Hello dear forum users,
I have attached a soil pipe plan and have some questions about it. I hope to receive helpful answers.
We received the following plan from our builder. I have added some measurements for reference. The blue circle indicates the soil stack from the upper floor. The vent pipe is routed through the roof there.
From my research, the main drain (soil pipe) is quite long, and I am concerned that the kitchen (on the far right) might face drainage issues.
Due to the upper floor layout, there is no real possibility to install an additional vent pipe through the roof at the end of the branch.
How would experts assess this situation overall?
I have attached a soil pipe plan and have some questions about it. I hope to receive helpful answers.
We received the following plan from our builder. I have added some measurements for reference. The blue circle indicates the soil stack from the upper floor. The vent pipe is routed through the roof there.
From my research, the main drain (soil pipe) is quite long, and I am concerned that the kitchen (on the far right) might face drainage issues.
Due to the upper floor layout, there is no real possibility to install an additional vent pipe through the roof at the end of the branch.
How would experts assess this situation overall?
Thank you for the answers provided so far. It initially sounds like it’s not entirely unreasonable...
@ypg: The channel connection is located at the top left corner of the building, about 3 meters high (10 feet). The connection under the stairs is a backup in case the homeowner requires any wastewater-related installations on the left side of the kitchen or if a need arises in the storage room.
@Knöpfchen: I looked into that as well. The dishwasher drain could be equipped with a vent, or it’s also possible to have a valve installed on the inside, directly under the sink, right?
I don’t see a utility sink in the technical room, but there is an option to connect to the wastewater pipe for technical purposes.
Besides, a small sink is planned in the laundry room in front of that.
@ypg: The channel connection is located at the top left corner of the building, about 3 meters high (10 feet). The connection under the stairs is a backup in case the homeowner requires any wastewater-related installations on the left side of the kitchen or if a need arises in the storage room.
@Knöpfchen: I looked into that as well. The dishwasher drain could be equipped with a vent, or it’s also possible to have a valve installed on the inside, directly under the sink, right?
I don’t see a utility sink in the technical room, but there is an option to connect to the wastewater pipe for technical purposes.
Besides, a small sink is planned in the laundry room in front of that.
D
derdietmar20 Mar 2026 08:36Hello,
There is nothing wrong with the planning. It is also not a problem to run the pipes underneath the house. You have cleanouts on both sides and 45-degree branches. From both sides of the building, you can easily access all points of the main sewer pipes for cleaning. Smaller connection pipes are usually accessed from above or inside.
The only theoretical risk would be a full pipe fill of the main line, which could potentially cause siphons to be emptied. However, this risk only exists if a drain with the same pipe diameter (DN) as the main line is connected and actually transports enough water to fill the pipe completely. The main line up to the respective floor slab penetrations is DN100. The supply pipes from the sink and others are only DN40, sometimes DN60. This ensures that air is supplied through the pipes even without extension to a roof vent. Only the toilet is connected with DN100, but it has an air supply through the roof.
If you want 100% certainty, for example, the connections from the utility room and staircase under the slab can be extended to the kitchen connection on the right. This way, you have a double connection and thus ventilation. No black water flows in this ventilation pipe (the highest point of the ventilation pipe is at the center of the ventilation system). For peace of mind, you can also specify the collector pipe as DN125.
A leak test should be performed before pouring the floor slab (this is usually also required to be submitted to the authorities).
Best regards
There is nothing wrong with the planning. It is also not a problem to run the pipes underneath the house. You have cleanouts on both sides and 45-degree branches. From both sides of the building, you can easily access all points of the main sewer pipes for cleaning. Smaller connection pipes are usually accessed from above or inside.
The only theoretical risk would be a full pipe fill of the main line, which could potentially cause siphons to be emptied. However, this risk only exists if a drain with the same pipe diameter (DN) as the main line is connected and actually transports enough water to fill the pipe completely. The main line up to the respective floor slab penetrations is DN100. The supply pipes from the sink and others are only DN40, sometimes DN60. This ensures that air is supplied through the pipes even without extension to a roof vent. Only the toilet is connected with DN100, but it has an air supply through the roof.
If you want 100% certainty, for example, the connections from the utility room and staircase under the slab can be extended to the kitchen connection on the right. This way, you have a double connection and thus ventilation. No black water flows in this ventilation pipe (the highest point of the ventilation pipe is at the center of the ventilation system). For peace of mind, you can also specify the collector pipe as DN125.
A leak test should be performed before pouring the floor slab (this is usually also required to be submitted to the authorities).
Best regards
M
MachsSelbst20 Mar 2026 10:37What are the advantages of running the wastewater pipes for a length of 10 to 12 meters (33 to 39 feet) beneath the concrete slab?
You’ll never be able to access them again.
In our case, the wastewater pipes come out of the concrete slab at the exterior wall and are routed directly outside. This is the most cost-effective solution for the homeowner, since the underground contractor handled the piping to the soil stack, which wasn’t included in the construction scope.
It is a bit more expensive, but if there’s an issue with the pipes, you can access them. You’ll of course have to break up curbstones, lift paving, etc., but if a pipe under the concrete slab gets blocked or breaks for any reason, you’re basically out of luck—you might as well tear the whole house down.
You’ll never be able to access them again.
In our case, the wastewater pipes come out of the concrete slab at the exterior wall and are routed directly outside. This is the most cost-effective solution for the homeowner, since the underground contractor handled the piping to the soil stack, which wasn’t included in the construction scope.
It is a bit more expensive, but if there’s an issue with the pipes, you can access them. You’ll of course have to break up curbstones, lift paving, etc., but if a pipe under the concrete slab gets blocked or breaks for any reason, you’re basically out of luck—you might as well tear the whole house down.
M
MachsSelbst20 Mar 2026 10:48And then all the pipes connect into one main pipe. That’s really a poor solution. Simpler, yes. Cheaper, yes. But still poor.
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