IntroductionThe concrete slab foundation is not the cheapest foundation but it is a very sound foundation. If you are building on wet, soft or unstable ground, this type of foundation is ideal. It is much stronger than the Pier and Beam foundation and will last substantially longer.PreparationMake up a timber box to the exact dimensions of your proposed floor slab using 6" or 4" X 2" timber (measure from the inside of the frame).

I normally do this on a level part of the garden. You can ensure the box frame is square by measuring the diagonals - they should be exactly equal. When you are sure the frame is square, nail a length of timber on top of the frame from Flange wood screws corner to corner. This will keep the frame square and prevent it from moving.Lay the box on the ground and position it exactly where you want it. Take your time doing this as this is your last chance to get the position right. Secure the frame temporarily with wooden pegs.Using a spade, mark the ground all around the box and then move the box out of the way.Clear the ground within the marks of all vegetable matter and topsoil and roughly level the ground. Reposition the box and make sure the diagonals are equal which will mean the box is square.Level the box with a spirit level and secure with wooden pegs. It is important to make sure the box frame is level if you want a level floor. However some of you may want to deliberately incorporate a small slope in the frame if you are going to have a wet floor and need the water to run off.Dig a small small trench 8" wide X 4" deep. This will help strengthen the slab when it is poured.Use a long, straight piece of timber and set it across the box at one end. 

the timber progressively over the box frame and measure frequently to ensure you have 4" depth all over.Pour the concrete.Mixing and pouring concrete will be covered in another article.