Once a house is completed there are many building components sealed in walls and no longer visible. This includes structure, exterior wall openings, cooling and heating, electrical and plumbing. After moving into the house if there is a problem with these hidden systems then interior walls have to be opened up to gain visual access.
A pre-drywall inspection involves observing what eventually will be hidden inside the walls of your house. It is done before the walls are insulated and the drywall, also known as sheet rock or gypsum board, is installed. Hence the use of the term “pre-drywall”.
I find components fail over time for several reasons. Either because of age, a material defect or by it being improperly installed. Components come with installation instructions. Typically the builder is required to install the components per those installation instructions. This may not happen. So if the installation does not look plausible based on the inspectors experience, then this is the time to ask questions of the builder before it gets sealed up in a wall.
In addition to considering having a pre-drywall inspection it can be beneficial for the buyer of the home to take photos of the interior of the open structure. These photos should then be placed in a photo album for future reference. When a stain shows up on an interior surface you can refer to your photos as to whether there is an obvious water source there. Or some day, should you want to make an opening in a wall you will know what lies behind it. The photos can be a time saver for locating the cause of the problem and reduce the number of access holes you need to make.