The Normal levels of damp –

Plaster – .2-1%

LIghweight concrete – >5%

Timber 10 – 20%

Test for Damp:   By sight, smell, possibly taste, touch and sound.  A calibrated damp meter should be used as follows:  Electrical Capacitance as a guide to damp below the surface and when located by electrical resistance as a guide to damp on and just below the surface. 

Pictures below show spatter caused damp, a dripping overflow pipe has caused substantial damage to this property, rising damp, penetrating damp etc