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All about Concrete Foundations for Smaller Walls

Before starting to dig any trenches, however, there are a couple of points to be taken into consideration. You shouldn’t dig any trenches too close to the house, as they could undermine the building. This is especially so in the case of older houses, where concrete might not have been used in the foundations. Also, check with your local Authority to see if you need planning permission. I’ve heard of instances where they have insisted on walls being demolished because of the fact that planning permission was not sought beforehand.

For single thickness walls, which are 100mm (4in) thick, 300mm (12in) strip foundations should be adequate. For double thick walls, you’ll need to dig out a trench about 450mm (18in) wide. Remember that for a single thickness wall over 450mm (18in) high, you’ll need to build piers alongside it every so often to strengthen it. Those should be about 1.8m (6ft) apart, and you should widen the trench by 200mm (8in) to accommodate them. Dig down about 350mm (14in) or further until you find hard ground. You should begin the brickwork below ground level, so the top of your concrete should be set accordingly. If it’s going to be a fairly long wall, it might be worth your while hiring a skip. You can get rid of the soil straightaway, so that it won’t be in your way later on. If the ground has a slope in it, you’ll have to step the foundations–work out the height and the length of each step by using the size of the bricks that you’re going to use, plus allowing a bit extra for the mortar joints and beds. If you’re going to have a 90 degree angle in your wall, use a builder’s square, (remember your geometry from schooldays), which has sides of 3:4:5

For the concrete, you can use ready mix, but it can be expensive if you’re doing a large area. You can easily mix your own, using ballast that is made up of gravel and sharp sand along with ordinary cement. For a small amount of concrete, you can mix it by hand, but if there’s a large area, it’s probably better to hire a mixer. If mixing by hand, have your ballast on concrete or a sheet of plywood, so that it will be easier to shovel. Set some bricks at the required height (top of the foundation) a couple of metres apart, tip in your concrete, working it in with a spade to get rid of any air bubbles, and use a stout piece of wood to smooth it out between the bricks. Repeat this for the whole length of the foundation.

Remember that weather conditions can affect your well-laid plans. If you’re not going to lay the concrete straightaway, it’s advisable to cover the trench with boards and sheeting to keep the rain off.