Originally Posted by
POBJOY
When the metal monoplane became the standard design of most aircraft, the wing loading and weight coupled with smaller wheels made grass an unreliable surface due to its loss of bearing strength when wet. The RAF realised this during the expansion period and could see that it was no good having a modern fighter force that could easily be grounded by the local weather conditions, so the classic 3 runway airfield became the new standard for them.
I am not sure if you are referring to hard runways there? Fighter Command operated from grass in the Battle of Britain and it wasn't until August 1942 that the standard RAF pattern for a 3 hard runway configuration was laid down, although this was primarily for Bomber Command.