Q. What are the reasons for cracks/gaps at the joint of doorframe and wall?

Wood absorbs moisture from air when the moisture content is high in the atmosphere and as a result it expands causing cracks in the wall. During dry atmosphere the wood contracts and a gap is formed between the wall and the frame of the door.

Q. Which wood should be used for making of doors and windows?

A good quality wood like, Holyoke, deodar, teak etc should be used. The wood should be properly seasoned.