Trims and flashings

What trims and flashings do

Trims and flashings are folded sheet-metal profiles that close every edge, corner and opening of the building envelope. They have three jobs: keep water and wind out where sheets stop or change direction, hide cut edges and framing for a clean appearance, and accommodate movement between the cladding and the structure. Although they represent a small part of the steel tonnage, poor flashing details are responsible for a large share of leaks in steel buildings. A complete PEB envelope can include a hundred or more different trim pieces, all scheduled on the roof and wall sheeting drawings.

Main trim types

Closures, sealants and fasteners

Trims are only weather-tight with the right ancillaries:

• Profiled closures (foam or rubber shaped to the sheet ribs) fill the voids under ridge, eave and high-side trims and keep out wind-driven rain, insects and birds.
• Sealants – non-skinning butyl tape mastic at laps and closures, and neutral-cure silicone or polyurethane where exposed – must be compatible with the coating.
• Fasteners – self-drilling screws and stitch screws or sealed blind rivets matching the trim colour, with EPDM washers; spacing typically 300–450 mm along trims. See structural bolts and cladding fasteners.

Detailing rules

1. Shingle principle: every upper piece laps over the lower piece in the direction of water flow; never rely on sealant alone.
2. Laps: typically 100–150 mm between trim lengths, with sealant and stitch fasteners; stagger laps away from sheet laps.
3. Turn-ups and stop-ends: turn up roof sheet pans under ridge and high-side flashings to stop water being blown uphill.
4. Drip edges: fold hems at the lower edge so water drips clear of the face below.
5. Movement: on long buildings and standing seam roofs, use sliding or floating flashings and expansion joints so thermal movement does not tear fixings.
6. Compatibility: avoid contact between coated steel and copper, lead or treated timber run-off; isolate aluminium.
7. Wind: trims at corners, eaves and rakes sit in high-pressure zones; fastener spacing must suit the wind design.

Special flashings and trim schedule

Beyond the standard edge trims, many buildings need special flashings where structures meet or where the roof changes plane. These pieces are detailed individually and checked carefully because they are where leaks most often start.

Trim schedule. Each trim is identified by a mark number, a cross-section with girth and bend angles, material and colour, length and quantity. Common girths range from about 150 mm for small corner trims to 600 mm or more for ridge, valley and parapet capping pieces; trims are typically supplied in lengths of 3–6 m to suit handling and packing. A clear schedule makes site sorting faster and allows replacement pieces to be ordered accurately years later.

Materials, installation and maintenance

Trims are press-braked or roll-formed from the same pre-painted metallic-coated coil as the cladding, commonly 0.45–0.6 mm, so colour and corrosion resistance match. Cut edges should be kept short and burr-free, and swarf removed the same day. During inspections (see maintenance and inspection) check for loose fasteners, sealant breakdown, displaced closures and damaged trims around doors where forklifts operate.

World Steel folds all trims and flashings in-house as part of the cladding and accessories scope, with each piece marked to the erection drawings; related details for gutters and fascias are coordinated in the same drawing set.