Retrofit snow guards are for installing on existing roofs. You do not have to remove the slates, tiles, or shingles in order to install these snow guards! There are two general types: ones with notched shanks that slide up underneath the shingle and hook on the shingle nail (Berger, Gough, etc.), and ones with a hooked top that slide over the head of the slates (Mullane, Salvo, etc.).