Rising damp and salt attack: effects and solutions

Image showing exterior of heritage building impacted by rising damp

Understand the effects of rising damp and salt attack, know how to spot the signs of damage and learn what’s required in a remediation plan.

Article by Mark Milevski

Rising damp is a serious matter that can cause significant damage to a building’s structure and lead to health issues due to mould growth. To minimise these deleterious effects, signs of rising damp – such as deteriorating stone or render, flaking paint and musty smells – should be investigated promptly.

Rising damp involving soluble salts can lead to salt crystal growth resulting in a mode of decay commonly known as salt damp. This article explores the process and effects of rising damp and salt attack, signs to look out for in your structure, and how testing and analysis by Stone Initiatives can help to determine the severity of the problem.

What is rising damp?

Rising damp is the process of moisture from the ground (groundwater or surface water) travelling upwards through the masonry by capillary action (often walls that have inadequate damp proofing). The water is effectively absorbed up through the porous masonry wall like water into a sponge, or oil into a wick. If left unaddressed, rising damp can weaken the walls and foundation of a building, which can lead to serious and costly damage. 

What are the effects of rising damp? Is it dangerous?

The presence of damp can cause materials to deteriorate, paint to flake, bubble and peel, plaster to soften, nails to rust, timber to rot, and mould and mildew to grow. When salts are also present (see ‘What is salt attack?’ below), masonry can decay, which may result in structural instability. Without proper remediation, the effects of rising damp and salt attack can be disastrous to a building, leading to expensive repairs. In addition to structural danger, mould and mildew in internal walls can lead to personal respiratory illness and other health hazards. 

What are the signs of rising damp?

Rising damp signs include:

  • Deteriorating stone, mortar and masonry finishes, especially close to ground level (generally up to hip height)
  • Peeling and flaking of paint and other decorative finishes (internal and/or external)
  • The formation of powdery white salts forming on the surface (efflorescence) 
  • Mould or musty smells, especially indoors
Stone Initiatives performed testing and analysis at the remand wing of Old Adelaide Gaol, damaged by salt damp. The investigation included a moisture survey to determine the source and extent of the moisture, testing to determine levels of salt present, and development of a remediation plan.

What causes rising damp?

Events that may lead to rising damp include poor drainage, overwatering, overspray, poor ventilation, landscaping and design deficiencies, poor maintenance, and the use of inappropriate coatings and membranes that prevent evaporation from the walls.

Other sources of moisture:

  • Penetrating damp: This occurs when water from outside the building penetrates through walls, roofs, or windows – this is also known as lateral damp. Examples include driving rain, sprinkler overspray and ocean spray (especially near surf coasts). 
  • Leaks: Leaks from pipes (e.g. underground irrigation or above ground downpipes), gutters, or other sources can contribute to dampness in buildings. Leaks from gutters, roofs or downpipes can contribute to damp; this is often referred to as falling damp.
  • Poor ventilation: Lack of proper ventilation can exacerbate the effect of rising damp, leading to an accumulation of moisture inside the building. This is often a precursor to mould growth.
  • Condensation: This occurs when warm, humid air comes into contact with a cold surface and condenses into water droplets.  This may appear similar to rising damp, but is really a separate and superficial issue.

What is salt attack?

“Salt attack” describes damage to dimension stone through salt crystal growth. It is a serious issue, and many iconic buildings and monuments, such as the Sistine Chapel and the Egyptian Pyramids, have decayed because of salt damage. Salt attack is largely thought to occur via two mechanisms: crystal growth (a build-up of crystallization pressure) and changing hydration states. The porosity and strength of the stone play a large part in controlling the extent of salt damage. You can learn more about salt attack in our article “Understanding Salt Attack in Dimension Stone.” 

What is salt damp?

