ᐅ Single-family house without a basement: Assessment / Larger foundation slab than planned?
Created on: 19 May 2013 19:21
M
majestixHello everyone, we are building a single-family house (approximately 123 sqm (1,324 sq ft) of living space, 1.5 stories, without a basement) and started the week before last.
Here is the situation:
I am doing the shell construction together with my friend, who is a master bricklayer and does good work. However, yesterday while excavating the foundations, we made a mistake that only became apparent when measuring the sewer connections inside the house.
The surveyor marked the exterior dimensions including 16 cm (6 inches) of exterior insulation (ETICS).
According to the structural engineer’s system detail, however, the 24 cm (9.5 inches) exterior masonry walls are flush with the concrete slab. This means the foundation slab should measure 8.11 m x 9.68 m (26.6 ft x 31.8 ft).
Therefore, we should have deducted 16 cm (6 inches) for the insulation from the surveyor’s dimensions before starting the foundations. Instead, we used the surveyor’s measurements for the strip footings, and now the slab measures 8.43 m x 10 m (27.7 ft x 32.8 ft), which is about 32 cm (13 inches) larger on each side.
My friend proposed the following:
We place a 6 mm (0.24 inch) exterior insulation on the strip footings (80 cm (31.5 inches) wide, unreinforced) instead of 4 mm (0.16 inch), and omit the 4 mm (0.16 inch) interior insulation. This would give the footing about 2 mm (0.08 inch) more concrete bearing area.
(He had already suggested omitting the interior insulation, because we are insulating the foundations down to the ground. The slab itself is planned with 20 cm (8 inches) of lava aggregate 16/32, a 10 cm (4 inch) blinding layer, 10 cm (4 inch) insulation, and a 25 cm (10 inch) structural slab. The slab is about 50 cm (20 inches) above ground level.)
He said that this way the ETICS would align flush with the foundations.
My questions for you are as follows:
In the current situation, I could then move the masonry, for example, by only 12 cm (5 inches) of ETICS about 4 cm (1.6 inches) further outward. This would give us a little more living space. The only concern I see here is that the interior dimensions would differ from the original plan, which could mean a slightly larger span for the intermediate floor slab.
Of course, I don’t want to make a mountain out of a molehill if it’s just a minor issue. There are DIN standards and other guidelines, but also reasonable judgement. That is what I would appreciate from you—how would you, as a homeowner, professional, or structural engineer, approach this?
Here is the situation:
I am doing the shell construction together with my friend, who is a master bricklayer and does good work. However, yesterday while excavating the foundations, we made a mistake that only became apparent when measuring the sewer connections inside the house.
The surveyor marked the exterior dimensions including 16 cm (6 inches) of exterior insulation (ETICS).
According to the structural engineer’s system detail, however, the 24 cm (9.5 inches) exterior masonry walls are flush with the concrete slab. This means the foundation slab should measure 8.11 m x 9.68 m (26.6 ft x 31.8 ft).
Therefore, we should have deducted 16 cm (6 inches) for the insulation from the surveyor’s dimensions before starting the foundations. Instead, we used the surveyor’s measurements for the strip footings, and now the slab measures 8.43 m x 10 m (27.7 ft x 32.8 ft), which is about 32 cm (13 inches) larger on each side.
My friend proposed the following:
We place a 6 mm (0.24 inch) exterior insulation on the strip footings (80 cm (31.5 inches) wide, unreinforced) instead of 4 mm (0.16 inch), and omit the 4 mm (0.16 inch) interior insulation. This would give the footing about 2 mm (0.08 inch) more concrete bearing area.
(He had already suggested omitting the interior insulation, because we are insulating the foundations down to the ground. The slab itself is planned with 20 cm (8 inches) of lava aggregate 16/32, a 10 cm (4 inch) blinding layer, 10 cm (4 inch) insulation, and a 25 cm (10 inch) structural slab. The slab is about 50 cm (20 inches) above ground level.)
He said that this way the ETICS would align flush with the foundations.
My questions for you are as follows:
- The exterior dimensions do not change, so we are still complying with the building setbacks, right?
- Do you see any issue with the slab being flush with the exterior insulation?
- The slab is now about 5 sqm (54 sq ft) larger. Do you see any structural problems or is this within tolerance?
- We had considered using a simple BIMS thermal insulation block, for example with a U-value of 0.12 or 0.13 (instead of 0.16), and possibly reducing the ETICS thickness from 16 cm (6 inches) to around 12 cm (5 inches) or 10 cm (4 inches).
In the current situation, I could then move the masonry, for example, by only 12 cm (5 inches) of ETICS about 4 cm (1.6 inches) further outward. This would give us a little more living space. The only concern I see here is that the interior dimensions would differ from the original plan, which could mean a slightly larger span for the intermediate floor slab.
- Is that still permissible under the building regulations (exemption from planning permission according to §67 of the building code)?
- Structural safety: is it within tolerance or not?
Of course, I don’t want to make a mountain out of a molehill if it’s just a minor issue. There are DIN standards and other guidelines, but also reasonable judgement. That is what I would appreciate from you—how would you, as a homeowner, professional, or structural engineer, approach this?
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