Double top plates are used in wood frame construction. The top plate itself is the horizontal frame that runs along the top of joists. Depending on when the building itself was constructed; it will have a single or double top plate. The modern industry standard is a double top plate as it is critical to maintaining the integrity of a structure and provides support to joists for additions.
A top plate is typically visible at the top of the studs of each floor if it is stripped down to frame. If the house has several stories, the top plate will be seen supporting the framework joists themselves. When the house is finished, it is covered with interior walls and siding, making it invisible to the eye.
Double top plates (or even single ones) are vulnerable to bending, twisting or bowing beams. This can result in poor alignment or instability in a home. Thin beams can also result in insect infestation, heavy mold or mildew infestation or rot. This means that framed homes should be regularly inspected by a professional home inspector to confirm if any of these problems are occurring and to suggest immediate repairs.