If you are thinking of boarding your loft one of the main considerations is the thickness of the insulation. The recommended level of insulation now sits well above the level of the joist and this poses a problem, just how do you fix the boards into place when you can't see the joist?
The simple answer is to install raised flooring. This is constructed from 4x2 timbers laid at right angles to the existing joist work allowing the insulation to breathe and maintain airflow. Compressing the insulation too much can lead to a number of future problems and a small outlay initially to have a floor laid correctly is far cheaper than correcting potential future issues by not doing it.
Additionally raising the floor will generally lead to a much cleaner finish for the floor, rising over the top of obstacles that would usually have to be cut around i.e. pipework, binder beams or excessive cabling.