ᐅ Is an 80 cm gravel layer necessary beneath a 25 cm thick concrete slab for frost protection?

Hello,

My soil report states: "The area is located in frost exposure zone I. Foundations must be protected against frost according to DIN 1054. For external foundations, a frost-free foundation depth of 0.8 m (0.8 meters) must be maintained."

I have now received a quote from the civil engineer including an 80 cm (31.5 inches) gravel layer under the 25 cm (10 inches) thick concrete slab. Our neighbor across the street built a similar house with only 20 cm (8 inches) of gravel. Is an 80 cm (31.5 inches) gravel layer necessary, or can the slab be protected against frost in another way?

Good luck
Regina1991

Regina1991 schrieb:

Is an 80cm (31.5 inches) gravel layer necessary?

Yes. 80 cm (31.5 inches) is frost-free.

Regina1991 schrieb:

…or can the slab be protected against frost in other ways?

Take a look:

Regina1991 schrieb:

My soil report states … a frost-free foundation depth of 0.8 m (31.5 inches) must be observed.

Either a frost protection barrier or gravel. I think gravel is the more cost-effective option.

The impact of frost is extreme. Many people cannot imagine these tremendous forces.

While most materials contract when cold, water expands. This force cannot be measured. It is the anomaly of water.
Frost-resistant foundations are generally only possible starting at about 1.20 m (4 feet) deep, depending on the building area. 80 cm (31 inches) is too shallow. However, for a slab foundation, it can be sufficient, but it also depends on where you are building.

If you have layered or groundwater that freezes at 50 cm (20 inches), it will lift your house if the frost protection depth is insufficient.
To put it simply: cracks in the house and damage to structural elements result.

In spring, when the temperature rises, the soil subsides, and the house moves with it, causing it to shift. This acts like a small earthquake.

So, I would definitely recommend at least 80 cm (31 inches), probably a bit deeper.

I believe in 1996 we had a very cold winter here in central Germany with continuous frost from November to early April. The ground was frozen to a depth of between 1.20 m and 1.50 m (4 to 5 feet). Greetings to the heat pump supporters!

This caused massive damage, especially to water pipes, which often burst at a depth of 1.00 m (3 feet), also due to soil expansion and movement.

It always depends on the soil, the environment, etc. Sometimes 30 cm (12 inches) is enough, sometimes 1.20 m (4 feet) is not deep enough.

It depends on where your foundation slab is located. If the top edge of the slab is level with the ground surface, then 60cm (24 inches) of gravel is sufficient. If the bottom edge of the slab is at ground level, then at least 80cm (31 inches) of gravel should be used. When building with a frost skirt, the thickness of the gravel layer is less critical (provided you don’t have a high water table). The soil just needs to be stable. For example, if you have solid ground, theoretically a lean concrete layer would be enough, followed by the frost skirt. Do you have a soil report? It usually contains the foundation recommendations, which the excavator can also use to prepare the quote.

For example, we have a 1.00m (3 ft 3 in) frost skirt, with 1.00m (3 ft 3 in) of gravel on one side and 0.20m (8 inches) of gravel on the other. It really depends on the soil conditions… the excavator decides based on experience.

Oberhäslich schrieb:

Do you have a soil report?

That’s how the post starts! The recommendation of the geotechnical survey applies.

Regina1991 schrieb:

My soil report states

Regina1991 schrieb:

Is an 80cm (31.5 inches) gravel layer necessary, or can the slab be protected against frost in other ways?

A frost skirt or frost shield are also options. Each method has its own advantages and disadvantages. For example, a thick gravel base is suitable if the upper soil layer is not very load-bearing. A frost shield (insulation extending beyond the slab) can be implemented with little effort but has the drawback that water does not drain well due to the insulation overhang.
Usually, the geotechnical engineer will recommend the most appropriate solution.