Q: Do you really need flashing under stone masonry or precast sills?
Yes. Even one-piece stone or precast sills may take on moisture at the interface of the sill and masonry jamb, window failures, material imperfections, or porous material structure. The primary purpose of flashing under sills is to protect the wall below. If there is a moisture penetration problem, it is easier to repair a sill than to fix the wall. In other words, it is preferable to
collect and divert moisture to the outside as quickly as practical rather than to hope it finds through-wall flashing elsewhere.
In general, whenever flashing is installed in a masonry wall you should also install weep vents – masonry sills are no exception. Without weeps, water may become trapped behind or under the sill. This water could eventually enter the building or deteriorate the mortar and/or sill through freeze thaw cycles.
Weeps can be installed in sill head joints or between the sill and the flashing. Examples of under sill weeps may include: rope wicks, tubes, half-tubes, or cell vents lying on their sides. Of course, care should be taken to keep mortar from clogging the weep vents…
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