Ponding Water on a Metal Roof: Slope, Seams, and the Limit

Metal roofs are designed to shed water, not hold it. Every system I install assumes water is moving off the roof, and the slower it moves the harder the system works. Ponding, water that sits after the rain stops, is where most metal roofs meet their limit, and it is worth understanding where that limit is before you put metal on a low-slope section of a house.

Why standing water matters

A seam that is never under water only has to shed rain. A seam that sits under a puddle for hours has water pressing on it from above, freezing in it in January, and wicking into any gap by capillary action. Debris settles in a puddle and rots there. An exposed screw in a puddle is a screw with water on its washer all day.

On a steep roof none of that happens. On a roof at 1/12 or 2/12 pitch, or a roof that has sagged between rafters, or a valley that drains into a wall, it happens every rain.

Which systems tolerate it

Exposed fastener. Not well. Every screw in the flow path is a seal sitting in water. Most manufacturers limit their warranty below a 3/12 pitch, and we will not install it below 4/12. Details on the exposed fastener page.

Snap-lock standing seam. Not well either. The seam is snapped, not closed, and standing water, snow, and ice find the open side. It belongs on steep, well-draining roofs.

Interlocking metal shingle. Needs 3/12 or steeper. Below that, water moves too slowly for a shingle-style product, and I will not put one there.

Stone-coated steel. Carries a minimum pitch as well and is not the answer on a low-slope section.

Mechanically seamed standing seam. This is what handles low slope. The seam is folded shut by a machine, so there is no open side for water to enter, and the concealed clips keep every fastener out of the weather. For very low slopes we build the same 4Ever Armored panel with a tighter Vice Lock fold made for exactly those areas. How the seam is formed matters, and nowhere more than here.

What a low-slope section needs

A lot of houses have one: a porch roof, a shed dormer, an addition, a roof over a bay. They are often the place the old roof leaked first. On those sections:

• A mechanically seamed panel with a double-lock or Vice Lock seam, and full-width peel-and-stick underlayment beneath it, which some manufacturers require for a low-slope warranty.
• Continuous panels. No end laps. Panels roll-formed to the full run so there is no horizontal joint for water to sit on.
• Sealant in the seam where the manufacturer calls for it on very low pitches. This is the one place sealant belongs inside a seam rather than smeared over a joint.
• Drainage that works. Water has to have somewhere to go. A low-slope roof that drains into a sidewall or a blocked gutter ponds no matter what is on it.

It is normal for a house to end up with two systems, a shingle profile on the main roof and standing seam on the porch. That is a good outcome, not a compromise.

Where metal is the wrong product

A genuinely flat roof, or one under about half an inch of rise per foot, is not a metal roofing job. That is a membrane roof, TPO or EPDM, installed by someone who does that work. I would rather tell you so than sell you a panel that will sit in water. The same is true of a roof that has sagged so much that water pools mid-span; the fix is structural before it is roofing.

The existing roof that is ponding now

If your metal roof holds water, it is one of three things: debris damming a valley or a drain, which is maintenance; a roof that has sagged, which is structural; or a system installed below the pitch it was made for, which is the install. The first is a broom. The second and third are assessments, and on a roof with life left in it, a repair contractor's job. If the ponding is part of a roof giving up in several places, the repair-or-replace guide is the place to start.

Now you know where the limit is, and which seam moves it.