Capillary action (roofing)

Capillary action in a roofing context is the physical effect by which water travels along a thin gap against gravity, drawn by the interaction of surface tension and adhesion to the gap surfaces. It is the failure mode behind most metal-roof side-lap leaks below the manufacturer’s minimum pitch, behind tile head-lap leaks at the bottom edge of a tile course, and behind the persistent leak at parapet copings when the cap detail leaves a thin gap to the masonry below.

The two conditions that produce capillary action in a roof junction:

A thin gap (commonly less than 2 mm) between two surfaces that water can wet (steel, paint film, ceramic glaze).
A water source above or beside the gap that supplies water continuously while wind, pressure differential, or simple wicking pulls water along.

Once the gap and water source align, capillary action moves water upward, sideways, or around obstacles independent of the slope. A perfectly-laid 5-degree Custom Orb roof can still leak at the side lap if the lap dimension is below the manufacturer’s minimum, because the capillary path is faster than the lap can shed.

The two anti-capillary devices a roofer uses:

Anti-capillary fold (also called a capillary break or anti-cap groove). A folded or rolled lip in the side-lap of a metal sheet that breaks the thin-gap path: instead of a flat parallel gap, the water encounters a sharp change of direction it cannot follow. Built into the profile of standard metal sheets at the manufacturer.
Minimum-pitch and minimum-lap rules from the manufacturer. Below the minimum pitch, the side-lap gap geometry overrides the anti-cap fold’s effectiveness. Below the minimum end-lap dimension, water gets through despite the anti-cap fold.

Concrete and clay roof tiles have their own capillary risk at the head lap (top edge of the tile, where the next tile course sits over it). Head-lap minimums under AS 2050 vary by tile profile and pitch; below the minimum, capillary action draws water back under the tile course above.

Builder takeaway. Capillary action is invisible until it leaks, and the leak appears upslope from the source, which is why diagnosing it from inside the ceiling is hard. Prevent it at design: stay above the minimum pitch for the chosen sheet or tile, use the manufacturer’s anti-cap detail, and never short the side-lap or end-lap minimums on cheaper sheets.

Also known as: capillary leak; capillary creep; wicking.

Category: Building science.

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