Strip Foundation: Everything You Need to Know
A strip foundation is a continuous reinforced concrete strip that runs along the entire perimeter of a building, including all load-bearing interior and exterior walls. This design evenly distributes the load from the structure onto the ground. Thanks to its reliability and relative simplicity of construction, a strip foundation is one of the most popular solutions in private home building, suitable for both light wooden houses and heavy brick and concrete homes.
Types of Strip Foundations
Strip foundations are classified according to several key parameters:
1. By Construction Method:
- Monolithic (Cast-in-Place): This type of foundation is poured directly on the construction site. The process involves tying a rebar cage and then pouring concrete, which ensures the integrity and high strength of the structure.
- Precast: Consists of precast reinforced concrete blocks that are delivered to the site and installed using a crane. The blocks are joined together with cement mortar.
2. By Depth of Laying:
- Shallow Foundation: Laid at a depth of 50 to 70 cm. This option is suitable for light structures such as wooden and frame houses, as well as for utility buildings on slightly heaving soils.
- Deep Foundation: This type of foundation is laid below the frost line. It is necessary for heavy houses with concrete or brick walls and heavy floor slabs, especially on heaving soils, to avoid building deformation due to frost heave.
Area of Application
Strip foundations are versatile and are used for the construction of:
- Houses with concrete, stone, or brick walls, the density of which exceeds 1000–1300 kg/m³.
- Buildings with heavy floor slabs, such as monolithic or precast reinforced concrete slabs.
- Houses with basements or crawl spaces, as the foundation walls form the ready walls of the basement area.
- Structures on sites with uneven terrain.
Advantages and Disadvantages
| Advantages | Disadvantages |
|---|---|
| Reliability and Durability: Capable of withstanding significant loads. | High Material Consumption: Requires a large volume of concrete and rebar. |
| Versatility: Suitable for most building types and soils. | Labor-Intensive: Requires significant excavation and formwork installation. |
| Possibility of Basement Construction: Foundation walls serve as walls for the basement or crawl space. | Higher Cost: Compared to pier or pile foundations. |
| Simple Technology: The construction technology is relatively simple compared to a slab foundation. | Complexity on Weak Soils: On very weak or peat soils, a combined foundation may be required. |
Key Construction Stages
- Layout: The axes of the future foundation are marked out on the site.
- Excavation: A trench of the required depth and width is dug.
- Creating the Base Layer: The bottom of the trench is filled with a layer of sand and gravel, which is compacted.
- Formwork Installation: The form for pouring concrete is assembled.
- Reinforcement: Inside the formwork, a frame of steel rebar is created to provide strength. Special attention is paid to the corners.
- Concrete Pouring: The formwork is filled with concrete mix.
- Waterproofing: After the concrete gains strength, it is covered with waterproofing materials to protect against moisture.
The correct choice of strip foundation type and adherence to construction technology are essential for the longevity and stability of the entire building.
How to Properly Build a Strip Foundation for a House Yourself
Building a strip foundation yourself is a complex task, but achievable with strict adherence to technology and the right approach. This step-by-step guide will help you understand all the nuances of the process.
Stage 1: Design and Calculations
Before commencing construction, thorough preparation is necessary.
- Geological Surveys: It is extremely important to determine the soil type on the plot, the groundwater level, and the frost penetration depth of the soil. This data directly influences the choice of strip foundation type (shallow or deep) and its parameters. Performing a full-scale survey yourself is difficult, so it is recommended to consult with specialists.
- Load Calculation: It is necessary to calculate the total weight of the future house, including walls, floor slabs, roofing, as well as snow and live loads. This will help determine the optimal width of the foundation strip.
- Project Creation: Based on geological data and load calculations, a foundation project is created. It specifies its dimensions, depth of laying, reinforcement scheme, and concrete grade.
Stage 2: Preparatory Work and Layout
- Site Preparation: The site must be cleared of debris, the topsoil removed, and leveled.
- Foundation Layout: Using stakes and string, the precise layout of the future foundation’s axes is marked according to the project. It is important to meticulously verify all angles (they must be exactly 90 degrees) and diagonals.
Stage 3: Excavation
- Trench Digging: A trench is dug along the marked layout. The trench walls should be as vertical as possible. The trench depth depends on the foundation type:
- Shallow Foundation: 50-70 cm.
- Deep Foundation: 20-30 cm below the frost line for your region.
- Creating the Base Layer: The bottom of the trench is filled with a layer of sand (15-20 cm), which is moistened with water and thoroughly compacted. A layer of crushed stone of the same thickness is then laid over the sand and also compacted. This base layer serves to evenly distribute the load onto the soil.
Stage 4: Formwork Installation
Formwork is the mold into which concrete will be poured.
- Materials: Most often, formwork is assembled from sawn timber boards 25-50 mm thick.
- Assembly: The formwork panels are installed along the edges of the trench. They must be securely fastened with braces and props to prevent deformation under the pressure of the concrete.
- Waterproofing: It is recommended to line the inner surface of the formwork with a dense polyethylene film or roofing felt. This will prevent the leakage of cementitious water from the concrete and protect the foundation from ground moisture.
