Hello everyone,
I am currently excavating about 45 sqm (484 sq ft) in my old farmhouse where there is no concrete slab (all other rooms already have a slab). I have noticed that one interior wall (non-load-bearing) is resting directly on clay soil. I had planned to support this wall section by section with a strip footing.
Overall, I want to excavate 60 cm (24 inches) below the finished floor level and construct the slab as follows:
1. 15 cm (6 inches) recycled concrete gravel
2. PE foil (polyethylene vapor barrier)
3. 8 cm (3 inches) XPS insulation
4. 15 cm (6 inches) concrete slab, mixed on site with a 1:4 ratio
5. Full-surface coating and torch-applied waterproof membrane over the entire slab, with a 10 cm (4 inches) overlap extending up the masonry
6. 20 cm (8 inches) space for installation zone, insulation, underfloor heating, and screed
There is a horizontal damp-proof course installed in both the exterior and interior walls. The plaster above the damp-proof course is completely dry. My slab will be about 15-20 cm (6-8 inches) below the damp-proof course—could this cause any issues?
How should I generally handle the transition between the exterior wall and the slab? Should I seal it with a waterproof slurry and a thin layer of XPS insulation, or can the concrete be placed directly against the exterior wall?
What are your thoughts on the overall construction method? Currently, there is only soil beneath the screed, and we have never had any moisture problems.
I’m open to tips and suggestions for improvements 🙂
I am currently excavating about 45 sqm (484 sq ft) in my old farmhouse where there is no concrete slab (all other rooms already have a slab). I have noticed that one interior wall (non-load-bearing) is resting directly on clay soil. I had planned to support this wall section by section with a strip footing.
Overall, I want to excavate 60 cm (24 inches) below the finished floor level and construct the slab as follows:
1. 15 cm (6 inches) recycled concrete gravel
2. PE foil (polyethylene vapor barrier)
3. 8 cm (3 inches) XPS insulation
4. 15 cm (6 inches) concrete slab, mixed on site with a 1:4 ratio
5. Full-surface coating and torch-applied waterproof membrane over the entire slab, with a 10 cm (4 inches) overlap extending up the masonry
6. 20 cm (8 inches) space for installation zone, insulation, underfloor heating, and screed
There is a horizontal damp-proof course installed in both the exterior and interior walls. The plaster above the damp-proof course is completely dry. My slab will be about 15-20 cm (6-8 inches) below the damp-proof course—could this cause any issues?
How should I generally handle the transition between the exterior wall and the slab? Should I seal it with a waterproof slurry and a thin layer of XPS insulation, or can the concrete be placed directly against the exterior wall?
What are your thoughts on the overall construction method? Currently, there is only soil beneath the screed, and we have never had any moisture problems.
I’m open to tips and suggestions for improvements 🙂
You are already approaching the sectional underpinning correctly. Be sure to ensure that there is an interlock between the individually poured concrete sections so that the new foundation remains structurally and mechanically connected. If in doubt, this can be easily achieved with simple notches in the existing soil, especially in clay – this helps to better link the entire system later and reduces the risk of settlement.
Regarding water management: Moisture redistribution in the subsoil in existing buildings can usually only be roughly estimated. One possible approach is to carefully document the layering, any existing old plaster or discolorations in the soil, as well as the capillarity of the surrounding clay. Beyond this, monitoring over several months through the creation of small test pits and temporary moisture sensors is often the best you can do on your own.
Pay close attention to penetrations, for example around installations, and do not forget the wall junctions in detail – are the planned risers accessible from outside, or are you working exclusively from the interior?
Good luck!
Regarding water management: Moisture redistribution in the subsoil in existing buildings can usually only be roughly estimated. One possible approach is to carefully document the layering, any existing old plaster or discolorations in the soil, as well as the capillarity of the surrounding clay. Beyond this, monitoring over several months through the creation of small test pits and temporary moisture sensors is often the best you can do on your own.
Pay close attention to penetrations, for example around installations, and do not forget the wall junctions in detail – are the planned risers accessible from outside, or are you working exclusively from the interior?
Good luck!
Oh wow, I will never manage the water drainage. I’ll have to come up with another solution or get a professional company involved. A complete renovation of an old farmhouse is definitely not easy.
I’m only working from the inside. Later, I will install a strip foundation on the outside to create space for insulation and a new brick veneer wall (double-layer masonry) on the affected wall.
I’m only working from the inside. Later, I will install a strip foundation on the outside to create space for insulation and a new brick veneer wall (double-layer masonry) on the affected wall.
There is also an interesting YouTube channel by someone who has been working on his construction site for years. He reinforced the timber-framed house in sections, removed soil, refilled it, and then poured several concrete foundations. You can take a look; maybe you’ll find something helpful.
For me, this building project is comparable to the construction of the pyramids—the guy has impressive perseverance.
For me, this building project is comparable to the construction of the pyramids—the guy has impressive perseverance.