Weep holes are holes in the exterior walls of a structure which allow water to vent to the outside, ensuring that it cannot accumulate behind the walls. They also permit air circulation, which makes the area inside the walls hostile to molds which can damage structural elements of a building. These holes are used on masonry and stucco homes, both of which are prone to water problems if they are not built and maintained properly.
The installation of weep holes occurs during the construction process, when the courses of masonry over the foundation are being laid. The holes are made by leaving open cracks unfilled with masonry cement. Some masons use tubes surrounded by mortar to create a less obvious crack, and the hole can be covered with netting or similar material to keep pests from getting inside the wall. As the name implies, these holes literally weep when water accumulates inside the wall, allowing the water an outlet.
The weep holes must be installed above grade. If the holes are below grade, water, dirt, and other materials can enter the walls through the holes, and this is not desired. This is why the holes are typically established at the foundation line, as the foundation comes up above grade. In the case of a house surrounded by grade changes, as might be the case with a house built into a hillside, it may be necessary to install weep holes at different heights.
In addition to being installed around the foundation, these ventilation holes should theoretically also be placed around windows and doors. These openings can provide a way for water to get in between the walls, making adequate ventilation and drainage necessary to prevent problems. While flashing is designed to prevent water penetration, it cannot be successful all of the time. Without weep holes, water which creeps in through the cracks will be trapped in the walls, where it can lead to mold, rot, and mildew.
Installation of weep holes may be mandated under the building code for some types of structures in certain regions. A knowledgeable contractor should be aware of when this feature is required. Once a structure is finished, installing these holes is extremely difficult, so it is important to confirm that they are or are not necessary before construction has progressed too far. In some regions, contractors who fail to meet code when they build a structure can be forced to fix the structure or to pay for remediation to bring the structure up to code.