secured to a frame in a manner known to those of ordinary skill in the art fig4 depicts an arrangement of non combustible core 412 and 414 supporting structure 422 424 and 426 and 430 in door 400 according to another embodiment of the invention according to one embodiment door 400 has a height of 8 feet 0 inches and a width of 4 feet and 1 16 inch in this embodiment supporting structure 422 may have a width of between 1 and u215c inch and 8 inches and preferably has a width of 5 and u00bd inches supporting structure 424 has a height of between 4 and 6 inches and preferably a height of 5 inches supporting structure 426 has a height between 5 and 12 inches and preferably a height of 10 inches or it may not be provided note that in this embodiment non combustible core 412 and 414 have different dimensions this is so that a doorknob can be located at an appropriate position thus supporting structure 426 may be positioned in door 400 accordingly for example structure 426 may be positioned with its centerline at 41 u2033 from the bottom of the door but this may vary as needed intumescent material 430 has a thickness of between 1 16 inch and u00bd inch and in one embodiment u215b inch cover 450 such as a hardwood veneer or plastic laminate covers intumescent material 430 in the stiles of door 400 and preferably has a thickness of 5 16 inch before trimming after door 400 is trimmed to finished size cover 450 may have a thickness.