Setting out for concrete footings or strip foundations

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This is an outline of a basic method of laying out a site prior to undertaking any small building work. It is aimed specifically at preparation for concrete strip foundations for block or brickwork, but it would also apply if you were using timber, for example. Besides involving concrete, this includes the other three banes of life; making accurate measurements, levelling and worst of all, digging! I have never made anything level in my life, but I don’t worry about it too much. Our natural environment isn’t flat, it has camber and undulates (up and down, side to side) so why place so much importance on it? Well, the first time I tried building something, I think I found out why. Basically, the more accurate you make this initial process, the easier it it is to make everything fit together later on.

Begin by imagining the site as a rectangle, looking down on it from above. If your site isn’t a rectangle, stop reading now! Choose a location to use as a base line, a reference from which you will establish the outline of the site. You might make the base line parallel to the main building, or at a completely different angle, it’s up to you. If your project comes under the scope of Building control and your local authority is involved, they should define this for you and it represents the boundary where no development is permitted.

Decide on the positioning of the front wall; this will normally fall on the base line. Measure its length. The side walls, obviously, will be at right angles to the base line wall. You can measure this angle with a builder’s square, which is a large right angled triangle. An alternative to this is to make measurements using the 3:4:5 ratio of a right angle triangle. I assume it was Pythagoras that came up with this. Drive a 2″x2″ post into the ground at the corner of the front and side walls. Knock a nail into the centre of the post to tie a line around.

Measure along the base line a distance of 3m. Drive in another post at this point and hook a tape measure over the nail on the top. Go back towards the corner, wrap the tape around the corner peg and continue down the side until you reach 7m. The 7m mark is the position of the third post. This gives you a right angle, the total outline length of the right angle triangle being 12m. Do this for the other three corners. Follow up each measurement by stretching a line between each post to mark out an outline.

Succeed this by installing permanent line markers using profile boards. The boards must be set at equal height to give you a perfectly level reference, if such a thing is possible. However, first you need to think about how wide and deep to dig the foundations. What are they for anyway? Strip foundations are fairly standard for building up to four storeys high but they are not suitable for all situations. They are placed centrally under the walls and help to dissipate the load, making the building more stable and less likely to move downwards or topple.

The thickness of the foundation should be at least equal to its projection beyond the width of the wall. The Building Regulations say that the thickness should not be less than 150mm or 6″. There is a table in the approved documents that recommends suitable dimensions for various ground conditions. The top surface of the strip foundation should be 400mm to 600mm below ground level to reduce the effects of frost heave (expansion).

Profile boards are placed at either side of each corner, approximately 2m away to give you room to work. They can be nailed together out of 5″x1″boards fixed to 2″x2″ posts. The profile of each board is defined by marking on it both the outer width of the channel and the width of the bricks that will form the wall. This can be done by various means such as saw kerfs, nails or paint. Stretch a line between the inside mark, the one that defines the position of the wall inside the foundation trench. When laying bricks, you can plumb down these lines with a spirit level. Once you have established the required width and depth and marked everything out on the profiles, it’s time to get digging.

You can make a gauge for checking the depth of the bottom of the trench by cutting a length of timber to the required length and placing it end to end from the site lines to the bottom of the trench. Therefore its length=trench depth+distance from ground level to site lines.

Penultimately, prior to mixing and laying the concrete, you must mark the depth of the top of the strip foundation. A simple way of doing this is to drive in posts at the bottom of the trench to the required height. Mix your concrete and get shovelling it into the trench. Level it out as required around the post markers. If you are using a wheel barrow, place a board at the edges of the trenches to support its weight. Potentially it could cause the edge to cave in, destroying your hard work.