Salt damp is a term that combines “salt attack” and “rising damp.” It is used to describe a type of damage to buildings, particularly in older structures, caused by the accumulation of salt, often in porous masonry walls, foundations or flooring structures that do not have adequate damp proofing. This can occur as a result of rising damp, where groundwater (or surface water) containing high levels of salt is drawn up into the walls, causing the salt to crystallize and damage the porous masonry. Effectively the processes of rising damp and salt attack are combined. 

Stone Initiatives materials testing specialist obtaining drilling samples from a heritage wall for salt content testing.

How does Stone Initiatives detect rising damp and salt attack?

  • Moisture survey: A portable moisture meter is used to survey the affected area, typically in a 300 x 300 mm or 500 x 500 mm grid. Moisture values are plotted onto a grid and a “heat map” is produced, showing the extent of damp and often epicentres and sources of moisture.
  • Salt content testing (TDS testing): Samples of poultice, drilling samples, soil or other masonry elements can be tested in the lab to determine the salt content. The salt content results are then used to aid in developing a desalination plan.
  • X-Ray Diffraction (XRD testing): XRD is a useful tool to determine the mineralogical composition of test specimens. It is often used to determine salt types present in efflorescence and to aid in mortar analysis.
  • Mortar analysis: Often salt damp may lead to decay of original mortar on heritage structures. Stone Initiatives can sample the original mortar and test it in the lab to determine its composition. A comprehensive report details mineralogy, sand sizing and description, and provides a mix ratio and “recipe” for recreating a close match to the original mortar. Read a fascinating case study about this here.
Example of a moisture survey presented as a heat map. The red indicates very high levels of moisture.
Stone Initiatives technician performing soluble salt testing.

How can you fix rising damp or salt damp?

It’s recommended to consult a professional builder or damp proofing specialist for an appropriate rising damp treatment. Stone Initiatives can help you determine the severity and extent of rising damp or salt damp, which can aid in developing a remediation plan. 

To fix rising damp or salt damp, the following treatments can be considered:

  • Remove the source of moisture: If there is a source of moisture, such as a leaky pipe, poor drainage, adjoining garden beds, sprinkler overspray etc. 
  • Install a damp-proof course (DPC): A damp-proof course is a barrier installed at the base of walls to prevent dampness rising from the ground. This is typically a physical barrier, but an injected chemical dampcourse may be successful in some locations. 
  • Re-plastering / re-rendering: If the plaster or render has been damaged by rising damp, it will need to be removed and replaced – a vapour permeable (breathable) coating should be used – lime based if possible. Non-permeable coating should not be used e.g. cement, acrylic paint etc. 
  • Desalination of the masonry: If salts have accumulated in the wall, they should be removed by applying a poultice or captive head washing.
  • Ventilation: Adequate ventilation must be provided to reduce moisture levels in the building.

In the repair of a building damaged by rising damp or salt attack, Stone Initiatives can perform testing that determines the durability of any replacement stone using the resistance to salt attack test. The test is useful in selecting a stone with the required durability but with a mode of decay that matches the existing stonework.

Sandstone samples that have undergone resistance to salt attack testing. Note the various forms of decay.

Maintenance and prevention tips

  • Ensure adequate drainage away from sensitive masonry structures
  • Ensure gutters and downpipes are clean and working effectively
  • Remove any vapour impermeable coatings (acrylic paint, cement render etc.) and replace with vapour permeable alternatives (mineral silicate paint, lime wash, lime mortar etc.)
  • Ensure pavement level is below the dampproof course (at least 100 mm, preferably up to 200 mm) 
  • Monitor the building for early signs of damp, some examples may include: discolouration / peeling of finishes, damp or musty smell, efflorescence, mould / mildew, rotting of timber / termites. 

It’s important to address rising damp as early as possible, as it can cause significant damage to the building structure and create health issues due to mould growth. A specialist such as Stone Initiatives should be engaged to determine the severity and extent of rising damp or salt damp, which can aid in developing a remediation plan with a professional builder or damp proofing specialist.