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 🙂
Hi manu,
your planned floor construction is basically solid, but the connection between the slab and the wall waterproofing is crucial. If the top edge of the slab is clearly below the damp-proof course, that is not a problem as long as you apply the waterproofing—such as a bituminous membrane—consistently and overlapping well above the damp-proof course.
The concrete can generally be placed directly against the exterior wall, but it is better to include a separating strip of XPS insulation (depending on space, about 2–3 cm (1–1¼ inches)) as edge insulation and seal it properly. This prevents thermal bridging and minimizes the risk of moisture being drawn capillarily into the wall. I would also recommend using a slurry waterproofing on the wall in the transition area—it creates a clean bond with the floor waterproofing.
Have you checked how this was handled with the existing slabs or how old the damp-proof course is? This could help determine the best execution now.
Good luck!
your planned floor construction is basically solid, but the connection between the slab and the wall waterproofing is crucial. If the top edge of the slab is clearly below the damp-proof course, that is not a problem as long as you apply the waterproofing—such as a bituminous membrane—consistently and overlapping well above the damp-proof course.
The concrete can generally be placed directly against the exterior wall, but it is better to include a separating strip of XPS insulation (depending on space, about 2–3 cm (1–1¼ inches)) as edge insulation and seal it properly. This prevents thermal bridging and minimizes the risk of moisture being drawn capillarily into the wall. I would also recommend using a slurry waterproofing on the wall in the transition area—it creates a clean bond with the floor waterproofing.
Have you checked how this was handled with the existing slabs or how old the damp-proof course is? This could help determine the best execution now.
Good luck!
Hi Nauer,
The horizontal damp proof courses are definitely present, including in the walls of the existing concrete slab. But as I already mentioned, there is no moisture in the plaster and the horizontal damp proof courses are clearly visible because I have already removed the screed completely. The walls appear to have been fully built on the existing concrete slab.
The horizontal damp proof courses are definitely present, including in the walls of the existing concrete slab. But as I already mentioned, there is no moisture in the plaster and the horizontal damp proof courses are clearly visible because I have already removed the screed completely. The walls appear to have been fully built on the existing concrete slab.
Hi,
your approach is generally understandable, but I would recommend paying close attention to possible sources of moisture beneath the slab. Even if no moisture has appeared so far, with older masonry you should not underestimate the effect of a large-scale floor opening combined with modern waterproofing. Especially when adding a waterproof concrete slab that resists hydrostatic pressure, the moisture behavior in the ground could change—for example, side water pressure pushing against previously open areas of the masonry.
Also, consider the chemical compatibility between the waterproofing membrane, XPS insulation, slurry waterproofing, and any existing older materials to avoid long-term interactions or delamination. Speaking of waterproofing membranes—are you planning a bitumen membrane according to DIN 18533 or a traditional building waterproofing system? The details in corners and at wall upstands are critical to prevent leaks in those areas.
Another question: how do you ensure that underpinning the existing walls (strip foundation) will not cause settlements or damage to the structure? Will sections be underpinned alternately, and what is the length of each individual step?
Good luck!
your approach is generally understandable, but I would recommend paying close attention to possible sources of moisture beneath the slab. Even if no moisture has appeared so far, with older masonry you should not underestimate the effect of a large-scale floor opening combined with modern waterproofing. Especially when adding a waterproof concrete slab that resists hydrostatic pressure, the moisture behavior in the ground could change—for example, side water pressure pushing against previously open areas of the masonry.
Also, consider the chemical compatibility between the waterproofing membrane, XPS insulation, slurry waterproofing, and any existing older materials to avoid long-term interactions or delamination. Speaking of waterproofing membranes—are you planning a bitumen membrane according to DIN 18533 or a traditional building waterproofing system? The details in corners and at wall upstands are critical to prevent leaks in those areas.
Another question: how do you ensure that underpinning the existing walls (strip foundation) will not cause settlements or damage to the structure? Will sections be underpinned alternately, and what is the length of each individual step?
Good luck!
Thank you for the quick response:
I would remove the soil in sections, for example in 50 cm (20 inches) pieces, install formwork, and pour concrete.
The point about the water pressure-resistant slab is correct, but it is also difficult to fully plan in advance where the water will flow. Do you have any ideas or approaches on how I can best check this?
I am planning to use a bitumen membrane.
I would remove the soil in sections, for example in 50 cm (20 inches) pieces, install formwork, and pour concrete.
The point about the water pressure-resistant slab is correct, but it is also difficult to fully plan in advance where the water will flow. Do you have any ideas or approaches on how I can best check this?
I am planning to use a bitumen membrane.
manu986 schrieb:
I am currently excavating about 45 square meters (484 square feet) at my old farmhouse I have already helped you twice (15:32 and 16:21), but somehow it keeps disappearing:
https://www.instagram.com/11antgmxde/
https://www.linkedin.com/company/bauen-jetzt/
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