How can I identify a false plinth?
A false plinth was a typical and accepted structural solution in the 1970s and 1980s in terraced and detached houses. The aim was to achieve a tight and thermally sound solution for the joint between the floor, wall and plinth/foundation wall.
It has been observed in hindsight, however, that the false plinth was a risky solution that has been classified as a high-risk structure in the KH 90-00394 guideline for condition inspections in connection with housing transactions, which includes instructions on how to carry out a condition inspection.
The instructions state that such a high-risk structure must be opened to determine its condition. A mere sensory and visual assessment or studying the surfaces with a moisture sensor is not a sufficient method when studying the condition of a high-risk structure.
What is a false plinth?
In a false plinth, the timber frame of the exterior wall is usually close to or even below ground level. The problem with a false plinth is moisture damage to the timber structures due to the effects of moisture from the soil and condensation from the indoor air.
If moisture from the soil and condensation from the indoor air are not controlled, trouble may arise. So, you need to make sure that:
- The underdrainage is in working order.
- Waterproofing of the foundation wall is in working order.
- The ground around the building is properly inclined away from the walls.
- Rainwater is properly directed away from the sides of the house.
How common a high-risk structure is a false plinth?
A false plinth has been observed in a total of 16% of the sites inspected by Raksystems in connection with a housing transaction. As mentioned earlier, the statistics also show that this high-risk structure is most common in houses from the 1960s to the 1980s.
Some 40% of the Raksystems inspection sites with a false plinth date back to the 1970s, some 30% to the 1980s and some 15% to the 1960s.
Between 2016 and 2020, Raksystems inspectors found damage in approximately 20% of the sites with a false plinth, or one in every five buildings. Further surveys were recommended for 25% of the inspected sites, or one in every four false plinth.
How do I recognise a false plinth in my home?
A false plinth is usually very easy to detect from the outside of the building. If there is a false plinth, the lower edge of the front door extends about 20 to 40 centimetres below the top of the foundation wall.
In the image below, taken from the door of a house, you can see that the plinth on the outside is higher than the bottom edge of the front door.
A false plinth is also very clearly visible in structural drawings; see the illustration below. You can see how the lower part of the timber frame is below ground level. You can also see that if there are deficiencies in the weeping drains or, for example, the ground slopes towards the building, water will run down to the foot of the building and the structures can easily get wet.
Should a false plinth be investigated?
It makes sense to check the condition of a false plinth to avoid problems or possibly fix the problem before it becomes a health hazard for the residents. If problems with the indoor air have been detected – the air is stale, the residents are often ill or the home has an unpleasant odour reminiscent of an earth cellar – the structure should be inspected.
A condition inspection in connection with a housing transaction includes an inspection of a false plinth through a 110 mm hole made with a hole saw, subject to the homeowner’s permission. In the case of a housing company, the opening of a structure also requires the permission of the property owner.
The hole made in the false plinth is used to make visual and olfactory observations and survey the moisture content of the structures, for example. The structure will not be opened if it can already be established in advance by other means that it is damaged.
Opening the structure during a condition inspection is not always sufficient, as the hole made is small and visibility may therefore be limited. For this reason, we often recommend wider opening of the structure, meaning making a larger access hole in the wall.
How can the structure be repaired?
If the structure is damaged, repairs will be recommended. The best way to repair the damage is to remove the damaged structure and replace it with a new one. When repairing a false plinth, repair costs often arise from exterior repairs in addition to the interior work. The exterior repairs may include improvement of the rainwater drain system, forming of the ground and repairs/renovation of the weeping drains.
The repairing of a false plinth on the inside costs around €300–500 per metre.* The repairing of a false plinth on the outside costs around €200–500 per metre.*
* The repair costs are a rough estimate, and the price can end up being considerably lower or higher depending on factors such as the size of the site and the extent of the damage. The prices are estimates that are based on a turnkey contract in the Helsinki metropolitan region.
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.
