ᐅ Should a drainage system be considered during building planning? How to implement it?

Good morning everyone,

We will soon be building a semi-detached house using prefabricated construction (or having it built). The city’s development plan states that rainwater must not infiltrate the property but instead must be directed by drainage to two large soakaway pits at the edge of the new development area.

I have a few questions that maybe some of you can help me with.

1) Who is responsible for this? The earthworks contractor? The prefab house company definitely won’t do it; they only handle the foundation slab and the house itself. Or should I contact a landscape gardener?

2) We are building a semi-detached house in Bauhaus style, which means a flat roof. It has four rainwater outlets: two at the back and two at the front. How do I get the water from the rear outlets to the front? Going along the left side of the foundation slab is not possible because the neighboring house is there, and the right side won’t work due to the strip footings for the garage. Should the pipes run underneath or through the foundation slab? I probably cannot go too deep because then the pipes won’t have enough slope to drain water toward the street.

3) How is the garden drainage supposed to work? Do pipes need to be installed throughout the garden?

4) And of course, the most important question: How much does this typically cost? The plot is about 400 square meters (4,300 square feet), with just over 200 square meters (2,150 square feet) designated as garden.

EDIT: I just realized I posted this in the wrong forum; maybe one of the moderators can move it? 🙂

MarkoW. schrieb:

Good morning everyone,

we are about to build a semi-detached house using prefabricated construction (or rather have it built). The city’s development plan states that rainwater must not infiltrate on the property but must be directed via drainage into two large infiltration pits at the edge of the new development area.

I have a few questions about this that perhaps someone here can answer.

1) Who handles this? The earthworks contractor? The prefab house company definitely won’t do it; they only handle the foundation slab and the house. Or should I contact a landscaping contractor?

2) We are building a semi-detached house in Bauhaus style, which means a flat roof. There are four rainwater outlets: two at the back, two at the front. How can I get the water from the rear outlets to the front? Running the pipes past the left side of the foundation slab isn’t possible because that’s where the neighboring house is, and not on the right because that’s where the strip footings for the garage will be. Under the foundation slab? Or through it? I probably can’t go too deep or there won’t be enough slope in the pipes to the street.

3) How is garden drainage supposed to work? Do pipes need to be installed everywhere in the garden?

4) And of course, the most important question: How much does this roughly cost? The plot is about 400sqm (4300 sq ft), of which a little more than 200sqm (2150 sq ft) is garden.

EDIT: I just noticed that I ended up in the wrong forum, maybe one of the mods can move this? 🙂

Hello,
Drainage, infiltration trenches, etc. are a current topic for us too, so here is my experience:

1) Drainage is usually the responsibility of the civil or earthworks contractor. In our case, the earthworks contractor did not do all the work on the property because they were not permitted to work on public land. It is advisable to find a contractor who can carry out the work both on your private property and on public land, for example as part of utility connections or sewer works. Whether a landscaping contractor can handle this depends and I cannot tell you for sure at this point.

2) I would expect that the pipes will be routed along the house and under the garage foundations. Our strip footings are about 80-90cm (31-35 inches) deep, so in your case the pipes could go beneath them.

3) Unfortunately, I cannot give you reliable information about that since on our property we use infiltration trenches within the garden. In our case, 1-2 underground pipes run along the house from the front to the back to direct the water to the infiltration trench.

4) The cost depends heavily on the region and the total length of trenching required, but earthworks tend to be quite expensive. For reference, for a 10x10 meter (33x33 feet) house, earthworks alone cost around $4,000-$5,000, involving about 30 meters (98 feet) of trenching.

MarkoW. schrieb:

1) Who handles this? The excavation contractor? The prefabricated house company definitely won’t do it, they only handle the foundation slab and the house. Or should I contact a landscaping contractor?

A civil engineering or excavation company. It’s possible that the municipality has a company on standby for this. At least that was the case for us.

