Foundations
Anton Pieterse writes: I want to know more about the construction of foundations.
Home DIY editor Ashley Stemmett replies: Foundations are a critical part of any building and therefore the plans for your house or structure must be drawn by a qualified architect or architectural designer who can properly specify the type and thickness of the foundations you need to pour, as foundations should take the loading that the structure needs to carry into account.
There are many types of foundation: differing ground conditions, the proximity of trees, soil types and even wind speeds will dictate what best suits the building. Generally, foundation trenches will be a minimum of 600mm deep (below natural ground level), 600mm wide for external walls and 450mm wide for internal walls. Double-storey foundations will vary in depth, and steel may also be used to deliver higher structural strength.
The thickness and height of the wall have a direct bearing on the proportions of the foundation. The general rule of thumb is that the width of the foundation should be twice that of the thickness of the wall (300mm-thick wall = 600mm-wide foundation). The thickness of the wall also depicts the thickness of the foundation, as well as the maximum height – here, the general rule is 10 times thickness of wall equals maximum height (300mm thick = max height of 3m). The thickness of the foundation is calculated on an equal ratio (300mm wall = 300mm thick foundation).
Once your foundation trenches have been dug (and approved), use a concrete mix of 2 pockets of cement, 3 wheelbarrows of 19mm concrete stone, 3 wheelbarrows of river sand plus about 80 litres of clean water. This ratio seems to vary from one source to the next but seems to be the average. It would be a good idea to check this with your architect beforehand. Remember that the strength of concrete is determined by the cement to water ratio. The more water you use, the weaker the mix. The ratio of cement, sand, stone, and water should be therefore be maintained during the entire process.
Foundations should ideally be allowed to cure for 28 days and should be 'watered' (kept wet) over the first few days. The slower the concrete dries, the harder it becomes.
