The construction of a car storage space always begins with the preparation of the foundation, even if we are talking about a lightweight metal structure. Many owners mistakenly believe that it is enough to simply level the site and install a purchased set of walls and roof. However, ignoring the requirements for soil and load distribution can lead to skewing of the posts, jamming of the gate and corrosion of the lower crown in the shortest possible time.
High quality foundation performs two critical functions: it evenly distributes the weight of the structure over the ground area and insulates the metal from moisture. Correctly chosen construction technology allows you to avoid costly repairs in the future. In this article we will analyze in detail all stages of work, from choosing the type of foundation to pouring concrete.
It is worth considering that even light metal garage creates a significant load on the soil, especially in winter, when frost heaving forces act. Unprepared soil can “float”, dragging along one of the supports, which will disrupt the geometry of the entire structure. That is why the issue of preparing the base cannot be ignored.
Soil analysis and selection of foundation type
The first step towards reliable car storage is to study the soil on your site. The choice of foundation design directly depends on the type of soil, freezing depth and groundwater level. If you ignore this step, then even the strongest concrete can crack due to seasonal movements of the earth.
There are several main types of foundations, each of which has its own advantages and disadvantages. For metal garages, strip, slab or column options are most often used. The choice depends on the weight of the structure and budget.
If the site has a high groundwater level, then buried structures will require high-quality waterproofing and drainage. Otherwise, water will wash away the base, causing corrosion of the reinforcement and destruction of concrete. Light soils allow for simpler and cheaper solutions.
- 🏗️ Strip foundation - ideal for heavy metal structures with a concrete floor inside, provides high stability.
- 🧱 Slab foundation - creates a finished floor covering, evenly distributes the load, but requires more concrete and reinforcement.
- 📐 Columnar foundation - the most economical option for light frame garages, installed on dense soils.
⚠️ Attention: Before starting digging, be sure to check the location of underground communications. Damage to a gas pipe or cable can lead to an emergency and large fines.
How to determine the type of soil yourself?
Take a soil sample from a depth of 1.5 meters. If you can roll a rope out of wet soil and roll it into a ring without cracks, it is clay. If the rope crumbles, it is sandy loam or sand. Clay soils are more susceptible to heaving.
Calculation of materials and preparation of tools
Proper preparation of estimates allows you to avoid downtime and unnecessary costs. To build a foundation, you will need not only concrete, but also materials for formwork, reinforcement and waterproofing. It is strictly not recommended to save on reinforcement or cement brand, as this reduces the load-bearing capacity.
The optimal grade of concrete for a garage foundation is considered M200 or M250. The use of lower grades is only possible for temporary structures or on very dense rocky soils, which is rare. For reinforcement, ribbed reinforcement with a diameter of 10-12 mm is best suited.
Don't forget to prepare all the necessary tools in advance. Lack of the necessary equipment in the midst of work can significantly delay the process. Check the serviceability of the concrete mixer, the presence of buckets, shovels and measuring instruments.
| Material | Purpose | Approximate consumption (per 10 m²) |
|---|---|---|
| Concrete M200 | Pouring the foundation body | 2.5 - 3 m³ |
| Reinforcement 12 mm | Reinforcing frame | 150 - 200 m.p. |
| Edged board | Formwork | 0.5 m³ |
| Gidroizol/Roofing felt | Cut-off waterproofing | 15 m² |
Order concrete with a reserve of 5-10%. Unforeseen losses during pouring, uneven trenches or shrinkage of the mixture can lead to a lack of mortar, which will create a cold joint and weaken the structure.
Site marking and excavation work
Accurate markings are the key to ensuring that the garage walls will stand level and the gate will open without distortion. Pegs and construction cord are used for this. It is important to maintain right angles by checking the diagonals of the rectangle: they must be absolutely equal.
After fixing the contours, they begin to remove the fertile soil layer. Vegetation, roots and humus must be completely removed, since organic matter rots over time, forming voids under the foundation. This can lead to subsidence of the structure.
The depth of the trench depends on the type of foundation and the depth of soil freezing in your region. For a strip base, they usually dig a trench 50-70 cm deep, but on heaving soils it may be necessary to dig below the freezing point. The bottom of the trench is carefully leveled and compacted.
- 📏 Mark the external and internal boundaries of the trench, taking into account the thickness of the formwork walls.
- 🌱 Remove the top layer of soil (15-20 cm) over the entire area of the future garage.
