To install window sills, door trim, or any other architectural details, be sure that all of the proper flashings have been installed before beginning mortar work. When finishing openings or penetration points such as light boxes or spigots, start by installing window sills first.
To ensure proper spacing for the sill and placement of the bracing you’ll need when mortaring and installing window sills, follow these simple guidelines:
Window sill installation tips: use a 2x4 ledger
Position a 2x4 straight-edge bracing board just under the pre-positioned sill. Check for level, and mark the location either at the top of the 2x4 or the bottom of the sill.
Now you’re ready to attach the bracing board to the wall. Once correctly placed, screw the level 2x4 bracing board to studs—16 inches on center.
Continue by installing the sill, using the secure 2x4 bracing board as a rest for it, which will support it during the drying period.
Next, apply mortar to the entire back of the sill, pressing it securely into the wall, and check for level.
Once mortar has been applied, and the window sill installed, gently scrape excess material from the joints. Allow the mortar to dry on the sill entirely before removing the 2x4 brace.
Trim stone tips: work from the outer edges to the center
Begin with the top and center of the window or door frame using a keystone or center stone. Next, work from outer edges of the frame inward toward the center, cutting pieces to fit as needed.
Be sure that all proper flashing around door and window frames have been installed before beginning mortar work.