Q: What is the maximum you can have between the bottom of door and the thresholds for a fire-rated door assembly? I have some stairwell doors with 1 inch to 1 ¼ inch gap between the bottom of the door and the threshold.
A: That will be a problem. According to section 184.108.40.206 of the 2012 Life Safety Code, you must install fire-rated door assemblies in accordance with NFPA 80-2010. Section 220.127.116.11 of NFPA 80-2010 says the clearance under the bottom of the door shall be a maximum of ¾ inch. Also, section 18.104.22.168.2 of the 2012 Life Safety Code says you must maintain the fire rated door assemblies in accordance with NFPA 80.
Any clearance under the bottom of the door (while it is in the closed position) that exceeds ¾ inch will be considered non-compliant. There are after-market devices available that you can install on the door to fill that gap, but you must be very careful as you can only install devices that have been listed by an independent testing laboratory (i.e. UL, FM Approval, Intertek) that have been approved for that purpose.
Keep in mind that there are limitations on the door material, amount of clearance, and the rating. Here are links to information on 3 products suggested to me by Lori Greene (see her website at www.idighardware.com), but there may be more: