Hello,
We are in the process of purchasing an older (1958) townhouse, built by Borohus in a terraced layout.
In the terraced floor, over the past 10 years (according to the sellers, an older couple), a significant bump has formed in the middle of the living room floor. The bump measures approximately 3m in length, 1m in width, and 70mm in height.
Neither IF nor the AntiCimex inspector had "ever seen anything like it," so we hired a builder to open up the bump during inspection. The herringbone parquet was removed, and the floor concrete, about 30mm thick, was chiselled away. Underneath, there was an air pocket, about 30-40mm above the structural concrete slab and the floor concrete. The air pocket was completely dry and without any odor.
What do you think could have caused this bump? One theory is that the walls, also made of concrete, have been pushed together/against each other slightly, creating tensions that have lifted the upper concrete layer.
Has anyone experienced similar "construction defects?"
Thanks in advance!
We are in the process of purchasing an older (1958) townhouse, built by Borohus in a terraced layout.
In the terraced floor, over the past 10 years (according to the sellers, an older couple), a significant bump has formed in the middle of the living room floor. The bump measures approximately 3m in length, 1m in width, and 70mm in height.
Neither IF nor the AntiCimex inspector had "ever seen anything like it," so we hired a builder to open up the bump during inspection. The herringbone parquet was removed, and the floor concrete, about 30mm thick, was chiselled away. Underneath, there was an air pocket, about 30-40mm above the structural concrete slab and the floor concrete. The air pocket was completely dry and without any odor.
What do you think could have caused this bump? One theory is that the walls, also made of concrete, have been pushed together/against each other slightly, creating tensions that have lifted the upper concrete layer.
Has anyone experienced similar "construction defects?"
Thanks in advance!
In my ears, it sounds like a moisture-related problem. If the basement floor has an uninsulated slab at the bottom, much can happen. The outer walls in the basement are likely made of concrete block, i.e., masonry. That they would have been pushed inward due to surrounding soil pressure sounds a bit far-fetched. Especially since probably even the intermediate floor is made of concrete. This should not be difficult to ascertain. However, I am quite skeptical of the competence of Anticimex inspectors.
Thank you Justus,
Exactly right, the exterior walls are made of concrete hollow blocks. The moisture meter has been used on two occasions without giving any indications.
Then we are also thinking about how we should redo the floor. We plan to remove the raised concrete and replace it with new concrete - i.e., either skim coating or casting a new floor about 30mm thick. Is it recommended to have some kind of moisture barrier between these two layers of concrete?
Grateful for any advice.
Exactly right, the exterior walls are made of concrete hollow blocks. The moisture meter has been used on two occasions without giving any indications.
Then we are also thinking about how we should redo the floor. We plan to remove the raised concrete and replace it with new concrete - i.e., either skim coating or casting a new floor about 30mm thick. Is it recommended to have some kind of moisture barrier between these two layers of concrete?
Grateful for any advice.
I believe you need to wait for access so that you can access all the drawings. What is crucial for how the problem should be addressed is what lies beneath the slab. If there is insulation under the slab, no moisture barriers are needed. If there is no insulation under the slab, any moisture barriers must be placed on top of other insulation; otherwise, the materials will wade in moisture. One alternative is ventilated floor constructions. However, until we know how it looks, it's all just speculation.
Thank you for the detailed answers, Justus!
We have already gained access to the original drawings from 1957 (available digitally at the city archive), and it states that the "Ground conditions" are "Rock, Pinnmo. Gravel, Clay" and that "Foundation method" is "Leveling of rock. Spread plates at frost-free depth. H=25cm, B adjusted according to the ground's bearing capacity. Reinforcement 3 ø 12. Concrete in mix. 1:3:3"
Is it usually specified in any way if there is special insulation also under the slab?
We have already gained access to the original drawings from 1957 (available digitally at the city archive), and it states that the "Ground conditions" are "Rock, Pinnmo. Gravel, Clay" and that "Foundation method" is "Leveling of rock. Spread plates at frost-free depth. H=25cm, B adjusted according to the ground's bearing capacity. Reinforcement 3 ø 12. Concrete in mix. 1:3:3"
Is it usually specified in any way if there is special insulation also under the slab?
