Flashings & junctions: what actually keeps a wall dry

Flashings are the shaped sheet-metal pieces that bridge every junction and opening to throw water back out of the wall. Junctions are where claddings leak, so the flashing — not the cladding sheet — is what actually keeps a building dry.

The three rules behind every flashing

Whatever the junction, the same three principles apply:

• Overlap upper-over-lower — like roof tiles, so water always runs onto the piece below.
• Keep a fall — so water runs off rather than sitting.
• Finish with a drip edge — so water can’t track back underneath.

Material gauge by exposure

Pick the gauge and coating for the exposure, and match the metal to what it meets:

• Minimum 0.55 mm steel inland.
• Step up to 0.7 mm in coastal Zone D, with the matching exposure-grade coating or stainless.
• Match the flashing metal to the cladding or roof metal to avoid galvanic corrosion between dissimilar metals.

The reason the gauge matters: 0.55 mm in Zone D will corrode through within 5–10 years, so always use 0.7 mm on the coast.

Window & door flashings

Openings are the most common leak point, and each flashing has its own reach, fall and lap:

• Head flashing — extends 50 mm beyond the jamb each side, 15° fall, drip edge to the front.
• Sill flashing — extends 35 mm beyond the jamb each side, 5° fall.
• Jamb flashing — overlaps the sill flashing, with the vertical leg behind the cladding.
• Apron flashing — where the base of vertical cladding meets a roof: 100 mm vertical leg, 35 mm horizontal lap onto the cladding.

Roof flashings

Roofs carry their own set, each covering a specific edge, apex or penetration:

• Barge flashing — the gable end edge, covering the raw cladding edge.
• Ridge flashing — the roof apex.
• Valley flashing — between two roof slopes (0.55 mm minimum, often 0.7 mm).
• Pipe flashing — a Dektite or similar around vent and plumbing penetrations.
• Skylight flashing — a proprietary kit (Velux, etc.).