As we all know UL is a very high standard for fire doors which is established in USA. Certainly there is also standard size for the product which is widely circulating around the market such as 36 in. x 80 in. and 36 in. x 84 in..
The mentioned dimensions above are inner frame size which is measured from inside the frame left to right and bottom to top. Which means the actual dimension of door slab is always smaller than what’s mentioned above cause according to standards of SDI, 1/8 in. is required for each gap between door slab and frame in left, right and top. On the bottom, a common choice for undercut is 3/4 in. or bigger in order to install a proper threshold or marble.
In the market of North America, most hollow metal frames are wrapping the construction wall no matter it’s a dry wall or masonry wall. Hence the construction opening is 2 in. wider than door width and 1 in. higher than door height. For a 36 in. x 80 in. door, the construction opening is 38 in. x 81 in. for door installation.
Additionally, the depth of wall in USA market is usually 4 3/4 in. so the total jamb width of hollow metal frame is 5 3/4 in. With simple anchors, site worker could install the frames very easily onto the wall and usually only one person is able to make this done.