Building a garage for a car is a serious investment of money and effort, where each stage requires a precise engineering approach. Foundation is the basis of the entire structure, and errors when laying it can lead to cracks in the walls or even to the collapse of the structure. Garages from foam blocks popular due to their low thermal conductivity and lightness of the material, but they are extremely sensitive to movements of the base.
Many novice builders are wondering: is it possible to save on excavation work and make a shallow belt, or is it necessary to dig below the freezing level? The answer to this question depends on many factors, including the type of soil, groundwater level and the weight of the structure itself. In this article we will look at all the nuances that will help you make the right decision and build reliable storage for your car.
Before you pick up a shovel, you need to conduct a thorough analysis of the area. Ignoring the geological features of the area is the most common cause of future foundation problems. You must understand that freezing depth soil in your region is a key parameter on which the entire project will be based.
Factors influencing the choice of depth
Determining the optimal trench depth is not fortune telling, but an engineering calculation. The first and most important factor is soil type on your site. Heaving clays expand in volume when they freeze and can push a lightweight garage foundation upward, creating dangerous stresses.
The second critical parameter is the level of occurrence groundwater. If the water comes close to the surface, you will have to not only dig deeper, but also organize a complex drainage system. Ignoring this fact will result in the garage standing in water in the spring, and in winter the concrete will collapse due to freezing and thawing cycles.
⚠️ Warning: Never rely solely on your neighbors' stories about the depth of their foundations. Soil can change its characteristics even within one area, especially if there are elevation changes or bodies of water nearby.
The weight of the structure also dictates its conditions. Foam block is a lightweight material, but if you are planning a two-story garage with an attic or a heavy reinforced concrete floor slab, the load on the base will increase many times over. In such cases, it is absolutely impossible to save on laying depth.
Calculation of soil freezing depth by region
The standard freezing depth is a calculated value, which is determined using formulas that take into account the climatic characteristics of the region. For different cities, these values differ significantly, and you need to build a garage in Moscow and Sochi in different ways.
Below is a table with approximate freezing depths for various types of soil in some regions. This data will help you form an initial idea of the scale of the excavation work.
| Region | Clay and loam (cm) | Sandy loam (cm) | Sands (cm) |
|---|---|---|---|
| Moscow and region | 120-130 | 140-150 | 160-170 |
| Saint Petersburg | 120-130 | 140-150 | 160-170 |
| Yekaterinburg | 180-190 | 200-210 | 220-230 |
| Novosibirsk | 190-200 | 210-220 | 230-240 |
| Krasnodar | 70-80 | 90-100 | 110-120 |
| Moscow and region | 120-130 | 140-150 | 160-170 |
| Saint Petersburg | 120-130 | 140-150 | 160-170 |
| Yekaterinburg | 180-190 | 200-210 | 220-230 |
| Novosibirsk | 190-200 | 210-220 | 230-240 |
| Krasnodar | 70-80 | 90-100 | 110-120 |
It is important to understand that the actual freezing depth depends not only on the climate, but also on the presence of snow and vegetation. Snow cover acts as a heat insulator, and the ground freezes less under the snowdrifts. However, when building a new garage, you cannot count on snow, since in winter it may simply not be there or it will be removed.
Types of foundations for garages made of foam blocks
The choice of foundation type directly affects the required digging depth. For garages made of foam blocks, strip or slab bases are most often used, each of which has its own deepening characteristics.
Strip foundation is a classic solution. It consists of a continuous reinforced concrete strip under all load-bearing walls. This type of foundation can be buried (below the freezing level) or shallow. The first option requires digging deep trenches and installing formwork to a great height, which is labor-intensive, but reliable for heaving soils.
Slab foundation (floating slab) is often chosen for difficult soils. In this case, the pit is dug throughout the entire area of the garage, but not necessarily very deep. The slab “floats” with the soil, compensating for its movements. For a lightweight garage made of foam blocks, a slab thickness of 20-30 cm and a sand cushion are often sufficient.