- 🔨 Compact the bottom of the trench with a vibrating plate or manual tamper until it is solid.
The width of the trench should be 10-15 cm greater than the width of the future foundation on each side. This space is necessary for the installation and subsequent dismantling of formwork boards. If the walls of the trench are crumbling, they can be temporarily reinforced with sheets of plywood.
Installation of formwork and reinforcement
The formwork gives shape to the concrete monolith and prevents the mortar from spreading. For its manufacture, edged boards, plywood or special boards are used. The structure must be rigid to withstand the pressure of liquid concrete, so it is strengthened from the outside with supports and screeds.
Reinforcement is the foundation skeleton that absorbs tensile loads. Without a metal frame, a concrete stone will easily rub when the ground moves. Reinforcing rods are tied with wire into a spatial frame, avoiding welding, which weakens the metal at the seams.
It is important to provide a protective layer of concrete around the reinforcement. The metal should not touch the ground or the walls of the formwork, otherwise it will quickly rust. To do this, use special plastic clamps or linings made of broken bricks. The frame is assembled directly in the trench or knitted separately and lowered entirely.
☑️ Check before pouring
It is recommended to cover the inner surface of wooden formwork with plastic film or lubricate it with waste oil. This will prevent moisture from being absorbed from the concrete into the wood and will make it easier to dismantle the structure after hardening. The boards must be tightly fitted so that cement laitance does not leak out through the cracks.
Pouring concrete and caring for the monolith
The filling process must be continuous. If you take long breaks, layers will form that will not adhere into a single whole, which will sharply reduce the strength of the foundation. Therefore, it is better to order a mixer with ready-made concrete or organize work with several concrete mixers at the same time.
After laying, the mixture must be compacted. To do this, use a deep vibrator or, in extreme cases, bayonet with a reinforcing bar. This allows you to remove air bubbles and ensure a tight fit of the mortar around the reinforcement. The surface of the foundation is leveled with a rule or trowel.
Strengthening concrete is a long process. In the first days, it is critical to prevent the surface from drying out. If concrete dries too quickly, it will not have time to gain its design strength and will become covered with a network of cracks. Therefore, in hot weather, it is covered with film and periodically watered.
⚠️ Attention: Full load on the foundation can be applied only after reaching 100% strength, which takes 28 days under normal conditions. Early installation of walls can lead to deformation of fresh concrete.
The main secret of durability is not the speed of pouring, but proper care of the concrete in the first 7-10 days. Regular hydration is more important than the brand of cement.
Waterproofing and garage installation
After the concrete has completely dried and the formwork has been dismantled, waterproofing begins. Concrete has a porous structure and is able to absorb moisture from the soil, transferring it to the metal structures of the garage. To prevent corrosion, the upper plane and side walls of the foundation are insulated.
The most common method is pasting with bituminous materials (roofing felt, hydroisol, technoNIKOL). First, the surface is coated with bitumen mastic, then the rolled material is fused or glued. It is important to overlap the canvases at least 10 cm.
Only after completing all waterproofing work can you begin installing a metal garage. The lower frame of the structure is installed strictly horizontally. A layer of waterproofing must be laid between the metal and concrete, even if it is already on the foundation.
- 🧹 Clean the surface of the foundation from dirt and dust before applying mastic.
- 🔥 Apply bitumen primer to improve waterproofing adhesion.
- 🧱 Check the horizontality of the upper plane with a level before installing the walls.
The metal frame is fastened to the foundation using anchor bolts, which are often installed at the pouring stage, or large-diameter dowel-nails. Reliable fixation will prevent the roof from being torn off by the wind and ensure the stability of the entire structure.
Is it possible to fill the foundation in parts?
Theoretically it is possible, but this requires strict adherence to technology. If you stop without waiting for the layer to set (about 2-3 hours), then a “cold seam” will form - a weak point. When resuming work, the surface of the previous layer must be clean and rough. However, for a garage, it is highly recommended to pour the foundation in one go.
Do I need to insulate the garage foundation?
If the garage is heated, this is a must. Insulation (usually extruded polystyrene foam) prevents the floor from freezing and reduces heat loss. For a cold metal garage, insulating the foundation has no practical meaning; high-quality waterproofing is sufficient.
How thick should the slab be?
For a passenger car, the optimal thickness of a monolithic slab is 15-20 cm. If you plan to store heavy equipment or a truck, the thickness should be increased to 25 cm with reinforced reinforcement.