Creep is a material property which is a simple way of saying that some materials creep and others don’t. For instance, steel exhibits very little creep, so little that it can be ignored in almost every application. On the other hand, wood and reinforced concrete creep a lot.
So what is creep and why should you care? The simple explanation is that what you apply a load to steel or reinforced concrete beam there will be an immediate deflection. There will also be a creep deflection that will become apparent over time. In fact, as time goes by the creep deflection can actually be greater, sometimes much greater, that the immediate deflection.
Look at the picture above and note that the horizontal wood member (called a rail) is sagging. The rail was horizontal when the fence was built. The sag due to creep is many times the sag that occurred initially when the only load carried by the rail was the dead load of the rail itself and the wood attached to it.
A funny thing about creep
Two funny things actually. As creep distortion grows the stress in the rail does not increase; it remains constant. So it is not a structural issue in a load-carrying sense. It is more of a cosmetic issue.
Creep is also not normally taken into account in the design process. It is basically ignored. Why? It does not become noticeable until at least 10 years and mitigating creep would have a significant adverse effect on the cost of building a house.