SU-4

('su' in the fourth tone)

There are 30 standard Chinese characters with a 'su' pronunciation. There are no predominant sound-indicating components.

宿 = to stay overnight

宿舍 = su(4)she(4) = dormitory

舍 = she(4) = house, hut

At least, the association with a dormitory is easy to remember for the character 宿. It is composed of a roof, 宀 , a sideman, 亻, and the character for 'one hundred', 百 .  It's obvious that when there are one hundred people under what is just a roof, then it must be a dormitory. The simplicity of the place is emphasised by the character for hut,舍 .