Retrofit snow guards are installed 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: notched shanks which slide underneath the shingle and hook on the shingle nail, or hooked shanks that slide over the head of the slates.
Note: the notched retrofit snow guards have a "reach," which is the maximum distance they will slide under a slate and still hook a nail.

Measure your slate from the bottom edge to the nail hole in order to determine what reach is required for your particular slate. Some imported slates have high holes and some longer slates (22" long and 24" long) may be too long for the snow guards you're looking at. If your reach is too high, use Mullane or Salvo retrofit snow guards, or custom order Gough retrofit snow guards with extra shank length. 

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