gutter blocked with leavesNow that the autumn leaves have stopped falling it is time to check and clean out your gutters.   Empty them of leaves, mud and debris and hose clean where possible.

Cleaning your gutters means that rain can drain away easily and it won’t overflow as quickly.   This prevents water damage to your eaves, inside your roof or below on your pavers and foundations.

Check your gutters for rust spots, holes and breaks also and fix and/or replace if required.

  1. Check your down pipes

Whilst you are up on the ladder it is a great chance to check all your homes down pipes as well.    Are they connected properly?  Are there any holes or breaks?   Do they drain away to storm water at the base.

By ensuring that your down pipes are attached properly you prevent incidences of what is commonly called falling damp.  Continual moisture against your masonry from a leaking pipe can cause rising damp type symptoms such as salt stains, blistering paint, plaster and erosion of mortar.   In extreme cases you can also get mould.

Fix any issues if required – you may need to call a specialist for this.

  1. Check your roof

And finally, check your roof for any cracked, broken or missing roof tiles.   Ensure that the flashing is in place and that ridge tiles are secure and unbroken.

If you have any damp patches on your ceilings that could be a sign that you have a cracked tile, ridge or a leak somewhere in your roof.  It is worth at this point calling in a roof expert to thoroughly check over the area and determine where the leak is occurring.

  1. Check drainage around your home

Are there any areas that water pools up against your home?  Or do you find water seeping in during heavy rain?

If so now might be the time to improve the drainage around your home.  You may need to investigate installing a series of Ag drains to direct water away from the area of concern.

Do you have drains blocked with debris?    Clean them out to allow water to drain away easily.

  1. Rain Water Tank

Is your rain water tank securely attached?  Do you know where it overflows?   Does it flow down and away from your house?

Check all connections for your rain water tank.  Are they free of debris and in good working order and is the overflow working as it should.  If your tank is full it could be worth using some of that water now or draining some of the water in preparation for heavy rains.

  1. Prune your trees

It is worth pruning any branches that are dead or overhanging the house or power lines now.  Better to do it now than have heavy branches fall on your roof in the middle of a storm.

  1. Chimney

If you have a working chimney make sure it is cleaned out prior to winter. Every couple of years get it inspected by a professional to prevent safety issues and fire hazards.

These maintenance steps will obviously not protect you from a severe thunderstorm event or from flooding but it may help to prevent a damp house this year.      To help you take measures to protect your family and pets during severe storms then click on this link to the SES website for further information.