In wood frame construction, the top plate is the horizontal framing which runs along the top of the joists. Depending on when a building was constructed, it may have a single or double top plate, with the double version being the modern industry standard. This particular aspect of frame construction is critical to the integrity of the building, providing support to joists for additional floors, along with beams and rafters in the roof.
If a house is stripped down to the framing, the top plate will be readily visible at the top of the studs on each floor of the building. If the stripped building has multiple stories, the top plate can be seen supporting the framework of the joists, while the framing on the uppermost story will provide support to the rafters of the roof. As the house is finished, the top plate will be covered with siding and interior walls, becoming invisible to the eye.
Like all of the framing in a house, the top plate is vulnerable to an assortment of problems. If wood which has not been properly cured is used, the beams may bend, bow, or twist, pulling the house out of alignment and causing structural instability. Thinner beams may be prone to breakage or weakening which could cause the floor or roofline to sag, contributing to structural problems, and the top plate can also be eaten away by rot, insect infestation, or heavy mold and mildew.
Framed houses are routinely inspected once they have been framed to confirm that all of the framing is in good shape. A deformed top plate or a beam with noticeable problems such as large knotholes should be removed and replaced so that it does not cause problems in the future. Reputable contractors should be using wood which has been thoroughly cured, and the wood may need to be treated so that it will resist rot and insects.
When inspecting a home which has already been built, it can be challenging to identify problems with the framing, because the framing is not visible from the outside. A structural engineer can sometimes inspect a home and identify problems such as a deformed roof line or sagging floor which could signal issues with the framing, and inspections of this type are highly advised when making the decision to purchase a structure, so that critical flaws can be discovered before all of the paperwork has been signed.