ᐅ Foundation for an Extension: Is This the Right Approach? Any Experiences?
Created on: 10 Jun 2012 23:49
H
Huti
Hello.
This is my first post here, so I’ll include a brief introduction.
My name is Stefan, I’m 31 years old, and I recently bought a house built in 1951. I am currently renovating and remodeling it.
Although I’m not a professional (trained as a business administrator, not a tradesperson), I’m doing a lot of the work myself during the renovation.
Right now, I’m gathering information for a planned extension at the front of the house (the main entrance will be enlarged / extended outward by 1.5m (5 feet) and the front door will be rotated 90°).
I’ll describe how I have planned this construction project so far and would appreciate advice on whether this is correct or how it might be better / more practical / feasible at all.
As I mentioned, I’m not a professional, so please excuse any missing technical terms or often amateurish expressions.
The extension itself is planned to be 1.5m (5 feet) deep and 3.5m (12 feet) wide, with the front door located on the short side at a 90° angle to the usual front of the house.
The main entrance (now and in the future) is about 50cm (20 inches) above ground level and is reached by 3 steps.
The structure will be built with aerated concrete blocks or Poroton blocks, 17.5cm (7 inches) thick.
For the foundation, a trench 80cm (31 inches) deep will be dug around the outside and filled with concrete to ensure frost protection.
Here is my first specific question: Is it sufficient to build this foundation up to ground level and then build the walls on top (with appropriate moisture protection), or should the foundation be cast significantly above ground level using formwork? Possibly even the full 50cm (20 inches) that will need to be backfilled later?
How should the connection to the existing house and basement wall be made? Should it simply be concreted together (“cast in place”) or is a moisture barrier required between the basement wall and the foundation for the extension?
Of course, it is clear that insulation is necessary at the transition to the outside (toward the soil).
The 50cm (20 inches) above ground to match the house entrance level will be filled with construction rubble. Above this, screed, underfloor heating, top screed, and tiles will be installed (the same continuous flooring as in the adjacent hallway extended by the new extension). Any objections?
I hope I have explained my project clearly, and even if my planning is misguided, I kindly ask for well-intentioned advice.
Thanks in advance and best regards
This is my first post here, so I’ll include a brief introduction.
My name is Stefan, I’m 31 years old, and I recently bought a house built in 1951. I am currently renovating and remodeling it.
Although I’m not a professional (trained as a business administrator, not a tradesperson), I’m doing a lot of the work myself during the renovation.
Right now, I’m gathering information for a planned extension at the front of the house (the main entrance will be enlarged / extended outward by 1.5m (5 feet) and the front door will be rotated 90°).
I’ll describe how I have planned this construction project so far and would appreciate advice on whether this is correct or how it might be better / more practical / feasible at all.
As I mentioned, I’m not a professional, so please excuse any missing technical terms or often amateurish expressions.
The extension itself is planned to be 1.5m (5 feet) deep and 3.5m (12 feet) wide, with the front door located on the short side at a 90° angle to the usual front of the house.
The main entrance (now and in the future) is about 50cm (20 inches) above ground level and is reached by 3 steps.
The structure will be built with aerated concrete blocks or Poroton blocks, 17.5cm (7 inches) thick.
For the foundation, a trench 80cm (31 inches) deep will be dug around the outside and filled with concrete to ensure frost protection.
Here is my first specific question: Is it sufficient to build this foundation up to ground level and then build the walls on top (with appropriate moisture protection), or should the foundation be cast significantly above ground level using formwork? Possibly even the full 50cm (20 inches) that will need to be backfilled later?
How should the connection to the existing house and basement wall be made? Should it simply be concreted together (“cast in place”) or is a moisture barrier required between the basement wall and the foundation for the extension?
Of course, it is clear that insulation is necessary at the transition to the outside (toward the soil).
