How Custom Aluminium Parapet Cappings Prevent Leaks in Flat Roof Extensions

The Physics of Water Management: Drip Edges and Falls

Preventing leaks is not just about blocking water; it is about managing how water behaves. Custom aluminium parapet cappings from cills.ie are engineered around the physics of water runoff. They feature a minimum 1.5 to 2-degree fall directing water inward towards the roof drainage system, and a 40mm overhang with a return drip bead on the underside of the nose. This bead breaks the surface tension of water, forcing it to drip clear of the wall instead of running down the render face.

How Secret-Fix Bracket Systems Eliminate Leak Paths

Traditional cappings are often screwed directly through the top surface into the wall. As the metal expands and contracts, these screw holes widen and the rubber washers degrade, creating direct leak paths into the masonry core. cills.ie uses internal secret-fix brackets secured to the plywood soleplate. The capping clips onto these brackets, leaving the top weathering face completely smooth, continuous, and unpenetrated.

Welded Corners: Solving the Corner Leak Risk

The corner of a parapet wall is its most vulnerable point because thermal expansion forces are concentrated there. Cutting cappings at 45 degrees on-site and sealing them with silicone is a recipe for failure, as the silicone joint will fail within 12–24 months. cills.ie factory-welds corner units in our Dublin workshop, creating a single, solid metal unit that is structurally sound and completely watertight before it arrives on-site.

Protecting External Wall Insulation (EWI) from Delamination

When EWI is wrapped over a parapet wall, water getting behind the insulation boards can ruin their thermal efficiency and cause the adhesive to fail, leading to the render peeling away. Custom wide-profile cappings from cills.ie act as an umbrella, keeping EWI dry and structurally secure.