ᐅ Is the price for the concrete slab justified?

Hi!
In our rural area of NRW, this could be done significantly cheaper with a bit of organizing:

- 1 day of excavation + delivery, installation, and compaction of 50 cubic meters of recycled concrete aggregate about 2500€
- 1 cubic meter of concrete including delivery about 150€ (for you about 15-20 cubic meters = 2250€–3000€?)
- a few working hours of 2 concrete workers/bricklayers = 500€

I would commission these separately: hire a subcontractor or similar for the earthworks, try to find one or two people for half a day for the concrete work, and order the concrete directly from the plant. If you help out yourself, you only need one “expert.”
With formwork and reinforcement (material + installation), that would probably be an additional half day (+2500€). Overall, around 8500€.
In 2020, I poured a slab at my place, but without earthworks (I dug by hand). Back then, it cost around 2500€ (materials and labor) for a 59 square meter (635 square feet), 16 cm (6 inches) thick, and properly reinforced slab. All prices including VAT. A concrete pump was also included.

LukPfaffendorf schrieb:

I also wonder whether this is necessary and reasonable. For the neighboring house, a soil survey was carried out (80-90cm (31-35 inches) of soil group OH above the topsoil).

For the foundation of the house, the recommended measures in the report were to remove the topsoil and use strip foundations. Whether this is necessary for a garage, I cannot assess either. We plan to build the garage ourselves and intended to have the concrete slab done by the shell contractor. Therefore, there are no requirements from a garage manufacturer.

Topsoil must be removed in any case. Topsoil contains organic material that decomposes over the years and is also considered a protected resource in Germany. However, 90cm (35 inches) is quite a high figure; usually, it is not that deep (20-40cm (8-16 inches) is normal). A plowed field typically has 50-60cm (20-24 inches). But it doesn’t really matter; I am a supporter of slab foundations anyway.

Do you have structural calculations for the garage? If yes, have the engineer calculate a slab foundation (this is not very complicated). Then use F1 gravel as described above, a 1m (3.3 feet) cushion layer, and omit the strip foundations.

Are you still in the building phase? How is or was the house founded? This also needs to be taken into account...

Cronos86 schrieb:

However, 90 cm (35 inches) is quite a high measurement, of course, 20–40 cm (8–16 inches) is normal. A plowed field has 50–60 cm (20–24 inches).

Either someone dumped material there at some point, or the area was treated with sod or peat fertilization.

Cronos86 schrieb:

Topsoil definitely needs to be removed. Topsoil contains organic matter that can decompose over time, and in Germany, it is also considered a protected resource. However, 90 cm (35 inches) is quite a deep estimate; typically, 20-40 cm (8-16 inches) is normal. A tilled field usually has 50-60 cm (20-24 inches). But either way, I’m a fan of slab foundations.

Do you have structural engineering plans for the garage? If so, have the engineer calculate a slab foundation (which isn’t very complicated). Then, as mentioned above, use a type F1 gravel base, a 1 m (3 ft) cushioning layer, and skip the strip footings.

Are you still in the building phase? How is or will the house be founded? That definitely needs to be taken into account…

- There used to be a nursery or tree farm on the plot. That might explain it.
- I need to request the structural engineering plans. The planner created them together with the house design.
- The house is currently under construction. The soil report recommended 90 cm (35 inches) replacement and strip footings at 90 cm (35 inches) depth as frost protection.