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!!
