Standing Seam Metal Roof Assembly

The covering is a set of long metal trays running down the slope, turned up at their edges and folded together into a raised seam. Because that seam stands proud of the drainage surface and is closed by folding rather than by sealant, the joint between trays is not the low point of the roof, which is what allows the type to work on shallow falls.

The trays are not fixed through. Clips engaged in the seam are fastened to the deck, and a proportion of them are designed to slide, so the metal can lengthen and shorten with temperature while remaining attached. This is the organising idea of the whole assembly: the covering has to be continuous and mobile at the same time.

Everything the metal sits on is chosen with that in mind. The deck has to be continuous, even and dry enough to receive clips and to avoid telegraphing its irregularities into a reflective surface. The separation layer between metal and deck manages contact, abrasion and incidental water. The layers below decide whether the underside of a thin, conductive covering will attract condensation.

The nearest relative is the built-up twin-skin metal roof assembly, and a glance settles which is which. A standing seam roof is carried on a continuous deck and shows no fasteners on the weathering surface; a twin-skin roof spans between purlins and is fixed through both sheets, so its outer sheet carries visible lines of fixings. This entry is also not the sheet product described in the materials library, because the system is the deck, the separation layer, the clips and the seams acting together.

Movement organises the assembly. A tray anchored at a chosen position and free along the rest of its run will travel in a predictable direction; anchored at each end, or restrained accidentally by a badly formed detail, the same tray has nowhere to put its expansion and it either buckles between clips or works its fixings loose.

The seam and the clip are a single component in separate parts. The clip holds nothing unless the seam is folded closed over it, and the seam has no restraint unless the clip is properly seated, so a length of seam left unfolded or a clip missed at a change of direction produces a local failure with no visible cause on the surface.

The deck controls both how the metal looks and how it behaves. A depression in the deck becomes a visible dish in a reflective surface, a proud fastener becomes a wear point as the tray slides over it, and a deck closed in wet condition releases moisture upward into the interface between metal and separation layer, where it has no easy route out.

The separation layer stands between components that would otherwise damage each other - metal that moves and a deck that does not. Where it is omitted, torn or stopped short, the tray abrades on the deck, incidental water sits at the interface, and any drainage function the layer was performing towards the eaves quietly disappears.

Beneath the deck, air tightness and vapour control decide whether the roof is wetting itself from the inside. A thin, conductive, impermeable covering runs cold on clear nights, so moist air that reaches its underside through a break in the layers below condenses there, and the resulting water is inside the build-up rather than on the roof.