Through-wall flashing is the hidden layer that collects the water that gets into a masonry wall and sends it back out — and when it is detailed wrong, the wall leaks in ways that are expensive to find.
Masonry veneer is not waterproof — it is a drainage-wall system that expects water to get into the cavity and relies on through-wall flashing to collect that water and direct it back out through weeps. When the flashing is detailed or installed wrong, the water it was supposed to expel instead finds its way into the building, and the source is buried in the wall where it is expensive to diagnose.
The failures are predictable and repeat from project to project. The sections below cover the four that ACE sees most — missing end dams, poor lap and seam sealing, blocked weeps and drainage, and termination at openings — and how to detail each so the drainage wall actually drains.
These challenges rarely stay in one silo: the same discipline runs through how we approach kickout flashing failures, our field notes on counterflashing and reglet failures, and the same playbook behind insulating glass unit seal failures, and it all ladders up to ACE’s broader forensic building-envelope investigation.
Missing end dams are one of the most common through-wall flashing failures, because flashing that simply stops at the end of a run lets the water it is carrying run off the edge and back into the wall instead of out through a weep. Turned-up end dams at the terminations of every flashing run keep the collected water contained until it reaches a drainage point.
The problem is invisible from outside, since the flashing looks continuous, and the leak often shows up well away from the actual defect, which is why end dams are a frequent and costly oversight.
Detailing and verifying end dams at window heads, sills, shelf angles, and the ends of every horizontal run is a small step that prevents a large share of masonry leaks.
Representative of the flashing-failure locations ACE finds in masonry investigation and peer-review work. Illustrative distribution, not a published statistic.
Poor lap and seam sealing turns a continuous flashing into a series of leaks, because flashing comes in lengths that must be lapped and sealed, and every unsealed or shingled-backward lap is an entry point for the water the flashing is carrying. Laps shingled in the direction of drainage and sealed per the manufacturer keep the run continuous.
Corners and transitions are the hardest laps to get right and the most likely to be rushed, so preformed corners and careful sealing at changes of direction are worth specifying and inspecting.
Because the flashing is concealed, a bad lap is not found until water appears inside, so getting the laps right during installation is far cheaper than chasing the leak later.
A cavity wall with defective through-wall flashing collects water it cannot expel, and it leaks inside far from the actual defect.
Detail it as a drained system: end dams at every run, sealed shingled laps, protected open weeps, and continuous termination at openings.
The wall drains as designed, water exits at the weeps, and the leaks — and the forensic search for them — stop.
Blocked weeps and drainage defeat even perfect flashing, because the water the flashing collects has to exit through weep openings at the flashing line, and mortar droppings, missing weeps, or too few weeps trap that water in the wall. Weeps at the correct spacing, protected from mortar blockage, keep the drainage path open.
A mortar-collection device or drainage material at the flashing line keeps droppings from damming the cavity and blocking the weeps above the flashing.
Because the weeps are the only exit for collected water, treating them as an integral part of the flashing system — not an afterthought — is what keeps the wall draining.
Get the through-wall flashing reviewed for end dams, laps, weeps, and opening terminations before the wall closes.
Schedule a consultationCall (866) 389-8883Termination at openings is where through-wall flashing most often fails, because window and door heads, sills, and jambs interrupt the drainage plane and must be tied into the flashing continuously. Flashing that dead-ends near an opening without integrating into the window’s own flashing leaves a gap that leaks.
The sequencing between the through-wall flashing and the fenestration flashing has to be coordinated so water is always directed outward and downward across the transition.
The most valuable move overall is treating the through-wall flashing as a continuous, drained system with proper end dams, laps, weeps, and opening terminations, and verifying it in the field before the masonry closes it in.
In ACE’s field work, most through-wall flashing failures problems trace back to a few recurring locations — Missing / short end dams, Unsealed or reversed 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.
Show end dams, shingled sealed laps, weep spacing with mortar protection, and continuous flashing terminations at every opening.
Inspect end dams, laps, weeps, and opening terminations while the flashing is exposed and before the masonry covers it.
Trace leaks to the flashing line, confirm whether end dams and weeps are working, and correct the drainage path.
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