|
You will need:
- Measuring tape
- Timber stakes
- Timber or square tube for profiles
- Nails and hammer
- String and plumb bob
- Spirit level and line level
- 12mm clear tube water level or dumpy level (optional)
- Builder's square
- Spade
Here's how:
1. Erect timber profiles 1m beyond the outline of the job. Tie string lines to the nails in the profiles to act as guides to the width of the foundation trench and walls.
2. Ensure the corners are 90 degrees with a builder's square or use the 3-4-5 method. From the corner measure down one side 3m and down the other side 4m (or any multiple of these). The hypotenuse (or diagonal) should equal 5m if you've made a right angle.
To be accurate, measurements should always be taken on the level, even on a slope. Use a spirit level to ensure the tape measure is level.
Establishing levels
3. Accurate levels are essential. Put a timber stake in the ground at each corner of the job. Put a mark on one stake and carry it around to the other stakes, using your choice of level to adjust the string lines to the same level.
Hire a qualified surveyor for large projects and any work near boundaries.
Foundation trenches and formwork
4. Concrete footings are usually twice the width of the wall and as deep as they are wide.
To find the level for the top of the footings (about 100mm below ground level), measure down an equal distance from the level mark on each stake. If the ground is not level, step the footings.
5. Dig within the string lines, keeping the sides of the trench vertical and straight.
6. If the footing rises above ground level, you'll need to erect temporary formwork to hold the concrete in shape until it hardens. The formwork foundation trench should be 100mm wider than the footings.
Footings - Building And Pouring
With the trenches dug, the next step is to mess around with concrete.
You will need:
- Timber for formwork
- Timber stakes, nails and hammer
- Steel reinforcement (mesh or bars, stirrups and bar chairs)
- Tie-wire and pliers
- Cement, sand and coarse aggregate
- Concrete mixer (optional)
- Wheelbarrow, shovel and gumboots
- Straight edge
- Steel and wooden floats
Steel reinforcement
Concrete should be reinforced with two layers of steel bars or trench mesh, placed as near the top and bottom of the concrete as possible and at least 60mm from the edges. Hold the bars in place with stirrups suspended by wire from timber beams and overlap joins by 450mm.
The concrete mix (4:2.5:1)
When mixed, the concrete should hold its shape without slumping (too wet) or crumbling (too dry). The usual mix is four parts coarse aggregate, such as gravel, two and a half parts sand and one part cement (4:2.5:1), with enough water to get it to a plastic state.
You can have ready-mixed concrete delivered in multiples of 0.2 cubic metres. When ordering, advise the type needed (whether it's for footings or slab, etc).
Quantities
Per cubic metre of concrete you'll need 0.8 cubic metres coarse aggregate, 0.5 cubic metres sand, six and a half bags (40kg) cement.
Hand mixing
To mix small quantities of concrete, you'll need a shovel, a metal wheelbarrow or a flat, easily cleaned surface.
1. Thoroughly mix the cement, sand and coarse aggregate together while they are still dry.
2. Make a well in the centre of the dry mix. Add water and combine until the mix has achieved an even colour and texture.
Pouring the footings
Prepare any formwork the day before and have all the equipment you'll need to carry, spread and level the concrete ready before you start. Get going early to have concrete set before dark.
Here's how:
1. Start pouring concrete at the lowest point on the site and spread it with a shovel. Regularly check the level as you go. Make sure the concrete is packed under the steel reinforcement so it doesn't drop. If the stirrups are suspended from a beam, cut the tie-wire with the shovel as you work. Step the footings if necessary.
2. Finish the concrete about 100mm below ground level and smooth with a float. A steel float gives a smoother finish than a wooden float.
3. Leave to cure for at least two days or the concrete may crack.
|