The 50cm (20 inches) above ground to match the house entrance level will be filled with construction rubble. Above this, screed, underfloor heating, top screed, and tiles will be installed (the same continuous flooring as in the adjacent hallway extended by the new extension). Any objections?
I hope I have explained my project clearly, and even if my planning is misguided, I kindly ask for well-intentioned advice.
Thanks in advance and best regards
Hello,
In a concrete footing, the load spreads at an angle of about 60 degrees. Below the footing, depending on the soil conditions, the load spreads further at an angle of around 45 degrees. If you want to prevent the load from the extension pushing against your basement wall, the foundations need to be correspondingly deep.
If you don’t have any existing documentation for your house, it’s quite useful to dig a trial trench at the spot where the extension is planned. This is done by excavating a narrow trench about 40cm (16 inches) wide, roughly the size of a small excavator bucket. This way, you can see how deep your existing foundations are, where there might be any foundation overhangs, and so on. Alternatively, the planner may assume the typical case for your building’s age based on experience, but this carries the risk that once excavation starts, adjustments may be necessary. Such changes can be costly and potentially blow the existing budget. However, at that point, you may already be at a "point of no return."
What is a stepped foundation?
Yes, but not along the entire length. Usually, the foundation steps up in increments of about 30cm (12 inches), ensuring that the load distribution angle does not extend beyond the concrete footing.
Regards,
BratacDD
In a concrete footing, the load spreads at an angle of about 60 degrees. Below the footing, depending on the soil conditions, the load spreads further at an angle of around 45 degrees. If you want to prevent the load from the extension pushing against your basement wall, the foundations need to be correspondingly deep.
If you don’t have any existing documentation for your house, it’s quite useful to dig a trial trench at the spot where the extension is planned. This is done by excavating a narrow trench about 40cm (16 inches) wide, roughly the size of a small excavator bucket. This way, you can see how deep your existing foundations are, where there might be any foundation overhangs, and so on. Alternatively, the planner may assume the typical case for your building’s age based on experience, but this carries the risk that once excavation starts, adjustments may be necessary. Such changes can be costly and potentially blow the existing budget. However, at that point, you may already be at a "point of no return."
What is a stepped foundation?
So does the foundation for my extension also need to be 1.5m deep?
Yes, but not along the entire length. Usually, the foundation steps up in increments of about 30cm (12 inches), ensuring that the load distribution angle does not extend beyond the concrete footing.
Regards,
BratacDD
Ahh, so the picture becomes clear quickly
Thank you!
Could you also tell me something about the damp proof course? Is the foundation of the extension simply poured flush with the house foundation, or does something need to be placed between them? I only know from masonry that a roofing membrane is laid between the foundation and the lowest course of bricks.
Thanks again for the quick and straightforward help!
Thank you!
Could you also tell me something about the damp proof course? Is the foundation of the extension simply poured flush with the house foundation, or does something need to be placed between them? I only know from masonry that a roofing membrane is laid between the foundation and the lowest course of bricks.
Thanks again for the quick and straightforward help!
I think you are mixing up a few things here. The separation of the foundations is not directly related to the waterproofing. From a structural point of view, I would place about 2 cm (0.8 inches) of insulation between the two foundations and pour concrete against it. Settlement is an important factor here. This prevents an unintended interlock between the two foundations. However, you will most likely find an overhang on the existing foundation, and in that case, I personally would not set the new foundation directly on the old one. Because then you will almost certainly get cracks in the existing structure.
Regards, Ronny
Regards, Ronny
Huti schrieb:
How deep and how wide is already stated in the first post. What about reinforcement? That's exactly the purpose of this thread—to answer my questions and to point out anything I didn’t mention that needs consideration. It confuses me when an architect asks me, as a layperson, how the reinforcement should be done in such a case...You didn’t understand me. I never expected you to actually answer these questions.
What I wanted to say is that building a foundation and a slab requires many questions to be answered.
