Valleys and internal gutters on industrial roofs: detailing the drainage path

Draw flow arrows and collection points on the roof plan. This highlights where a conventional flashing cannot handle the incoming water. A bespoke folding may be part of the answer, but neither the blank width nor the finished width establishes hydraulic capacity on its own.

At the valley, the profiled outer facing and the core of the roof panel terminate. A flat strip slipped under a cut panel does not by itself close that edge. Draw the closure, the seals and the transition that takes water off the facing into the valley.

Provide the bearing and fixing points required by the panel. Do not treat a thin sheet metal channel as a structural support for its cut edge. If the shape of the roof calls for extra framing, identify it in the design before installation rather than assigning the load to the flashing on site.

The valley should carry water downslope without pockets or raised obstructions. Coordinate the side upstands with the ends of the roof panels. Set their heights and clearances by the detail, not by the desired appearance of the visible gap. Excessive panel overhang can restrict both discharge and cleaning access.

An internal gutter may differ considerably from a valley flashing at the junction of two slopes. It may have separate supports, insulation and a dedicated sealing arrangement. Do not copy a small-roof detail without assessment. Identify each drainage type on the drawings and in the supply and installation schedules.

Long lengths of sheet expand and contract with temperature. Design the joints for this movement as well as the direction of flow. Rigidly sealing every junction is not a general solution. Take laps, seals and any movement breaks from the applicable technical detail.

Locate fixings so that they do not make unplanned holes where water collects. Do not replace missing brackets with extra screws through the channel base. Choose the fasteners for the support and structure, and check their positions before adjoining panels or flashings cover them.

Valleys and internal gutters may stay wet longer than the adjacent roof surface. Specify a coating for that exposure. Zinc-coated, aluminium-zinc-coated and colour-coated steels provide different protection. A sheet listed in a price catalogue is not necessarily suitable for every drainage channel.

Check compatibility with the seals and neighbouring materials. Nearby chimney flashings or service penetrations may affect the deposits washed into the channel. Ridge caps, verge trims and wall starter sections face different exposures. Specify material for each location instead of assigning one finish to every folded component.

Before handover, clear offcuts, swarf and excess sealant. Check the panel transitions, end details and outlets. The gutter apron and downstream drainage must receive the flow from the valley. A tidy-looking roof does not establish that water has an unobstructed route to discharge.

Record how maintenance staff can reach the channel for cleaning. A blockage can impair even a correctly sized gutter. Include coating and seal condition in inspections. Identify the components on the building drawings so staff can find, inspect and service the right section.