Timber floor structures above a concrete slab are considered a high-risk structure
A timber floor built over a concrete slab was a common construction method, especially from the 1940s to the 1980s. Since then, the structure has been classified as a high-risk structure in the KH 90-00394 guideline, or the guideline for condition inspections in connection with housing transactions (Kuntotarkastus asuntokaupan yhteydessä; suoritusohje; 2007).
A timber floor on top of a concrete slab refers to a concrete structure cast in situ on the ground with a timber-framed floor structure on top of it. Sometimes you may also encounter such a structure in the basement of a detached house build after the Second World War for the families of soldiers who fought at the battlefront (rintamamiestalo in Finnish), where the intermediate floor structure is a concrete slab and there is a timber floor on top of it.
Existence of the high-risk structure can be verified from the construction drawings or by opening the structures. Tapping the floor can also give the resident an indication of the type of floor structure.
The condition of a high-risk structure must be determined at the latest when the house is being sold
In accordance with the guideline for condition inspections in connection with housing transactions, the condition of a timber floor structure on top of a concrete slab must be determined by opening the structure. A simple superficial and sensory inspection or using a moisture sensor to study the surface are not sufficient, because in such a structure, the damage starts at the interface between the thermal insulation of the floor and the concrete slab underneath.
What kind of problems can the structure cause?
The risks posed by the structure can be divided into two types:
- If a concrete slab is in contact with the soil, moisture can rise from the soil by means of capillary action to the concrete slab and the structures on top of it. When moisture rises from the soil, thermal insulation and timber structures can be damaged.
- Another damage risk is caused by the condensation of the moisture in the indoor air, i.e. water vapour from the indoor air can condense at the interface between the concrete slab and the thermal insulation. Moist air can also enter the base floor structures from a crawlspace or an unheated basement under the concrete slab through improperly sealed spots.
Pipes are usually hidden between the concrete slab and the floor. This means that it can take a long time before a leak is detected. In such a case, the prolonged leak may have caused extensive damage to the structures.
Moisture build-up is usually caused by an inadequate underdrainage system or the lack of underdrainage and excessively fine-grained artificial fill having been used below the concrete slab. The lack of dampproofing between the timber structure and the aggregate structure contributes to the rise of moisture from the soil to the structures.
How should you go about repairing the structure?
It is best to leave the surveying of the condition of the high-risk structure to professionals. Determining the type of structure, examining its condition and identifying any associated risks requires the opening of the structures and inspection measures appropriate to the site.
A condition inspection is a good way to start studying the condition of a timber floor structure on top of a concrete slab. Based on the inspection, the expert will be able to recommend appropriate condition surveys for the structure, and the surveys will verify the condition of the structure.
The surveys will help in selecting the right repair methods for the structure and determining the extent of the repairs.
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.