It is essential that this job is done correctly, a concrete slab, dead level and square. It should be 6″/150mm longer and wider than the overall size of the garage. Made up of 4″/100mm of consolidated hardcore. 4″/100mm concrete on top of well prepared ground.
Set the first corner of the shuttering at 90 degrees. Check by marking 3ft along one edge and 4ft along the other edge. Measure across the two marks. This should measure 5ft. If under, move one edge out; if over move to exact dimension.
Set the rest of the shuttering to the correct dimension, check diagonals A and B which should be equal. It is important the base is level as well as square. We would recommend that you allow a toe of concrete approximately 9″/225mm wide around the base edge. This will stop the hardcore leaking out after the shuttering is removed. Make sure when tamping the concrete you get it as smooth as possible, no humps or hollows. Smooth trowel the perimeter edge.
A badly laid base will only lead to problems when it comes to erecting the building. An out of square base can lead to the garage running off the edge. A base sloping in its length, with a slight fall, will give staggered joints and an uneven line. A severe slope could mean none erection. A slope across the width could create problems for fitting doors, again if too severe, a none erection. Humps and bumps will create staggered joints and an uneven roof.