Where a Metal Roof Keeps Water Out: Flashings, Valleys and Penetrations

A steel panel does not leak. Water has nowhere to go through a sheet of coated steel. When a metal roof lets water in, it almost always does so where the panels stop: at an edge, in a valley, at a wall or around something that passes through the roof.

Those details take a large share of the time on any roofing job, and they are where installers differ. Here is what each one does.

The layer under everything

Weatherproofing starts before the first panel goes up. A roofing underlayment over the whole deck is the second line of defence, there for wind-driven rain, condensation and the occasional bit of snow that gets past the panels. At the eaves and in the valleys, where ice and water collect in winter, a self-adhered membrane is the usual choice.

We go through every layer in what is under a metal roof.

Eaves and rakes

The edges of the roof are where wind gets its grip and water first meets the structure.

• Drip edge at the eaves throws water clear of the fascia and into the eavestrough, rather than letting it run back behind.
• Gable trim along the sloped edges closes off the panel ends and stops wind lifting them or driving rain sideways under them.

Both are part of Highland's standard contract, not extras.

Valleys

A valley is where two slopes meet and their water runs together, so it carries more water than any other part of the roof. It also collects leaves and needles, and in winter it holds snow and ice longer than anywhere else.

On a metal roof the valley is a formed metal channel laid over membrane, with the panels on each side cut to it and sealed. It should be wide enough to carry the water off both slopes in a heavy storm. Keeping valleys clear of debris is one of the few regular jobs a metal roof asks of its owner.

Chimneys, walls and skylights

Anywhere the roof meets something vertical, water running down the slope hits an obstacle and has to be turned aside.

• Chimneys are flashed in two parts: a base flashing that turns up the side of the chimney, and a counter-flashing let into the masonry that laps down over it. Wide chimneys need a cricket, a small peaked diverter on the uphill side, so water and snow do not pond against the brick. A chimney cap on top keeps rain out of the flue itself.
• Sidewalls, where a roof runs along a dormer or a second-storey wall, need flashing that tucks behind the wall cladding so water runs over it, not behind it.
• Skylights need their own flashing kit worked into the panels. A skylight leak is one of the most common callbacks in roofing, so this is a detail worth asking about.

Vents, pipes and everything else

Plumbing stacks, exhaust vents and other pipes pass through the roof on a flexible boot sealed to the panel. On a profiled roof the boot has to follow the shape of the panel, so placement matters. A boot sitting in a valley of the profile seals differently from one straddling a rib.

These boots are made of materials that age faster than steel, so they are among the first things to check as a roof gets older.

The ridge

At the top, the ridge cap closes the roof while letting the air gap under the panels breathe out. Closures under the cap keep out snow, rain and insects without blocking that airflow. Why a metal roof needs an air gap explains why that airflow matters.

What to ask before you sign

• What goes under the panels, and is there membrane at the eaves and in the valleys?
• How will the chimney be flashed, and will it get a cricket?
• Are the skylights being reflashed or replaced?
• Is the ridge vented?
• Who handles a leak at one of these details in the first few years? Highland's own workmanship warranty is five years, separate from the manufacturer's warranty on the coating.

If you already have a leak, finding a roof leak explains why the stain is rarely under the hole. If you are planning a new roof, tell us about the house.