A solid core door is just as the name implies, solid instead of hollow like many modern doors. A solid core door is more expensive than a hollow core door and it also has a higher fire rating. Solid core doors are commonly used in areas where more privacy is preferred. There are four basic core materials used for making solid core doors. Particle cores are the most common, and are made from pressed wood fibers, similar to hardboard or fiberboard. Stave lumber cores consist of many small wooden blocks joined together with glue, while structural composite lumber cores are made from layered wood chips, giving them a similar appearance to plywood. Doors used in fire-rated walls have a mineral-based core. These cores are typically made from pressed gypsum, which helps slow the spread of heat and flames during a fire.