Probably the biggest problem contractors have with void formers is being sure that they will maintain their proper and rigid location before, during and after the placing and vibration of the mix. A secondary problem, but one that is also very important, is to ensure that the concrete flows under and completely surrounds the former. Normally a designer will specify that the voids must run in the same direction as the main reinforcement to take advantage of the greater bulk of concrete between the formers. A 2 inch minimum concrete cover over a former is necessary as protection for secondary reinforcement and against impact penetration of the void. When two or more formers are used side by side, the minimum spacing between them should be 2 inches to allow penetration by a vibrator head. The mix should be spread evenly across the form during placing rather than being allowed to pile up at one or two points for working out with the vibrator. The manufacturers of void formers are all prepared to recommend the correct method for locating and typing down their product, and many of them also offer a proprietary type of chair to facilitate the task. These recommendations must be observed carefully, since unless tie down is rigid, the former will invariably tend to float on the concrete and move during vibration. With removable formers probably the easiest method of location is by means of small precast spacing blocks wired to the main reinforcement vial loops casts into the back of the block.