Short answer: flashing in a masonry cavity wall is not a single strip of metal — it is one link in a continuous path that collects water getting past the outer masonry and drains it back outside. Flashing, weep openings, and end dams work together, and each depends on being continuous and well executed. This is an above-grade detail, and it is a different problem from a below-grade barrier.

The idea: one continuous path

Masonry gets wet. Rather than assume the outer wythe will keep every drop out, a drained wall assumes some water will get through and gives that water a way back out. Concrete-masonry technical guidance on flashing describes exactly this: a coordinated system that collects water and drains it, made of flashing, weeps, and the junction details that connect them. The performance comes from the path being continuous — from where water is collected to where it is discharged — not from any one component in isolation.

Flashing, weeps, and end dams

Three elements do the work, and they only work as a set:

  • Flashing is the collection surface. It catches water that has entered the cavity and directs it outward, projecting outward at the designed discharge location so the water exits the wall rather than continuing down inside it.
  • Weep openings are the exits. Placed just above the flashing, they let collected water leave. Without them, the flashing has nowhere to drain.
  • End dams close the ends. Where flashing terminates, turning it up forms a dam so water cannot simply run off the end and back into the construction.

Junctions matter as much as the parts: guidance emphasizes coordinated details where flashing meets openings, terminations, and other systems, along with material compatibility. A gap at a lap or a blocked weep breaks the path and the water goes somewhere it should not.

Compatibility deserves its own mention. Flashing sits in contact with mortar, masonry, and adjacent metals and sealants, and the technical guidance treats the selection and pairing of these materials as part of the detail rather than an afterthought — materials that are wrong for the exposure, or that react with one another, can fail long before the wall around them does. The same guidance frames flashing, weeps, junctions, and their execution as a single coordinated strategy, which is why a drawing that looks complete on paper can still leak if any one link is substituted, mislapped, or left out on site.

The detail, drawn

Cavity wall flashing and weep detail A schematic section through an above-grade masonry cavity wall. Rain that passes the outer masonry runs down the cavity face, is collected on base flashing that turns up the backup and projects outward through the outer wythe, and leaves through weep openings. An end dam stops water running off the end of the flashing. Numbered markers one to six correspond to the key beneath the drawing. exterior interior 1 2 3 4 5 6
  1. 1Outer wythe — the exposed masonry veneer; some rain gets past it.
  2. 2Cavity — the drainage space behind the veneer where water runs down.
  3. 3Base flashing — collects water and directs it outward at the designed discharge location.
  4. 4Weep opening — lets the collected water leave the wall.
  5. 5End dam — flashing turned up where it terminates, so water cannot run off the end.
  6. 6Water drains back outside rather than continuing into the construction.
Conceptual section through a drained cavity wall: rain that passes the outer wythe runs down the cavity, is collected on continuous base flashing, and is discharged outward through weep openings; an end dam closes the flashing at terminations. Schematic only — no dimensions, spacings, or products, and not a construction drawing.

Not every wall is built this way

This detail describes a drainage-type cavity wall — an outer wythe, an air space, and a backup, with room for water to drain. Not all masonry is built that way. Solid or single-wythe walls, and many older assemblies, do not have a cavity or weeps, so the same detail cannot simply be assumed or retrofitted. Just as important, this is an above-grade rain-management detail. It is a different question from managing water against a below-grade wall, where the strategy is a continuous barrier plus drainage — covered in damp proofing vs waterproofing.

Above grade vs below grade

A cavity wall drains rain back out through flashing and weeps. A below-grade wall resists soil moisture and possible water pressure with a continuous barrier and drainage. Don’t borrow one detail to solve the other.

Execution and limits

Because performance depends on continuity, execution is decisive: laps joined or sealed as required by the selected compatible flashing system, end dams formed, weeps kept open and clear, and compatible materials used throughout. This article gives no dimensions, no weep spacing, and no product selection on purpose — those belong to a specific design and to the applicable standards for a given wall. For how a coordinated model can make this detail’s intent, responsibility, and interfaces explicit before the wall is built, see BIM & wall-detail coordination.

Sources

Primary technical and preservation sources consulted for this article. Each was read at the URL below and checked on 2026-09-07. Links open the publishers' own pages; listing a source is not an endorsement of any product referenced there.

  1. Flashing Strategies for Concrete Masonry Walls (TEK 19-04A) — Concrete Masonry & Hardscapes Association. https://www.cmha.org/resource/tek-19-04a/