MarkoW. schrieb:

2) We’re building a semi-detached house in Bauhaus style, which means it has a flat roof. There are four rainwater outlets: two at the back, two at the front. How can I route the water from the rear outlets to the front? Going left past the foundation slab isn’t possible because of the neighboring house, and the right side is blocked due to strip foundations for the garage. Should the pipes go under or through the foundation slab? I probably can’t go too deep because otherwise there would be no slope in the pipes leading toward the street.

It could also be possible to the left by creating a shared drainage connection with the neighbor. That way you’d only have to dig once from back to front. However, this would require appropriate easements. Otherwise, routing under the garage is probably the way to go.

MarkoW. schrieb:

3) How is garden drainage supposed to work? Do pipes need to be installed all over the garden?

Are you sure this is not only required for impervious or paved surfaces?

MarkoW. schrieb:

4) And of course the most important question: roughly how much does this cost? The plot is about 400sqm (4,300 sq ft), with a little more than 200sqm (2,150 sq ft) allocated to the garden.

That’s really hard to say here. It also depends on how high or low the plot is compared to the connection point in the public ground. If you have to lay the pipes 3m (10 ft) deep, costs can quickly reach five figures.

MarkoW. schrieb:

1) Who does that? The earthworks contractor? The prefab house company definitely won’t do it; they only handle the foundation slab and the house. Or should I contact a landscaping contractor?

As @RE-1407 writes, that is done by the civil engineer or groundworks contractor. If they lay the pipes up to a connection point at the street, that is also fine; it doesn’t have to be the same company as the utility installers. It’s better to coordinate these (water, sewage, telecommunications, etc.) so the street only has to be dug up once, which also reduces costs.

MarkoW. schrieb:

2) We are building a semi-detached house in Bauhaus style, which means a flat roof. There are four rainwater drains on it: two at the back and two at the front. How do I get the water from the rear drains to the front? The left side of the foundation slab is not possible since that’s where the neighbor’s house is; the right side is also not possible because that’s where the strip foundations for the garage will be. Underneath the foundation slab? Or through the foundation slab? I probably cannot go too deep either, as otherwise there wouldn’t be enough slope in the pipes for the water to flow toward the street.

Flat roof = Bauhaus style is comparable to a vibrant picture of expressionism.
Regarding your question: routing pipes under the foundation slab is a practical and cost-effective solution. The slope doesn’t have to be very steep, but it must be taken into account.

MarkoW. schrieb:

3) How should the garden drainage work? Do pipes need to be laid everywhere in the garden?

I can hardly imagine that. Usually, the goal is to drain water from impermeable surfaces. The exact wording of the local regulations will provide clarification.

Thank you all in advance for the detailed responses 🙂

hampshire schrieb:

I can hardly imagine that; usually, it concerns the drainage of water from sealed surfaces. The exact wording of the regulation will provide clarity here.

The development plan basically states the following:

"Due to the soil conditions (loess loams and loess over terrace deposits), proper and professional infiltration can only take place in the deeper sandy loess or the lower gravel sands (infiltration basins with gravel breakthrough)...

According to § 44 of the Water Act for the state of North Rhine-Westphalia (State Water Act – LWG), rainwater from properties being developed for the first time must be disposed of in accordance with § 55 (2) of the Federal Water Act. According to the provisions of § 55 (2) of the Federal Water Act, rainwater should be infiltrated, dispersed, or discharged directly or via a sewer system without mixing with wastewater into a body of water, provided that neither water rights nor other public regulations or water management concerns prohibit this...

Since the planning area lies outside of water protection zones, rainwater infiltration can be carried out through two central infiltration systems. Decentralized systems are excluded. Further detailed planning will be coordinated with all parties involved.

For rainwater infiltration, both above-ground infiltration surfaces such as infiltration basins, and underground infiltration facilities such as soakaways are possible."

How should this be interpreted?

MarkoW. schrieb:

How should this be interpreted?

This is unclear. Two main infiltration zones for the area can be established. Can does not mean must. Furthermore, it refers to coordination with the parties involved. So, go to the local water authority and arrange a discussion.
Section 44 of the state water law contains an economic feasibility clause. Read that in preparation for your conversation.