Flashing, Weep Holes
And Related Anchorage
Proper flashing and weep holes are essential elements in exterior masonry walls. Together, they provide a means to control moisture in a wall. If not addressed, moisture can have damaging effects on exterior walls. Excessive moisture within masonry can lead to crazing, efflorescence and spalling in some cases. Improper flashing can lead to moisture in the interior of a building. An effective system to deal with exterior moisture penetration is necessary for a properly functioning Cast Stone wall.
A drainage wall, also known as a cavity wall, is the most effective solution for a Cast Stone wall exposed to the elements.
The Drainage Wall
The exterior wythe provides first resistance against moisture penetration. Cast Stone should be laid with full joints in mortar meeting the requirements of ASTM C 270, Type N mortar. (See Bulletin #42.) Care should be taken when laying the stone to ensure the cavity behind this wythe stays clear. A tapered bed joint can help minimize mortar droppings and protrusions into the drainage cavity. A minimum 1 in. cavity or air space is recommended. Cavities of 2 in. or more are easier to keep clear of mortar and debris. Cavities over 4 in. may require special ties and anchors. When insulation is specified, the clear space of the cavity is measured from the outer face of the insulation to the back of the exterior wythe. (See Detail 4.)
Through-wall flashing and weep holes should be used at the base of the drainage wall and at all interruptions in the cavity, such as at window heads and relieving angles. Flashing must be continuous and properly lapped and sealed at the base of the wall and at relieving angles. When flashing is used over openings, such as at windows, end dams are required. (See Detail 1.) Weep holes direct water from the drainage cavity to the outside. Open head joints of at least 1 in. in height are recommended. Open weep hole joints provide the best drainage. They should be spaced no more than 24 in. apart. Rope wicks can also be used, but weep holes should be placed closer together, at 16 in. o.c., since this type does not drain as quickly. Plastic tubes are not recommended because they are easily clogged by mortar or by insects. In stones over 24 in. in length, a 3/8 in. wide by 1 in. high notch through the base of the stone is recommended for drainage. Unnecessarily long lengths of stone are discouraged because adequate drainage between weep holes can be a problem. Moisture retained in the wall can lead to crazing of the Cast Stone.
Flashing at Bases
Flashing is also recommended below all Cast Stone belt courses
and watertables that sit on a relieving angle or occur at a change
in material, i.e. stone to brick. In most cases, flashing and weep
holes should be placed directly below the Cast Stone course for
proper drainage of the cavity. In cases where stone and clay brick
are used together in the same wythe, the flashing also serves as a
bond break between the Cast Stone and the brick.
Flashing Over Openings
Flashing at Copings and Caps
At chimney caps, step flashing from below the Cast Stone coping down through the first course of supporting masonry below the weep holes should be located in the head joints of the first course of supporting masonry. (See Detail 3.)
Anchor Penetrating through Flashing
Flashing materials used successfully with Cast Stone include stainless steel, copper, copper laminates, EPDM, and rubberized asphalt. Polyvinyl chloride (PVC) and galvanized steel flashing should be avoided because of their questionable long-term performance. (See the Brick Industry Associations Engineering & Research Digest, “Through-Wall Flashing”, for a detailed discussion.) Table 1 lists some advantages and disadvantages of each of the recommended flashing materials that must be considered in making a final selection.
Table 1: Recommended Flashing Materials
Table printed with permission from the Brick Industry Association Engineering & Research Digest, "Through-Wall Flashing".