Huti schrieb:
Good question, what do you recommend? If I now tell you "Foundation 1.0 m (3.3 ft) deep, 50 cm (20 inches) wide. Reinforcement with 12 mm (1/2 inch) concrete bars, slab thickness 20 cm (8 inches) with reinforced steel mesh Q257A on top and bottom," would you then just start digging your foundation?
My point is that someone needs to calculate this. No architect will tell you, "Just pour in plenty of concrete and steel, it will hold," even though that might probably be true.
Huti schrieb:
Do you mean the ring beam that sits under the roof structure? Exactly!
Huti schrieb:
A ring beam must be constructed in the following cases:
- Buildings with more than two full stories (My main entrance is only one story)
- Buildings longer than 18 m (59 ft) (Dimensions are in the first post, 1.5 m x 3.5 m (5 ft x 11.5 ft))
- Walls with many large openings (One door and one window 1.0 m x 1.0 m (3.3 ft x 3.3 ft), should be acceptable)
- If the ground conditions require it (No)
Huti schrieb:
Apart from the fact that according to the same source, I COULDN’T decide that myself, but the necessity of a ring beam is determined by an architect, and its size and reinforcement by a structural engineer. And it’s the same with the foundation and slab.
Huti schrieb:
You mean except that some of my questions would already be answered? I mean: foundation, slab, and all other load-bearing components must be engineered. When the structural engineer does the calculations, they will already know the details.
For a layperson, this is not something to handle. That’s the only advice you can get from this forum.
@BratacDD: And once again, thanks for the quick and helpful answers and explanations.
@E.Curb:
By that, you wanted to say something without really saying anything, which is clear to me by now. Otherwise, I would have been able to read at the latest here—probably even more than a page ago—which questions actually need to be clarified (apart from the roof structure and insulation that affect my foundation, but even that could have been set aside).
Obviously not the only advice; BratacDD has already helped me a lot. Where there’s a will...
But precisely this “hemming and hawing” is the reason why I want to gather all the information myself first before calling the architect. If I have them standing at my door and ask how the damp-proof membrane on my foundation should look, and then something like THAT comes up, no thanks.
@E.Curb:
I wanted to tell you that many questions need to be answered when building a foundation and a slab.
By that, you wanted to say something without really saying anything, which is clear to me by now. Otherwise, I would have been able to read at the latest here—probably even more than a page ago—which questions actually need to be clarified (apart from the roof structure and insulation that affect my foundation, but even that could have been set aside).
I mean: foundation, slab, and all other load-bearing components must be calculated. And when the structural engineer does the calculations, they already know the exact details. This is not for laymen. That is the only advice you can get from the forum.
Obviously not the only advice; BratacDD has already helped me a lot. Where there’s a will...
But precisely this “hemming and hawing” is the reason why I want to gather all the information myself first before calling the architect. If I have them standing at my door and ask how the damp-proof membrane on my foundation should look, and then something like THAT comes up, no thanks.
Please forgive my well-intentioned advice to hire professional planning services, even for such a small construction project. For someone with no experience at all, this is not something to do alone.
In any case, there is (at least from me) no do-it-yourself guide for adding an extension to an existing building, especially when it involves structural engineering. As I mentioned before, it simply doesn’t work if you don’t know the exact conditions on site.
But by gathering the necessary information yourself from the internet, your extension project can succeed. And my colleagues will be happy soon when they get to answer questions like:
Why are there cracks in the masonry? Why is water getting in here? Why is there a draft? Why do I have mold in the corners?
Best regards – I’m off to watch some soccer now.
In any case, there is (at least from me) no do-it-yourself guide for adding an extension to an existing building, especially when it involves structural engineering. As I mentioned before, it simply doesn’t work if you don’t know the exact conditions on site.
But by gathering the necessary information yourself from the internet, your extension project can succeed. And my colleagues will be happy soon when they get to answer questions like:
Why are there cracks in the masonry? Why is water getting in here? Why is there a draft? Why do I have mold in the corners?
Best regards – I’m off to watch some soccer now.
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