Through-Wall Flashing: An Essential Detail That Drains the Water a Wall Lets In

Masonry and cavity walls are drainage walls: they accept that some water will get past the outer wythe and rely on the cavity and flashing to collect it and send it back out. Through-wall flashing is the flashing that spans the wall to catch that water at the base of the cavity, above openings, and at shelf angles, then directs it to weeps that drain it to the exterior. It is a humble, concealed detail, but when it is discontinuous, lacks end dams, or is poorly lapped, water that should have drained instead enters the building. Getting it right is essential to how a drainage wall works.

The value is in the drainage path. The sections below cover the four aspects that matter most — catching water inside the wall, end dams and drainage, flashing materials and laps, and weeps and the cavity — and how ACE approaches through-wall flashing.

These challenges rarely stay in one silo: the same discipline runs through our guidance on fiber cement cladding, what we have learned from dimension stone cladding, and how we approach glass fiber reinforced concrete panels, and it all ladders up to ACE’s broader remedial waterproofing and repair.

Catching Water Inside the Wall

Catching water inside the wall is the whole purpose of through-wall flashing, because a cavity wall expects some water to pass the outer wythe, and the flashing spans the wall at the cavity base, above openings, and at shelf angles to catch that water and carry it back toward the exterior. Intercepting water inside the wall and redirecting it out is what the flashing does.

The flashing is placed exactly where water collects in the cavity — at bases, heads, and horizontal interruptions — so it can catch the water before it continues down or inward.

Because the wall is designed to drain rather than to be perfectly watertight, the flashing that collects and redirects the water is not optional but the mechanism that makes the drainage concept work.

End Dams and Drainage

End dams and drainage are where through-wall flashing most often succeeds or fails, because the flashing must have upturned end dams at its terminations so water cannot simply run off the ends into the wall, and it must be sloped and continuous so the water it catches actually drains to weeps. End dams and a continuous drainage path are decisive details.

Flashing without end dams lets collected water escape at the ends back into the wall, which is a common and consequential defect, so the end dams are as important as the flashing itself.

Because the flashing has to move water to an exit, its continuity, slope, and terminations all have to work together, and a break in that path leaves water with nowhere to go but into the building.

Flashing Materials and Laps

Flashing materials and laps determine whether the flashing lasts and stays continuous, because through-wall flashing is made from durable materials — stainless steel, copper, flexible membranes, or composites — that must be lapped and sealed so the flashing forms a continuous, watertight collector. Durable material and properly sealed laps are what keep the flashing performing.

The laps between flashing pieces are potential leak points, so they must be shingled and sealed correctly, and the material has to be durable enough to last the life of the wall since it is concealed and hard to replace.

Because the flashing is buried in the wall, choosing a long-lasting material and executing the laps and terminations correctly at installation is essential — there is little opportunity to correct it later without demolition.

Weeps and the Cavity

Weeps and the cavity complete the drainage system, because the water the flashing collects must exit through weep openings at the flashing level, and the cavity must be kept clear so water can reach the flashing and the weeps rather than being bridged by mortar droppings. Functioning weeps and a clear cavity are what let the flashing actually drain.

Weeps that are omitted, spaced too far apart, or blocked leave the flashing holding water with no exit, and mortar bridging the cavity can carry water across it, defeating the drainage, so both are essential.

The most valuable point overall is that through-wall flashing works only as part of a complete path — catch the water, dam the ends, keep it continuous on durable material, and drain it through clear weeps — and every link in that path has to be right for the drainage wall to perform.

In ACE’s field work, most through-wall flashing problems trace back to a few recurring locations — Missing or improper end dams, Discontinuity & bad laps, and Blocked or missing weeps — rather than the open field of the wall or roof. The cost of resolving them climbs by roughly an order of magnitude at each stage: a detail corrected during design costs a fraction of the same fix during construction, which is itself far cheaper than a repair once the facility is operating. That economics is why we push envelope decisions as early in the project as possible.

How through-wall flashing should work

Sources & further reading

Frequently asked questions