Of the acceptable methods for repairing slate roofs, the use of strap hangers is not one of them. It’s ironic, then, that we see so many strap hanger “repairs” on slate, tile and hard-shingle roofs.
A strap hanger is a strip of copper, or any sheet metal, nailed to the roof where the roof shingle is missing.The bottom of the strap is left to hang below the replacement shingle, then it’s bent up to hold the shingle in place. The problem with this system is two-fold: the strap hangers look bad. They constitute a repair that leaves unsightly repair evidence on the surface of the roof. In addition, the strap hangers won’t work when something like ice or snow slides down the roof because the weight of the sliding material will flatten out the strap and the roof shingle will slide out.
Instead, use a slate hook or use the “nail and bib” repair technique. A slate hook is a copper or stainless steel hook that will not bend open and is virtually invisible on the roof. The nail and bib technique requires nailing the replacement slate in the slot between the overlying slates, then sliding a bib flashing under the slates but over the nail. Again, this repair is virtually invisible when done right.