C. Re-Roofing: FHA will accept a maximum of 3 layers of existing roofing. If more than 2 layers exist and repair is necessary, then all old roofing must be removed as part of the re-roofing. (4905.1 REV-1, 2-12)
http://portal.hud.gov/hudportal/HUD?src=/program_offices/housing/sfh/ref/sfhp1-24