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Most modern homes today that feature well-detailed and well-constructed footings and concrete slabs rarely experience issues with slab edge dampness. In cases where this problem occurs, it's usually a result of one or more causes. Performing a thorough investigation will help determine the course of action to help you fix the issue.

Many conditions cause slab edges to be occasionally damp, including garden watering, contact with the ground, and rain. Sometimes the dampness comes from the exterior part of the building, affecting the internal structure and other finishes like floor coverings in the home.

It's essential to know the preventive measures to take because repairing the damage caused by slab edge dampness can be expensive. This article will take you through all the critical things about slab edge dampness. Continue reading for more information.

Indications of Slab Edge Dampness

When the problem occurs, persistent dampness of the exposed surface is usually the first indication. This usually results in efflorescence just below the damp-proof course. There are also other indications to look out for, and they include the following:

  • The delamination render below the damp-proof course (DPC)
  • Pungent odours in floor coverings
  • External paint peeling or blistering below the DPC
  • Watermark stains on wall linings or joinery
  • Loss of bond of the floor tiles and adjacent wall
  • Rusting, oxidisation or surface corrosion close to the footing or slab

Why Slab Edge Dampness Occurs

As mentioned earlier, slab edge dampness is a result of many things. Among these include moisture absorption by capillary suction through air voids or pores within the concrete structure.

Generally, when the concrete experiences intermittent exposure to moisture, the surface area is the only part affected. When the concrete is of poor quality and is exposed to moisture for a lengthy period, moisture ingress and slab edge dampness will occur.

However, in most cases, the problem doesn't become apparent immediately, primarily because of abnormal factors like dry weather conditions. Some issues are caused by stormwater alterations or post-construction landscaping that can affect drainage conditions.

This includes the installation of paving or garden beds above the DPC, bottom cavity or flashings within the external wall. Four factors can cause moisture ingress and slab edge dampness. These factors include the following:

  • Concrete footing/slab is poorly constructed, which can cause it to be permeable to water.
  • The concrete footing/slab is in contact with the soil, which is also a source of moisture.
  • Poor detailing and the installation of moisture barrier systems around the external walls and slab edge.
  • Stormwater discharge or inadequate site drainage allows water to pond against the footing or slab.

Moisture Ingress

The Building Code of Australia has specified different requirements to be followed during construction. Among these is that buildings should be constructed to offer resistance to moisture from various sources. They also specify additional requirements for subsoil drainage and stormwater.

However, many other factors can affect how a building meets this safety requirement. For instance, in the case of a single-leaf walling system, moisture will need to cover longer before it can affect the internal finishes and walling materials.

Many conditions can cause moisture ingress, and to help you understand better, here's an overview of some of these conditions. Avoiding these conditions to ensure that your property meets BCA building requirements is essential.

Moisture ingress below the flashing/DPC

Moisture from this source can migrate through the concrete, and this can be due to many reasons, including the following:

  • Soil, like clay, draws up moisture and retains it
  • Poor site drainage in areas with insufficient fall away or regions that are adjacent to the building
  • Plant roots that block the stormwater outlets
  • Excessive watering adjacent to the footing and slab
  • Rapid changes in the flow of water due to gardening and landscaping
  • Plumbing services or leaking downpipes

Moisture ingress above the flashing/DPC

Moisture from this source that is not discharged from the cavity occurs for many reasons, including the following:

  • Plumbing services or leaking downpipes
  • Brick ties that are poorly installed can easily direct water inwards.
  • Flashings and roofing that are poorly installed can enhance the entry of moisture
  • Excess mortar arising from the masonry construction
  • Absence of adequate flashing
  • Inadequately installed, damaged, or inappropriate DPC

Construction issues

Issues that occur during and after construction can also cause moisture ingress. Effective planning and sound construction practices are always recommended to avoid moisture ingress. Here are some key recommendations to follow:

  • Ensure proper concrete curing during the construction process. Doing this will help reduce the permeation level that occurs on surfaces, reduce the risk of cracking, and prevent the direct entry of moisture.
  • Install adequate drainage, especially at the base of the cut.
  • Ensure that all the finished slabs comply with building regulations within your region. 
  • Proper compaction of concrete is also an essential thing you should ensure. Doing this will reduce the air voids within the concrete and further increase the permeability of the concrete.
  • Ensure that excess water is not added to the concrete when placed. This can only lead to less durable and more porous concrete.
  • Ensure that the steps within the slabs are adequately drained and have a damp-proof membrane.
  • Ensure the proper installation of a damp-proof membrane. It's also essential to ensure that it is correctly protected and lapped.
  • Ensure that the slope of the plumbing trenches is away from the house to effectively direct moisture away from the property and ensure that it's not directly to the footings.
  • Ensure that the subsurface drains can be inspected easily and they are free draining.
  • Use a well-draining fill to ensure that the rise of moisture through concrete slab is prevented.
  • Verify that the face of the footing or slab is smooth and does not retain water.

Post-construction issues

Apart from construction issues, there are also post-construction issues that you should note. The provision of components like turf, patios, driveways, paths, and other landscaping features should be made to ensure no moisture ingress. Here are some key measures to help you avoid the buildup of moisture against the footing:

  • Mortar fill cavities to ground level, especially when it is above the edge of the footing or slab. This is to ensure that moisture can quickly drain out of the cavity.
  • Avoid raising the adjacent ground level above the flashing or DPC.
  • Ensure that all the surfaces have adequate falls to direct water runoff easily.

Recommendations for Slab Edge Dampness

Every aspect of planning, post-construction landscaping, and construction is essential and should be considered to reduce the risk of moisture ingress and slab edge dampness. Here are some recommendations for you:

  • Consider performing thorough inspections of the property. This should include pest inspections, termite inspections, and overall building inspections. Doing this is a great way to quickly identify issues that can cause moisture ingress or slab edge dampness.
  • Ensure a proper fall away from the footing and slab for all the paved areas on the property.
  • Make provisions for installing sufficient stormwater and downpipes to minimise overflow of the gutters adjacent to the walls.
  • Avoid adding water to the concrete because this can increase its permeability and make it less durable.
  • Ensure all the concrete elements are adequately compact to ensure minimal air voids.

In addition to everything above, it's essential to consult a professional to handle slab edge dampness and other similar issues on your property.

Consult a Building Inspector for Slab Edge Dampness

Using a building inspector to find slab edge dampness is ideal when considering the purchase of a new home because dampness in the slab edge can cause significant structural damage and lead to costly repairs. A building inspector has the expertise and equipment to identify potential issues with the home's foundation, including dampness in the slab edge.

This information can be crucial in negotiating the purchase price or deciding whether to proceed with the purchase at all. By utilising building inspection services, potential homebuyers can make informed decisions and avoid unexpected expenses in the future.

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