Thank you, Thomas,
Very good info, then we shall not follow that path anymore.
However, we shall ensure that the drainage works exemplary in the future. Great tips!
Very good info, then we shall not follow that path anymore.
However, we shall ensure that the drainage works exemplary in the future. Great tips!
Well.thomas33 said:
My slab from 1958 is insulated, although it barely counts as it's about 5cm of polystyrene - rather a moisture barrier compared to today's insulation.
On the other hand, there's also underfloor heating in the house, so it was ahead of its time in general.
Widespread slabs mean concrete soles under all bearing walls, often cast directly onto unexcavated ground. In between, there are thin (5-7 cm) unreinforced concrete floors. Under these, there's a capillary-breaking layer of gravel. On top of the rough concrete, there's a leveling layer which often was a plaster layer. Concrete slabs in today's sense were not present at all.
There are two types of moisture problems that can occur in a foundation. One is moisture migration from the ground upwards, which is stopped with the gravel layer and drainage. The other is that warm indoor air condenses in the cold and uninsulated concrete. When this type of foundation construction is used in a basement, it does not create problems because the floors are often open and the indoor temperature is lower. However, in a split-level house where the "basement" is partially to be used as heated living space, problems arise if the condensation risks are not managed in some way, which was not unknown in the 1950s. There are indeed examples of insulation being used under the concrete floor precisely in such contexts.
In your case, there are several alternatives for solutions (in no particular order): 1) Break up the floors, add insulation, and pour again, 2) Place insulation on top of the rough concrete and then cast a thin concrete slab, 3) Place insulation on top of the rough concrete and then lay a floating wooden floor, and 4) Build a ventilated floor construction with a distance to the rough concrete.
There are two types of moisture problems that can occur in a foundation. One is moisture migration from the ground upwards, which is stopped with the gravel layer and drainage. The other is that warm indoor air condenses in the cold and uninsulated concrete. When this type of foundation construction is used in a basement, it does not create problems because the floors are often open and the indoor temperature is lower. However, in a split-level house where the "basement" is partially to be used as heated living space, problems arise if the condensation risks are not managed in some way, which was not unknown in the 1950s. There are indeed examples of insulation being used under the concrete floor precisely in such contexts.
In your case, there are several alternatives for solutions (in no particular order): 1) Break up the floors, add insulation, and pour again, 2) Place insulation on top of the rough concrete and then cast a thin concrete slab, 3) Place insulation on top of the rough concrete and then lay a floating wooden floor, and 4) Build a ventilated floor construction with a distance to the rough concrete.
5 cm of Styrofoam is sufficient to stop moisture migration from above. Moisture problems due to the lack of a capillary-breaking layer have not occurred in Sweden since the 1920s.
Many thanks for the thorough decision-making basis! The whole thing about condensation against the cold foundation concrete sounds very logical. That the floor (the putty layer) would react in this "bulging" way under large south-facing window sections suddenly seems justified. The ceiling height in this basement floor is lower (210), so I will probably try to find the "lowest" solution for moisture insulation measured in centimeters. We are consulting our builder for alternative solutions, including those we find on your excellent list, Justus. Warm thanks!
Hello Justus,
My local construction contact - a highly appreciated and experienced builder who has already helped us remove the bump - recommends a solution where we lay a ventilating Platon mat on the concrete foundation, followed by chipboard and then parquet on top.
Would this solution also work to keep "future moisture reactions" away?
The thickness of this solution would also be roughly the same as before, meaning thresholds, radiators, tiles in front of the fireplace, etc., would not need to be adjusted due to the new floor height.
Grateful for your thoughts on this,
My local construction contact - a highly appreciated and experienced builder who has already helped us remove the bump - recommends a solution where we lay a ventilating Platon mat on the concrete foundation, followed by chipboard and then parquet on top.
Would this solution also work to keep "future moisture reactions" away?
The thickness of this solution would also be roughly the same as before, meaning thresholds, radiators, tiles in front of the fireplace, etc., would not need to be adjusted due to the new floor height.
Grateful for your thoughts on this,
I think it should be able to work. You can't be completely sure without having tried it for an extended period on-site, but that's not realistic. An experienced builder you know is always a good entry point into a project.