Wood doors 
Wood doors come in many types. Some are solid; others may be solid panels, or hollow core. Wood doors commonly used in closet spaces are bi-fold, solid, and sometimes louvered sliding units.

Metal doors 
These doors are becoming quite popular, especially as main entry doors to the residence and as the fire separator door from the attached garage to the living space. Fire-rated door requirements also include fire-rated jambs. Check the door to see that it is properly fitted in the frame and that the weather stripping is appropriately mounted. Quite often, magnetic weather stripping is utilized in these doors. Check for poor fitting doors, locking hardware and hinge problems. Other things to look out for is: threshold height; that it is fit properly to the door and is secure; condition of the weather stripping; dents; scratches; and signs of rust or other oxidation. 

Locking Mechanisms
Locking mechanisms are secured with different locking mechanisms, including: (1) key-in-knob; (2) thumb latch and dead bolts; (3) security bars on patio windows; and (4) pins inserted through window casings, etc.

Defects

Binding from movement or improper fitting

Doors should be checked for binding, which can usually be corrected by planing and sanding, however, some situations may require cutting or trimming the door. Shimming the hinges may be necessary for better alignment.

Missing hardware

The most frequently discovered defects are missing or misaligned strike plates and loose locksets.

Holes or dents in the doors

Doors should be evaluated on both sides to detect holes or dents, particularly in paneled and hollow-core doors.

Rot and/or deterioration

Rot/deterioration from moisture and insects most frequently exists at exterior door jambs, sills and thresholds. 

Weather stripping

A good method to check for weather stripping is to close the door and observe from the interior. Signs of light reflecting through at the doorsill or jambs suggest that the door is not fit as well as it should be. Sunny days will reflect more light than cloudy days.

Interior doors are often hollow core or builder’s line quality.

Exterior doors are built and glued for exterior use.  It is important that interior doors not be used for exterior applications. Interior doors have a tendency to delaminate when exposed to exterior elements.  Delaminating of an interior door is typically caused by moisture; however, poor manufacturing may be the cause in some cases.