ᐅ When Is a Slope Considered a Slope? Basement vs. Slab Foundation
Created on: 23 Jun 2017 00:17
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BobinhoHello everyone,
My family and I are currently looking for a plot of land. In a prospective new development area, where the sale of plots will begin soon, I have identified a few spots (the plots have not yet been officially surveyed) that we really like. However, all of these locations have more or less significant slopes.
Attached is a screenshot from the development plan, which includes some elevation details. I do not yet have any information on the soil conditions.
In the upper part, you can see that the plots have a slope of about 4 meters over 20 meters (approximately 13 feet over 66 feet), and the slope decreases further down.
We actually want to build without a basement on a slab-on-grade foundation (for cost reasons). This raises the following questions, and I would really appreciate your input:
- Is this feasible in the upper area with additional costs remaining reasonable compared to a flat plot?
- What about in the lower area?
- To avoid starting the same thread with other plots next week, how much slope can a plot have before the extra costs for a slab foundation become excessive and a basement becomes advisable?
- Can the additional costs be roughly defined now (for example, can we reasonably assume clay soil)?
- Would building a basement, possibly partially or fully open on the slope side, incur significantly higher costs compared to a basement on flat land?
- I assume I should have a soil survey done before purchasing? Would a report from a nearby site be insufficient?
Thank you very much in advance!
Bobinho
My family and I are currently looking for a plot of land. In a prospective new development area, where the sale of plots will begin soon, I have identified a few spots (the plots have not yet been officially surveyed) that we really like. However, all of these locations have more or less significant slopes.
Attached is a screenshot from the development plan, which includes some elevation details. I do not yet have any information on the soil conditions.
In the upper part, you can see that the plots have a slope of about 4 meters over 20 meters (approximately 13 feet over 66 feet), and the slope decreases further down.
We actually want to build without a basement on a slab-on-grade foundation (for cost reasons). This raises the following questions, and I would really appreciate your input:
- Is this feasible in the upper area with additional costs remaining reasonable compared to a flat plot?
- What about in the lower area?
- To avoid starting the same thread with other plots next week, how much slope can a plot have before the extra costs for a slab foundation become excessive and a basement becomes advisable?
- Can the additional costs be roughly defined now (for example, can we reasonably assume clay soil)?
- Would building a basement, possibly partially or fully open on the slope side, incur significantly higher costs compared to a basement on flat land?
- I assume I should have a soil survey done before purchasing? Would a report from a nearby site be insufficient?
Thank you very much in advance!
Bobinho
Bobinho schrieb:
In the upper part, you can see that the plots will have a slope of about 4m over 20m (13 feet over 66 feet), Assuming this slope is roughly consistent and the house depth is between 8 and 12 m (26 to 39 feet), this would result in a height difference between the street side and garden side of about 1.60 to 2.40 m (5.2 to 7.9 feet). During heavy rain, you would not want the front door to be below ground level, nor would you want to have to pump wastewater extensively into the sewer. So, you will probably need to go even more than the mentioned height below your foundation slab (since the ground floor’s finished floor level will be at least at the upper height). It would be almost a waste not to use the space between these walls as living space.
On the other hand, this slope also provides a decent above-ground height on the valley side, making the basement suitable for living or sleeping areas. So, having a basement there has, like everything, two sides to consider.
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Approximately 6 m (20 feet) over a 20-meter (66 feet) height difference is quite significant. Without a basement and retaining walls, you won't manage. A foundation slab alone won't be sufficient; you need to stabilize the slope. So, you should easily expect an additional cost of around 30,000k, if not more.
Where we live, there is a 3-meter (10 feet) difference over about 22 meters (72 feet), and we had to use 4-meter (13 feet) long L-shaped concrete blocks on the front door side. However, we intentionally built with a basement to avoid having a dark, unused space.
Where we live, there is a 3-meter (10 feet) difference over about 22 meters (72 feet), and we had to use 4-meter (13 feet) long L-shaped concrete blocks on the front door side. However, we intentionally built with a basement to avoid having a dark, unused space.
If you want to keep costs down: Make the surface as level as possible. Minimize the slope as much as you can.
