ᐅ Sloping site, basement exposed at the front, waterproof concrete shell (tank) system
Created on: 18 Apr 2016 11:58
T
tabtab
Hello,
We are currently building (the permit is in progress) and have had a soil survey conducted. The house is located on a slight slope. The basement is planned as a living space at the front, while at the back it is embedded in the ground. The building is just slightly (about 50cm - 1m (20 - 40 inches)) above the backwater level. According to our general contractor (GC), a backwater valve is therefore not necessary.
Now to the main issue, the basement. This is causing me quite some concern. We originally insisted on a waterproofed concrete structure ("white tank"), but our consultant strongly advised against it, saying we don’t need it because water can be drained away through a drainage system connected to the sewer. Furthermore, half of our basement will be exposed to open air, so water cannot accumulate there. The drainage will be installed with a flush shaft and geotextile fabric. The basement will be cast in concrete (seismic zone 3) and will have perimeter insulation. We could save the costs of a waterproofed concrete structure or the more expensive Thepro basement system (just to explain, it’s a waterproof concrete system with perimeter insulation and protection against groundwater ingress).
Basement construction according to the scope of work description: Bituminous thick coating with a minimum dry thickness of 3mm (0.1 inches) on the concrete walls, extended above the ceiling edge. Exterior basement insulation made of perimeter insulation boards with tongue and groove profiles (WLG035). Boards are glued, and those above ground level are additionally secured with plugs.
Question 1: Is this really sufficient since the basement is exposed at the front? I keep reading online about durability of around 30 years, after which renovation would be necessary.
Question 2: The consultant told us that a waterproofed concrete structure ("white tank") would be pointless because the side windows and the concrete light wells would be problematic... somehow due to water running behind these windows, etc. Honestly, I forgot the exact reasoning.
The soil survey states regarding waterproofing recommendation:
Temporary inflow of groundwater layers is to be expected. The encountered subsoil is rated as slightly permeable, so especially during rainfall, rising seepage water is to be anticipated. After establishing the final foundation depth, it is recommended to check whether a drainage system laid under the slab can be connected to the drainage system. If this is possible and the outflow can be permanently ensured, waterproofing according to DIN 18195 Part 4 against rising humidity (capillary/adhesive water) is sufficient. In this case, it is recommended to lay a filter-stable geotextile (minimum GRK3) below the drainage layer. Otherwise, waterproofing against rising seepage water according to DIN 18195 Part 6 "Waterproofing against external water pressure and rising seepage water" up to the ground surface is required.
This is exactly what worries me. I should add that the soil is very clayey, and water tends to accumulate on the plots after rain.
My GC refuses to build a white tank; instead, he recommends the so-called Thepro basement. It is supposed to be better and resistant to backflow, which would be a major advantage compared to the white tank.
Should I now spend the extra cost (about 8,000€) or can I confidently forego it and assume the planned waterproofing will last more than 30 years?
Thank you in advance for your opinions!!
P.S. I am asking here because neither the soil surveyor nor the GC give me a clear recommendation. The GC says it’s fine, the soil surveyor says it depends... well... I would like to be sure if it’s okay or not!!
We are currently building (the permit is in progress) and have had a soil survey conducted. The house is located on a slight slope. The basement is planned as a living space at the front, while at the back it is embedded in the ground. The building is just slightly (about 50cm - 1m (20 - 40 inches)) above the backwater level. According to our general contractor (GC), a backwater valve is therefore not necessary.
Now to the main issue, the basement. This is causing me quite some concern. We originally insisted on a waterproofed concrete structure ("white tank"), but our consultant strongly advised against it, saying we don’t need it because water can be drained away through a drainage system connected to the sewer. Furthermore, half of our basement will be exposed to open air, so water cannot accumulate there. The drainage will be installed with a flush shaft and geotextile fabric. The basement will be cast in concrete (seismic zone 3) and will have perimeter insulation. We could save the costs of a waterproofed concrete structure or the more expensive Thepro basement system (just to explain, it’s a waterproof concrete system with perimeter insulation and protection against groundwater ingress).
Basement construction according to the scope of work description: Bituminous thick coating with a minimum dry thickness of 3mm (0.1 inches) on the concrete walls, extended above the ceiling edge. Exterior basement insulation made of perimeter insulation boards with tongue and groove profiles (WLG035). Boards are glued, and those above ground level are additionally secured with plugs.
Question 1: Is this really sufficient since the basement is exposed at the front? I keep reading online about durability of around 30 years, after which renovation would be necessary.
Question 2: The consultant told us that a waterproofed concrete structure ("white tank") would be pointless because the side windows and the concrete light wells would be problematic... somehow due to water running behind these windows, etc. Honestly, I forgot the exact reasoning.