Stage 5: Reinforcement
The reinforcing cage provides the concrete structure with strength against bending and tension.
- Materials: Ribbed steel rebar with a diameter of 10-16 mm is used for longitudinal bars, and smooth rebar with a diameter of 6-8 mm for transverse ties.
- Cage Tying: Longitudinal bars are laid in two tiers (top and bottom of the strip) and connected to each other with vertical and horizontal ties using tying wire. Welding is not recommended as it can weaken the metal.
- Protective Layer: It is important to ensure a gap of at least 5 cm between the rebar and the formwork walls (concrete cover). Special plastic ‘star’ spacers are used for this.
Stage 6: Concrete Pouring
- Concrete Selection: For strip foundations, concrete grades M200-M300 are typically used.
- Pouring: According to building codes, monolithic foundations must be poured in one continuous operation to avoid the formation of “cold joints,” which reduce the strength of the structure. For this purpose, it is optimal to order ready-mix concrete from a plant and use a concrete pump for its delivery.
- Compaction: During pouring, the concrete must be compacted with a poker vibrator. This will remove air bubbles and make the monolithic structure denser and more uniform.
Stage 7: Concrete Curing and Waterproofing
- Curing: After pouring, the concrete must be covered with plastic sheeting to prevent overly rapid moisture evaporation. In hot weather, it should be periodically watered during the first week.
- Formwork Removal: Formwork can be removed after approximately 7-10 days, once the concrete has gained sufficient strength.
- Waterproofing: After full drying (around 28 days), the exterior walls of the foundation must be treated with bituminous mastic or covered with rolled waterproofing materials to protect against groundwater.
Building a strip foundation is a crucial process on which the longevity of the entire house depends. If doubts arise at any stage, it is best to consult with professional builders.
Video Tutorial: How to Properly Build a Strip Foundation Yourself
Important Details Not to Miss
Appendix 1: Sleeves for Utilities
Why this is important: Laying sewer pipes, water pipes, or cables through an already cured monolithic foundation is an extremely laborious, expensive, and risky task. Drilling weakens the structure and can damage the rebar.
How to do it correctly:
- During the reinforcement stage: Before pouring concrete, it is necessary to plan for and install sleeves (sections of plastic or asbestos-cement pipes with a larger diameter than the utility pipe itself).
- Placement: Sleeves are installed in the formwork in strict accordance with the house’s utility network design. They should be placed below the frost line.
- Fixation: Sleeves are securely fastened to the rebar cage to prevent them from shifting during concrete pouring.
Appendix 2: Vents for Crawl Space Ventilation
Why this is important: If a house is planned with a wooden floor on joists (rather than a concrete slab on the ground), moisture will accumulate in the enclosed crawl space. This will inevitably lead to mold formation, rotting of wooden structures, and damage to the foundation itself.
How to do it correctly:
- Planning: In the above-ground part of the foundation (plinth/crawl space wall), it is necessary to provide ventilation openings — vents.
- Calculation and Placement: Vents are placed on opposite foundation walls to ensure cross-ventilation. The recommended distance is no more than 2-3 meters apart and at least 15 cm from ground level.
- Installation: Like utility sleeves, vents (most often sections of asbestos-cement or plastic pipes with a diameter of 100-150 mm) are laid during the formwork installation stage. In winter, they are covered with special grilles or covers to retain heat.
Appendix 3: Installation of a Drainage System
Why this is important: On sites with a high groundwater level (GWL) or on clayey soils that poorly drain water, moisture will accumulate around the foundation. Constant contact with water and significant frost heave can seriously damage or even destroy the concrete base.
How to do it correctly:
- Drainage Type: Perimeter footing drain is typically used.
- Methodology: Around the perimeter of the foundation, at its base level, a trench is dug with a slight slope. The bottom is lined with geotextile, a layer of crushed stone is laid, special drainage pipes (perforated) are installed, which are then covered with crushed stone and the remaining geotextile.
- Water Discharge: The collected water is diverted to a drainage well or storm sewer system.
Appendix 4: Backfilling and Insulated Apron
Why this is important: These two elements work in tandem to protect the foundation from frost heave forces and to divert surface water.
- Backfilling:
- Material: The spaces between the foundation walls and the trench should not be backfilled with previously excavated heaving soil (clay, loamy soil). Non-heaving material — sand or sand-gravel mixture (SGM) — is used for this purpose.
- Methodology: Backfilling is done in layers (20-30 cm at a time) with mandatory compaction of each layer. This will prevent future ground settlement around the house.
- Insulated Apron:
- Purpose: The apron diverts rainwater and meltwater away from the foundation walls. Insulating the apron (e.g., with extruded polystyrene foam boards) prevents the soil beneath it from freezing, which significantly reduces the forces of frost heave acting on the foundation.
- Construction: The apron is built with a slope away from the house (3-5%) and has a width of at least 80-100 cm.
Including these stages in the strip foundation construction plan will significantly increase its reliability and ensure the long service life of the entire structure.