How to Install Flashing and Weeps - Pro Masonry Guide
How to Install Flashing and Weeps

How to Install Flashing and Weeps

Moisture penetrating the walls of a masonry structure may lie in the installations of the flashing and weeps. 

By Gary Porter

In the spring of 2017, Chicago had a damp period that was cooler than normal. Our office received several calls related to moisture penetration. The cause for moisture penetrating the walls of our masonry structures may lie in the flashing and weeps.

First off,  when it comes to how to install flashing and weeps, we know all masonry, brick, concrete block, stone, thin veneers and mortar are going to let water through the outer wythe. The TMS 402 code, Building Code Requirements for Masonry Structures and Commentary, states a General Design Requirement: Design and detail flashing and weep holes in exterior veneer wall systems to resist water penetration into the building interior. Weep holes shall be at least 3/16 inch (4.8mm) in diameter and spaced less than 33 inches (838mm) on center.


On one particular job we saw, the weep ropes used were not letting the water drain from the cavity wall. Some weep rope used for these are not 100 percent cotton, so they do not wick moisture away as intended. The design was good; the material used was not so good.

A cavity wall is probably the best type of masonry wall. This cavity wall had an outer wythe of 4 inches of brick, 1 inch of airspace, 1 1/2 inches of rigid insulation, and 8 inches of concrete block.

After further investigation, the cause of water leaking into the structure was two-fold. There was no mastic, or not enough mastic, at the lap splices in the flashing. Because the weeps were not draining the cavity effectively, water built up in the cavity, and the lap between the flashing failed, causing water to leak into the structure.

The design intent of a cavity wall is not to hold water but to resist water. The recommendation for weep holes going forward should be to either leave the head joint open, or use a weep vent or a weep mesh vent. Rope weeps may contribute to moisture problems, as was the case in this project.

Another suggestion for a project for which you suspect the weep ropes may be a cause of moisture buildup would be to carefully drill a 3/8-inch hole in the mortar joint above the weep rope. Keep the drill slightly angled up, so that the flashing does not get penetrated. Don’t drill too deep. You will probably be able to pull the weep rope out of the mortar joint, so water can drain freely.

The flashing is probably one of the most important parts of a masonry wall. Flashing needs to be installed correctly and per the manufacturer’s instructions. A stainless steel drip edge needs to be caulked to the foundation. It is typical to prime the surface before a sticky-backed 30- or 40-mil flashing, with mastic installed at laps per the manufacturer’s instruction get attached to the substrate. A termination bar needs to be installed (anchored per manufacturer’s instructions and caulked properly) to ensure the flashing stays in place. It is necessary to install a mortar-collection device at the base of the wall and at the heads of openings, so the weep vents can work correctly and drain. The weep vents need to be spaced per the TMS 402/602 Code. An end dam is a must-have at heads of openings where the flashing terminates.


Perhaps builders and owners should request a picture of all flashing installations be submitted and approved before the masonry gets installed over it. Adding this simple step as a preventive measure would enable us to see problems before they occur.

Gary Porter is executive director of the Masonry Advisory Council and is called upon with construction concerns and for masonry advice from a variety of architects, engineers, contractors, developers and distributor sources. He is dedicated to ongoing education of masonry and shares helpful tips from his professional experience that may be beneficial to you. If you have questions or need help with a masonry issue, contact the Masonry Advisory Council, 847-297-6704.

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