A partition wall starting below the floor level is a sign of a high-risk structure
Partition walls starting below the floor level have been built from the 1950s to the present day. When a timber structure, in this case a load-bearing partition wall, starts below the floor level, the structure is at a higher risk of damage due to moisture.
Inadequate insulation and faulty underdrainage and rainwater drain systems often lead to water entering the structures in a damaging way.
Partition wall starting below the floor level is classified as a high-risk structure
A partition wall starting below the floor level has been classified as a high-risk structure in the KH 90-00394 guideline for condition inspections in connection with housing transactions (Kuntotarkastus asuntokaupan yhteydessä; suoritusohje; 2007), which includes instructions on how to carry out a condition inspection. When a partition wall starts below the floor level, it is susceptible to moisture from the soil.
This high-risk structure first appeared in the 1950s and 1960s, but has been steadily seen in condition inspections up to the present day. The high-risk structure is most prevalent in buildings built between the 1960s and 1980s.
Based on the condition inspections carried out by Raksystems, the structure was particularly popular in the 1970s, as it can be found in 42% of all the detached houses built in that decade.
“This structure is particularly interesting because it goes hand in hand with a couple of other high-risk structures. Quite often, timber studwork, i.e. sandwich structures, false plinths and partition walls that start below the floor level are found in the same building,” says Kim Malmivaara, Head of Inspections & Surveys at Raksystems.
Underdrainage and rainwater drain systems matter
Moisture and the consequent damage are the most common causes of risks. A partition wall starting below the floor level makes it easier for moisture from the soil to penetrate the structures. This is usually due to poorly functioning or non-existent weeping drains.
The estimated service life of an underdrainage system is approximately 40 years, but only if it is properly maintained. This is why the condition of weeping drains should be regularly checked. There may also be a defects in the inclination of the ground close to the walls, which can cause surface water and rainwater to flow towards the building, which causes a moisture damage risk.
“When the lowermost part of an external wall starts to move on top of the bottom concrete slab and the slab is subjected to moisture from the soil, for example, the moisture can easily rise by means of capillary action to the timber structures above, which will cause damage to the structures,” Malmivaara explains.
Risks caused by leaking pipes and moist soil
There are also other risk factors that can lead to moisture entering structures. There may be too fine-grained, i.e. capillary, artificial fill beneath the foundation of a partition wall.
This type of soil absorbs moisture more easily. The moisture is able to migrate from the fine artificial fill beneath the base floor to the lowermost parts of the partition wall.
In addition, dampproofing may be missing from between the sole plate of the partition wall and the aggregate structure beneath it, like the bitumen felt visible in this image. Partition wall structures can suffer from moisture damage caused by a pipe leak if it leads to moisture in the insulation space as the water drips downwards.
“If there are pipelines inside the structures, such as water and heating pipes, and they leak, the load-bearing partition walls will have their share of this moisture,” Malmivaara says.
Not all partition walls start from the top of the bottom concrete slab; normally partition walls start from the top of the top slab. Load-bearing partition walls often start deeper in the structure, which means that they are exposed to these risks.
The risks can be mitigated by, for example, using bitumen felt or felt strips and brushed bitumen beneath the lowermost part of the external wall to prevent moisture from entering the structure.
How can such damage be detected?
The term “high-risk structure” does not mean that the structure is automatically damaged. A high-risk structure is a structure of a certain type that has been identified as being highly susceptible to damage.
According to the instructions, the condition of a high-risk structure must be determined by opening the structure to get a sufficiently comprehensive view of its condition. Opening the structure is necessary to verify or exclude the defect.
The structure must be opened in such a manner that all the materials used, how the structure was built and their condition are revealed. This will also help in determining the location, extent and causes of any damage.
Furthermore, in many cases it is possible to determine the repair needs and repair methods at this stage. Damage to this structure type starts at the bottom of the sole plate of the partition wall, which is why the condition of the bottom of the sole plate must be verified.
A hole of at least 110 mm must be made in the high-risk structure with a hole saw, but only with the owner’s permission. The number and location of places in which the structures are opened must always be determined on a case-by-case basis.
If necessary, a condition survey may also include a variety of microbial studies. The need for microbial studies is always assessed on a case-by-case basis in connection with a condition survey.
What is a high-risk structure?
A high-risk structure refers to a type of structure which has been found in practice and based on structural surveys to be highly susceptible to damage. The structure usually complied with the building regulations and guidelines valid at the time when it was built, and the susceptibility to risk was not recognised until at a later point in time. As a result, the structure is no longer used.
Typically, damage to high-risk structures is caused by moisture entering the structure, either through the soil from the outside or from the indoor air in the form of water vapour. An example of a commonly observed high-risk structure is a false plinth.
The condition of the high-risk structure, i.e. whether the risk has been realised, should always be investigated. This is usually possible only by opening the structure.