The constructional hearth is the patch of fireproof material this may be concrete, stone or maybe a tiled surface. This should be present in every wooden floored property it generally should appear flush with the floorboards in front of where a fireplace and hearth is to be fitted.
The constructional hearth is there to stop the joists of the floor extending beneath the fireplace. If they did, then they would dry like tinder and any small ember finding its way down to them from the fireplace would cause a much larger fire than you ever intended. A constructional hearth will usually consist of fireproof material at least 6" deep, around 48" wide and extending out approximately 18".