- 🏗️ The strip foundation is ideal for garages with an inspection hole or basement, as the strip walls serve as the walls of these rooms.
- 🧱 A slab foundation requires less excavation work in depth, but more materials (concrete and reinforcement) in area.
- ⚖️ The columnar foundation for foam blocks is rarely used, only on very dense soils, since the foam block does not tolerate uneven shrinkage of the pillars well.
When choosing between tape and slab, also consider the need for access to the garage. A slab foundation immediately creates a ready-made foundation for a car to drive into, whereas for a strip foundation, the floor will have to be concreted separately.
Shallow vs deep foundation
This is the eternal dilemma of the private builder. A buried foundation, the base of which is below the freezing depth, is guaranteed to be protected from the forces of frost heaving. However, for a lightweight one-story garage made of foam blocks, such measures are often redundant and economically unfeasible.
Shallow foundation (MZLF) is laid to a depth of 40 to 70 cm. Its operating principle is different: it does not resist heaving forces, but works together with the soil. To prevent such a foundation from “sinking,” it is necessary to fulfill a number of conditions: replace the heaving soil underneath with a non-heaving cushion, make a high-quality blind area and insulate the perimeter.
⚠️ Attention: The use of a shallow foundation is unacceptable if the groundwater level is above the freezing depth. In this case, the water will soak the base, and when it freezes, the garage will simply “fly out” of the ground.
If you choose a shallow depth option, be sure to include reinforcement tapes. Four reinforcement bars with a diameter of 12 mm, tied into a frame, will allow the foundation to work as a single beam and resist breaking when the ground moves.
Step-by-step instructions for preparing the base
The process of creating a foundation for a garage requires sequential operations. Violation of technology at any stage can nullify all efforts. Start by marking out the area using pegs and string, following the design closely.
After removing the fertile layer of soil, start digging a trench. The bottom of the trench must be leveled horizontally to the nearest centimeter. This is critical for even load distribution.
A cushion of sand and crushed stone serves as a drainage and compensator. The sand is spilled with water and compacted to a “monolith” state. Crushed stone on top creates a rigid base. Only after this can formwork be installed and concrete poured.
Concreting should be carried out continuously to avoid the formation of “cold joints”, which reduce the strength of the structure. After pouring, concrete needs care: in hot weather it is watered and covered with film to prevent drying out.
Waterproofing and insulation of the foundation
Foam blocks are a hygroscopic material; they easily absorb moisture from the soil and air. Therefore, high-quality waterproofing foundation is not just a recommendation, but a mandatory requirement. Without protection, moisture will rise through the capillaries of the concrete and destroy the first rows of masonry.
For horizontal waterproofing, which cuts off the capillary rise of moisture, two layers of roofing material or modern bitumen-based built-up materials are usually used. The vertical walls of the foundation are also protected with coating waterproofing, especially if the garage is located on a slope or in a low area.
Insulating the foundation from the outside (with extruded polystyrene foam) has a dual effect: it retains heat in the garage (if it is heated) and, more importantly, reduces the depth of freezing of the soil around the foundation. This reduces the forces of frost heaving and extends the service life of the structure.
Do I need to do drainage around my garage foundation?
Drainage is necessary if the groundwater level is high or the garage is built on a slope where water can be collected. In dry sandy soils on a flat area, you can get by with high-quality blind area and waterproofing.
Is it possible to use used blocks for the foundation?
You can use used concrete blocks (FBC), but only if they do not have cracks or chips. However, for a lightweight garage made of foam blocks, it is often more profitable and easier to pour a monolithic tape than to search for, deliver and install heavy used elements.
How many days after pouring can foam blocks be laid?
Concrete strength gains within 28 days. However, you can begin laying walls after 7-10 days, when the concrete has gained 70% strength, sufficient to support the weight of the first rows of blocks. The foundation will only take full load in a month.