I am working on a 30-year-old building in the Chicago area. The limestone window sills run in 20-foot lengths under the windows. The mortar joints in the masonry wall directly beneath the limestone sills are deteriorating. This is the only location in the building that shows such deterioration. The mortar joints between the sections of limestone seem to have only minor cracks. The sills overhang the masonry by about 3/4 inch, but contain no drip. Is the deterioration caused by water leaking through the limestone itself?
The deterioration probably is the result of freeze/thaw damage, but it is unlikely that the water causing this deterioration is penetrating through the limestone itself. Even though the mortar joints show very few cracks, this is a likely source of entry. Flat or sloped surfaces leak more than vertical surfaces during rains. During cold weather, snow accumulates on the window sills. When the snow melts, water can seep through small cracks or separations between the limestone and the mortar. Because the sills have no drip, water also flows around and clings to the underside of the sill and reaches this wall. Not only will this area receive more water, but it also is more likely to freeze. Because it is exposed on the top and the sides, the top of the wall will cool faster than other areas. Flashings normally are installed beneath limestone sills to control water that penetrates the joints. If such a flashing was provided, it is not working effectively. Since the sill has no drip, the flashing underneath the sill should project out and bend down to form a drip. Any splices or penetrations in this flashing must be adequately sealed and end dams provided. I recommend removing the sills and installing through-wall flashing with a drip beneath them. A less expensive alternative would be to grind out the joints between the limestone sections and install sealant. Create a drip on the bottom by using the saw or other means. As long as the sealant is maintained, this alternate approach should be effective. The deteriorated joints beneath the sill should be ground out and removed down to sound mortar, then pointed in accordance with BIA Technical Notes on Brick Construction, Number 7F.