Customer Tarred The Flashing Now Leaks Elsewhere Decision Tree

Why this matters

Plastic roof cement, "tar," is the universal homeowner roof fix, and it has a universal failure mode: it stops water at one point and pushes it to another. When a customer trowels roof cement over a flashing and the leak reappears a few feet away, the tech has to recognize this as a redirected leak, not a brand-new fault. The tar built a dam or sealed a drainage path, and water took the next route. The correction is not more tar; it is removing the dam, restoring the drainage, and properly flashing the detail. Reading the new leak's position relative to the tarred area tells you exactly how the water was redirected.

Symptom presentation

The customer reports that after tarring a flashing, the original spot stopped leaking but a new leak appeared nearby, usually downslope or to one side. You find a thick, often cracked, layer of black roof cement smeared over a flashing, a valley shoulder, or a transition. The original detail is buried under tar; the new leak is at the next detail in the redirected flow path. Roof cement that has aged shows alligator cracking and lifting edges where it has shrunk and pulled away.

The signature is a heavy tar repair that stopped one leak and created another offset from it, with the new entry on the path the dam now feeds.

Quick checks

• Map the new leak relative to the tarred area with slope marked. The new entry is almost always on the downslope or lateral path the tar dam diverts water onto.
• Identify what the tar blocked. Roof cement across a valley shoulder, a flashing bottom, a weep, or a designed drainage channel reroutes water sideways into an unprepared detail.
• Inspect the tar for cracking and lifting. Aged roof cement shrinks and cracks, opening new entry points within the tarred area itself while still damming flow.
• Check the next detail in the diverted path. Whatever was the next-weakest flashing, boot, or seam on the new route is where the redirected water now enters.

Isolation tree

Branch 1: tar created a dam that diverts water laterally to a new entry. The classic redirected leak.

• Confirm by tracing the diverted flow from the tar edge to the new leak. Water that should have run straight down a valley or off a flashing now deflects around the tar mass into the side detail. The new entry is on that deflected path.
• Common case: tar across a valley or a step-flashing run forces water under adjacent courses or into a wall transition that was never meant to take concentrated flow.

Branch 2: tar sealed a drainage exit and water backed up to a new emergence point. Similar to a sealed weep, but with roof cement's bulk closing a larger relief.

• Confirm by finding the closed exit. A buried weep or flashing bottom with water backing up behind the tar, emerging at the next gap, is the mechanism.

Branch 3: the tar itself cracked and now leaks within the tarred area while still damming. Two faults: the original detail is still buried and failing, and the tar's own cracks admit water.

• Confirm by inspecting the tar surface for through-cracks over the buried detail and water entry at those cracks.

Branch 4: the new leak is independent and coincidental, not caused by the tar. Less likely but checked by confirming the new entry is not on the tar's flow path and was failing on its own.

Confirming diagnosis

• Controlled water test of the tarred area and the diverted path: water on the tar dam that runs off and enters the new detail confirms redirection on sight.
• Flow-path trace: with slope mapped, the new entry sitting squarely on the deflected path downslope or lateral of the tar confirms the dam mechanism.
• Tar condition check: alligator cracking and lifted edges over the buried detail show the tar is failing as a repair even where it has not yet leaked through.
• Exit audit: a designed drainage path (valley, weep, drip edge) buried under tar, with water backing up to a new emergence, confirms the sealed-exit mechanism.

Remediation

• Remove the roof cement from drainage paths and flashings. Tar is not a flashing; it dams and cracks. Strip it from valleys, flashing bottoms, weeps, and any designed water path.
• Restore the original detail to correct flashing practice. If the underlying flashing was the problem, replace or reset it so water laps over the surface below, not under a tar bridge.
• Repair the redirected new entry on its merits, but only after removing the dam, or it will keep receiving diverted water.
• Reserve roof cement for its legitimate, limited role (a temporary stopgap or an embedment for fabric in a specified repair system), never as a substitute for flashing across a moving or draining detail.
• Inspect what the tar hid before it is re-covered. A flashing buried under roof cement for a season often corroded or trapped moisture against the deck underneath, so check the metal and the sheathing below the removed tar and replace any flashing or sheathing the trapped moisture degraded rather than re-detailing over a compromised substrate.
• Re-test with controlled water after the proper flashing repair and confirm both the original and redirected entries are dry, watching the diverted path in particular to verify the dam is truly gone and water now runs its intended course off the detail.

References

• IBC Chapter 15 and IRC Chapter 9 (R903 flashing and drainage, R905 roof coverings), flashing and drainage requirements.
• ASTM D4586, asphalt roof cement, asbestos-free, standard specification.
• NRCA Roofing Manual, flashing standards and the limited, secondary role of roof cement.
• OSHA 29 CFR 1926 Subpart M, fall protection for roofing work.