The soil survey states regarding waterproofing recommendation:
Temporary inflow of groundwater layers is to be expected. The encountered subsoil is rated as slightly permeable, so especially during rainfall, rising seepage water is to be anticipated. After establishing the final foundation depth, it is recommended to check whether a drainage system laid under the slab can be connected to the drainage system. If this is possible and the outflow can be permanently ensured, waterproofing according to DIN 18195 Part 4 against rising humidity (capillary/adhesive water) is sufficient. In this case, it is recommended to lay a filter-stable geotextile (minimum GRK3) below the drainage layer. Otherwise, waterproofing against rising seepage water according to DIN 18195 Part 6 "Waterproofing against external water pressure and rising seepage water" up to the ground surface is required.
This is exactly what worries me. I should add that the soil is very clayey, and water tends to accumulate on the plots after rain.
My GC refuses to build a white tank; instead, he recommends the so-called Thepro basement. It is supposed to be better and resistant to backflow, which would be a major advantage compared to the white tank.
Should I now spend the extra cost (about 8,000€) or can I confidently forego it and assume the planned waterproofing will last more than 30 years?
Thank you in advance for your opinions!!
P.S. I am asking here because neither the soil surveyor nor the GC give me a clear recommendation. The GC says it’s fine, the soil surveyor says it depends... well... I would like to be sure if it’s okay or not!!
S
Sebastian7918 Apr 2016 12:08A drainage system requires ongoing maintenance (carried out by you) and can also fail (usually when you need it most).
You need to be aware of this – a true waterproof concrete tank ("white tank"), according to reports online, is not easy to construct and requires careful monitoring. If your general contractor (GC) is resisting this so strongly, it would raise red flags for me – either you trust the GC or you find another one who is willing to meet your requirements.
You need to be aware of this – a true waterproof concrete tank ("white tank"), according to reports online, is not easy to construct and requires careful monitoring. If your general contractor (GC) is resisting this so strongly, it would raise red flags for me – either you trust the GC or you find another one who is willing to meet your requirements.
I can’t look for another general contractor because we have a contract with this one. The only alternative would be to go with a Thepro basement. This general contractor is very well established here and has an excellent reputation. That’s why I think they are not refusing to install the waterproof concrete basement (“white tank” or “watertight basement”) because they are unfamiliar with it, but rather because they genuinely believe we don’t need it. For other customers whose expert reports were less favorable, they installed a Thepro basement. Now I’m wondering: can I trust that it will be sufficient, or should I opt for the more expensive solution? I just don’t want to spend an unnecessary 8,000€ but also don’t want to end up with a flooded basement later on...
B
Bauexperte18 Apr 2016 12:34Hello,
The execution your building surveyor intends to carry out is described exactly here. What exactly is your problem?
If your general contractor follows the geologist’s recommendation AND connecting the drainage to the drainage system is possible, no additional sealing or a so-called “white tank” (fully waterproof concrete structure) is necessary.
Best regards, Bauexperte
tabtab schrieb:
The soil report states the following recommendation for waterproofing:
Influx of groundwater, at least temporarily, is to be expected. The encountered subsoil is classified as poorly permeable, so it is especially likely that rising seepage water will occur during rainfall. We recommend, after determining the final foundation depth, to check whether a connection of a drainage system installed beneath the slab to the drainage network is possible. If this is feasible and the drainage flow can be permanently ensured, waterproofing according to DIN 18195 Part 4 against rising moisture (capillary water / adhering water) is sufficient. In this case, we recommend placing a filter-stable geotextile (minimum GRK3) beneath the drainage layer. Otherwise, waterproofing against rising seepage water in accordance with DIN 18195 Part 6 “Waterproofing against external water pressure and rising seepage water” up to ground level is required.
The execution your building surveyor intends to carry out is described exactly here. What exactly is your problem?
If your general contractor follows the geologist’s recommendation AND connecting the drainage to the drainage system is possible, no additional sealing or a so-called “white tank” (fully waterproof concrete structure) is necessary.
Best regards, Bauexperte
S
Sebastian7918 Apr 2016 13:15As mentioned, it is important to be aware that a drainage system requires maintenance; otherwise, it will stop working properly – this is something many people are not made aware of.
How much maintenance is required? You just need to flush it occasionally, right?
@Bauexperte: Connection to the drainage system is possible and permitted. However, I am concerned that the drainage might not function properly over time and that the drainage system could have problems during heavy storms. We are experiencing more and more extreme heavy rainfall and, as a result, huge volumes of water. Looking ahead, I wonder if this is still considered state of the art today or if it might become a problem in a few years.
@Bauexperte: Connection to the drainage system is possible and permitted. However, I am concerned that the drainage might not function properly over time and that the drainage system could have problems during heavy storms. We are experiencing more and more extreme heavy rainfall and, as a result, huge volumes of water. Looking ahead, I wonder if this is still considered state of the art today or if it might become a problem in a few years.
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