Whether a foundation is needed for a simple home extension or for each house in a new development, it must be sound and suitable for the property where it is laid. Poorly constructed foundations can have serious consequences such as subsidence, which can be very expensive to rectify. A well-constructed foundation will form the basis for what is to many their dream home.
- Skill level:
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Things you need
- Foundation designs
- Plumbing layouts
- Utility/service layouts
- Ground investigation reports
- Batter boards
- Concrete smoother
- Steel reinforcement
- Trench blocks
Establish the type of foundation your house will need, using the help of an engineer and consulting any ground investigation reports. Foundation type depends on the type of soil, presence of trees and other factors. Most houses require a simple strip foundation, which includes footings usually measuring 12 inches wide by about 6 inches deep. Some foundations are more complicated, such as pile or raft foundations, and require more intricate building techniques.
Ensure all building plans are approved and have the relevant permission. Hire a competent groundworks contractor to assist you and provide advice, if you are unable to do the work yourself. A groundworks contractor specialises in all aspects of substructure, such as foundation and drainage construction.
Mark the centre line of the foundation onto the ground using spray paint. You will need relevant coordinates and engineering layouts to do this. Batter boards are used to mark out the corners.
Excavate to the required depth and width using equipment operated by a qualified operator.
Lay reinforced steel mesh into the excavated trench, and fill to the required depth with concrete. This will take a few days to cure or dry. The mesh, also called rebar, is held off the ground by small metal supports known as "chairs," and the concrete is poured over it. The chairs can also be used to ensure the rebar is evenly spaced so as to give maximum reinforcement.
Build up to the required level on top of the solid foundation using trench blocks. Some foundation designs will specify the use of standard blocks and engineering bricks. Ensure that any pipe work or cabling that goes through this wall is laid. Backfill the trench with soil.
Building the Foundation
Place a few inches of gravel in the void between the top of the block work and the ground level inside the foundation perimeter. Some sites may use French drains, which are drainage pipes permeated with holes. These can be covered over in gravel as well to allow rainwater to filter through to the pipes that carry the water away.
Lay a sheet of waterproof plastic over the gravel and the blocks. This acts as protection against rising moisture from the ground, which, in turn, can cause rot.
Fill the area with concrete to the required depth, and smooth out. This can be done by a process known as 'tamping', where the surface of the concrete is repeatedly struck by a large piece of wood held by two people. This flattens the concrete, which is then smoothed to a flat finish by using a wooden or plastic float.
The drying time of concrete differs depending upon the moisture content and depth, but about two to six weeks should suffice.
Constructing the Floor Slab
Tips and warnings
- You must get planning for construction work before commencing.
- Employ competent, qualified contractors and engineers to provide advice. The excavator, for example, should hold a license indicating that he has had the necessary training required to operate the machinery. Those without the correct qualifications may provide a substandard service or not adhere to good health and safety practices.
- Without a foundation design, the footings could be insufficient to support the building, leading to subsidence.
- A contractor who is not well-qualified or experienced can cause problems costing time and money through poor quality work.
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