What is under the ridge cap
On a shingle roof the ridge is just the last course. On a metal roof it is a separate piece of formed metal sitting over the top of two panels, and what matters is not the cap you can see but the closures underneath it that you cannot.
- 1 The ridge cap. On metal it is a separate piece with closures under it. Blown snow finds the joint that was never closed.
- 2 The seams. A standing-seam roof lives at its seams and clips, not at its surface. The panel outlasts what holds it.
- 3 The valley. One channel taking what two slopes shed — and on the north side, holding snow long after the field is clear.
- 4 Where the ell meets the wall. A lower wing dying into the main house. Two roofs, one wall, and nobody sure whose seam it is.
- 5 The fasteners. Exposed screws through the panel, each with a rubber washer. The washers age; the screws work loose.
- 6 The eave. Drip edge, gutter, and the question every metal roof asks: where does a slab of snow go when it lets go?
The gap that has to be filled
Ribbed and seamed panels are not flat where they meet the ridge. There is a profile — ribs, seams, corrugations — and the cap sits over all of it, leaving a continuous gap along both sides. Closures are the shaped foam or metal pieces that fill that gap, and they are what stands between the attic and the outside.
They also cost very little and take a few minutes to skip. A cap screwed straight down over an open profile looks finished from the ground and is not finished at all.
Why it shows up in winter and not in a rainstorm
Rain falls down and the cap sheds it. Fine dry snow does not fall down: it travels along the surface and packs into any opening it finds, and the ridge is the highest opening on the building. It sits there, and when the attic warms it melts inside rather than outside.
That is why a roof can be watertight through three seasons and stain a ceiling in February, and why the owner is usually told it must be an ice dam at the eaves when it is nothing of the kind.
If the ridge is also the vent
Many metal roofs use the ridge as the exhaust for the space below, which means the gap is deliberate and the closure has to block weather while letting air out. That is a specific product and a specific detail, not sealant.
Worth asking directly: is this ridge vented or closed, and which product is going in? The two answers lead to different work, and the wrong one either blocks the airflow the roof needs or leaves the opening the snow wanted.