Roof Drain Leak or Drainpipe Leak? How the Repair Responsibilities Differ

A roofer should investigate the membrane-to-drain connection; a plumber should investigate the drain's connection to the piping and the downstream pipework. When the drain itself needs replacement, the two scopes may need to meet on the same visit.

That's the useful starting point when water appears near a roof drain inside your building. Calling everything a “roof leak” can send the investigation towards the membrane before anyone has checked the pipe. Calling everything a “plumbing leak” leaves the roof-side seal out of the picture.

For a Lower Mainland property manager, we recommend asking one question before approving a patch: which connection is leaking, and what evidence identifies it? A repair proposal should answer that clearly enough for you to understand why that trade is doing the work.

One drain, several separate connections

Think of a conventional clamping roof drain as a handoff between two water-management systems. The roof membrane keeps rain outside the building. The drain collects that water and delivers it into piping.

At the roof surface, the flange is the broad edge where the waterproofing connects to the drain. The clamping ring holds the flashing or membrane at the drain opening. The body or bowl receives water; its outlet connects to the leader, the pipe carrying storm water away.

These aren't interchangeable seals. The membrane connection can need repair even when the pipe connection is sound, and work on a pipe joint doesn't restore damaged roof flashing. A strainer dome keeps larger debris out; replacing the dome is not the same job as restoring either connection.

Drain models also differ. For example, Watts' RD-300 installation instructions show the flashing clamp separately from the outlet and describe several pipe-connection options. The contractor needs to identify the installed assembly rather than assume every roof drain has the same fittings.

A flanged insert drain is another arrangement: its short stem fits inside a leader. That creates a drain-to-pipe seal below the visible flange. Ask whether the investigation includes that interface, especially if earlier work addressed only the membrane around the opening.

A wet pipe is a clue, not the diagnosis

Rain timing alone doesn't settle the trade question. Both the roof surface and storm piping receive water during rain. More useful observations distinguish where water first appears from where it eventually drips.

• Water first appearing at a visible pipe joint: direct the investigation towards that plumbing connection, while checking whether water is arriving from above it.
• A visible opening or separation in the roof flashing: have the roofer assess that defect and its relationship to the interior leak. Its presence doesn't establish that it is the only source.
• Water on the outside of a pipe near the deck: keep both routes open. Record the highest visible wet point rather than naming the pipe as the source simply because it is wet.
• Leakage accompanied by slow drainage or backup: include a plumbing investigation of the drainage route, not just a roof-surface patch.

Give the attending contractor the time leakage started, its location and any safe interior photographs. Leave roof access and opening concealed areas to the people responsible for that work. If water threatens electrical equipment or an occupied area, follow the building's emergency procedures before pursuing the diagnosis.

Where the roofer and plumber should meet

The practical split is by component and disturbance, not by which company arrived first.

The roofer's investigation should address the surrounding membrane and the flashing into the drain. The plumber's should address the outlet connection, accessible piping and any suspected downstream restriction. Drain-body replacement is the point where a narrowly worded work order can leave a gap: disconnecting the piping and restoring the waterproofing are different tasks.

RCABC's SBS provisions for cast iron drains assign installation to the plumbing/mechanical trade and address the condition and flashing of the drain. Those are RoofStar system provisions, not a universal repair specification for every existing roof. They illustrate why replacing a drain should be coordinated with the roofing work rather than treated as a pipe swap alone.

Before a replacement visit, agree who disconnects and reconnects the pipe, who opens and restores the roofing, and who checks each completed interface. Put interior access and any ceiling reinstatement into the scope too. Otherwise, you can approve a complete plumbing repair and still have unfinished roofing work.

That coordination does not determine who pays. Keep repair scopes separate from decisions about contractual responsibility or coverage.

Why a surface patch can miss the job

Consider a hypothetical building with a drip beside an internal drain. An inspection finds the roof flashing intact but observes water emerging from the outlet coupling below the deck. A proposal to apply more roofing material around the flange does not address the observed leaking connection. The next scope should focus on the plumbing finding.

Reverse the example: the accessible pipework shows no leakage during the investigation, while the roofer identifies a failed membrane connection at the drain. Replacing a length of pipe without repairing that connection leaves the identified roof defect untreated.

The lesson isn't to book two full repair crews for every drip. It's to avoid buying a repair for the wrong interface. Start with the strongest evidence, and arrange a joint visit when the source remains uncertain or the proposed work crosses both trades.

Buy a defined repair, not “reseal drain”

“Reseal drain” is too vague to compare proposals. Ask for the affected component to be named: membrane flashing, drain body, insert seal, outlet coupling or a particular section of pipe. The proposal should explain what will be disturbed, what will be reinstated and what checks are included afterwards.

For roofing work, the repair needs to suit the existing membrane and drain detail. For plumbing work, it needs to suit the actual outlet and piping connection. If the proposal excludes one side, ask who will complete it before the area is closed up. Request photographs of the repaired connections while they are still accessible, along with any remaining investigation items.

Routine drain cleaning and maintenance is a separate task from diagnosing a leak at those connections. If you need a roof-side leak investigation, describe the interior location and whether piping is visible. That gives the attending team a better starting point than “the drain leaks” and helps you commission the right repair.