If you need to replace a damaged or warped door but you are keeping the existing frame, it is important to correctly measure the door to allow for a smooth transition from the existing door to the new door.
Start by taking the door opening and hardware location measurements off of the existing frame. This helps ensure the new door will fit into the existing opening. In some cases, the building has shifted, which causes the frame to shift and the frame to have non-standard dimensions. A frame that was once a 36” wide square frame when new may now measure 36” at the top, but 35 7/8” at the bottom of the frame (see diagram below). If this is the case, the new door should be ordered using the smallest frame dimensions to prevent a bind in the existing frame.
Measuring off of the existing door can also be beneficial. The existing door measurements can help assure the frame dimensions are correct. Additionally, the measurements from the existing door can lead to the understanding of the door’s construction. For instance, most doors have beveled edges, meaning the door width of the push side and pull side differ. On a typical 36” door (a door fitting into a 36” opening), the pull side will measure about 35 3/4”, whereas, the push side tends to measure at a smaller dimension than the pull side.
Furthermore, taking note of the hardware attached to the existing door and frame is important. The hinge size, type of lock and the hardware considerations should be considered to allow the new door to fit and operate properly within the existing frame.
Information provided by Tom VanEss & Nicole Bolssen