Concerning slate window stools: Is there any particular reason to choose a mortar setting bed over adhesive? (If it’s not obvious, this is for interior installation).
I’d have concerns about leveling and adhesion. Can the stool be clipped or otherwise anchored as well? I’ve had too many come loose over time.
You might want to contact one of the setting system manufacturers. I’d probably call someone at Custom Building Products. I believe Terry O’Hara still covers the DC area. His email address is ohara@cbpmail.net.
It depends on the substrate: I typically specify a mortar setting for stone sills set on masonry, since leveling is usually necessary & individual masonry units don’t typically provide a uniform substrate for thin adhesive setting. In a framed wall condition, it is more practical to provide a uniform, level substrate using cement board, plywood or lumber. A water-cleanable epoxy adhesive of the type used to adhered stone wall base to drywall or cement board can then be used to set the sill.
I also try to specify that sills be provided as single units without joints, if feasible with sizes indicated.
Thanks for your input Ken and Jeffrey!
Considering it’s framed construction, I’m thinking it’s best to go with adhesive, especially since this is the only stone in the project.
I would probably use medium-bed tile setting mortar as my adhesive. They can make up some degree of irregularity, and will adhere to stone and many substrates. You might check with one of the major manufacturers of these materials.