Weep

A weep, a weep hole, or a weep-brick is a small opening that allows water to drain from within an assembly. Weeps are located at the bottom of the object to allow for drainage; the weep hole must be sized adequately to overcome surface tension.

Weeps may also be necessary in a retaining wall, so water can escape from the retained earth, thus lessening the hydrostatic load on the wall and preventing moisture damage from freeze/thaw cycles. In such cases the weeps consist of small-diameter plastic, clay or metal pipes extending through the wall to a layer of porous backfill.

Typically, weeps are arranged to direct water which may have entered an assembly from outside back to the outside. Weeps may also be found in metal windows and glazed curtain walls to permit interstitial condensation to escape.

In building construction, weeps are typically found in a masonry veneer or cavity wall, just above the flashing. The cavity serves as a way to drain this water back out through the weep holes. The weep holes allow wind to create an air stream through the cavity. The stream removes evaporated water from the cavity to the outside. Weep holes are also placed above windows to prevent dry rot of a wooden window frame.

The open head joint process is done by leaving out mortar from the joints. This create open holes of the same size of the typical spacing of the joints. This is the most common method and most effective way to evaporate water from the cavity. The spacing between open head joints can be done at every 24 inches (61 cm). One of the problems with this method is that the open head joints create large holes which may not be appealing. Some maintenance workers may also inadvertently patch those holes up without realizing they are weep holes. There are some products such as aluminum vent and plastic grid that can be inserted into the weep holes to make them less conspicuous.

Cotton wicks can be used to form weeps. A rope of up to 12 inches (30 cm) in length is placed in the joints. The other end of the rope is extended up into the cavity wall. The cotton can absorb the moisture inside the wall and wick it to the outside, but the converse is also true - it is possible for the cotton rope to wick a small amount of moisture from the outside to inside the wall. The process of evaporation is slower than with weep holes. Also, the cotton could catch fire.

...
Wikipedia