Here’s an example: Our soil consists of topsoil, with clay and gravel underneath. Our lot is 30 meters (98 feet) deep at its longest point and 24 meters (79 feet) wide at its narrowest. The elevation change from street level to the back post is 1.2 meters (4 feet). The slope in the house area—our house is 10.5 meters (34.5 feet) wide—is about 1 meter (3 feet), and the house is set 5 meters (16 feet) behind the street boundary. The foundation slab was positioned so that we have 0.5 meters (1.6 feet) of fill at the front and a 0.5 meter (1.6 feet) cut at the rear, averaged out. The additional earthworks cost around 6,500 euros.
The takeaway: With the slope you indicated, you will likely face significantly more filling or excavation work.
Build as level as possible. Karsten
Here’s an example: Our soil consists of topsoil, with clay and gravel underneath. Our lot is 30 meters (98 feet) deep at its longest point and 24 meters (79 feet) wide at its narrowest. The elevation change from street level to the back post is 1.2 meters (4 feet). The slope in the house area—our house is 10.5 meters (34.5 feet) wide—is about 1 meter (3 feet), and the house is set 5 meters (16 feet) behind the street boundary. The foundation slab was positioned so that we have 0.5 meters (1.6 feet) of fill at the front and a 0.5 meter (1.6 feet) cut at the rear, averaged out. The additional earthworks cost around 6,500 euros.
The takeaway: With the slope you indicated, you will likely face significantly more filling or excavation work.
Build as level as possible. Karsten
tomtom79 schrieb:
No slab alone will be enough; you have to secure the slope. So you will easily need an extra 30,000 kg (66,000 lbs) or even more. Apart from the fact that for such a significant (also cost-related) effort "under the house," I would expect to have usable space in return: I wouldn’t even consider literally relying on a retaining wall to hold back a bank beneath my slab and thereby keep the foundation "dry."
A sloped lot and a slab-on-grade house (or vice versa) don’t really fit together. In the type of lot described, I only see the option of building with a basement; or conversely, a slab-on-grade house on a different lot.
In my opinion, any effort to manage without a basement on this lot will at best not make sense financially. That would be a Pyrrhic victory.
https://www.instagram.com/11antgmxde/
https://www.linkedin.com/company/bauen-jetzt/
Hello everyone, thank you so far. I realize that this plot with its 20% slope can only be reasonably built with a basement. The note about the ground level is also interesting; I hadn’t really considered that before. By the way, I’m not completely against a basement – on the contrary, I would like to have one on a slope – but I’m a bit hesitant because, according to some acquaintances, that’s where the biggest financial risks lie.
Can anyone roughly estimate how much it would cost me to build a residential basement there? We can assume a clay soil. I’m interested in all costs excluding site development, so that I can continue planning with a price for the top edge of the slab / basement.
By the way, if a 20% slope is too much for a slab foundation, what slope is acceptable? 10%? 5%? Or 5% only if the slope starts from the street due to ground level?
@Nordlys:
If I understand you correctly, you have a slope of about 0.8 m (31 inches) over 10.5 m (34 feet), which according to my smart calculator corresponds to approximately 7.6%. Is that correct or did I misunderstand you?
An extra cost of €6500 is of course always annoying, but personally, I still consider it acceptable (compared to a basement). I assume these additional costs were also not shown during planning? May I ask what total price you ended up with for the slab foundation?
Best regards
Bobinho
Can anyone roughly estimate how much it would cost me to build a residential basement there? We can assume a clay soil. I’m interested in all costs excluding site development, so that I can continue planning with a price for the top edge of the slab / basement.
By the way, if a 20% slope is too much for a slab foundation, what slope is acceptable? 10%? 5%? Or 5% only if the slope starts from the street due to ground level?
@Nordlys:
If I understand you correctly, you have a slope of about 0.8 m (31 inches) over 10.5 m (34 feet), which according to my smart calculator corresponds to approximately 7.6%. Is that correct or did I misunderstand you?
An extra cost of €6500 is of course always annoying, but personally, I still consider it acceptable (compared to a basement). I assume these additional costs were also not shown during planning? May I ask what total price you ended up with for the slab foundation?
Best regards
Bobinho
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