03-12-2012, 03:02 PM
You really just need to get into the jack stud. On a new pre-hung door they supply a 4" screw. Remove one of the old screws and take it to hardware store and match for size and type (i.e. # 10 flat head Phillips wood screw) and then buy a pack of 4" versions. You will want to drill the hole a little deeper with the proper drill bit (undersized) otherwise you will probably strip the head before you get it in.