Sheet Metal Fabrication and Installation
The Sheet Metal Fabrication and Installation domain tests whether you can assemble components that fit, fasten securely, allow for movement, and install to line and level without leaks or corrosion.
Fit-up and fastening
- Dry-fit before final fastening; correct twist and bow with clamps, not force.
- Rivet pitch is commonly 4–6 diameters, with edge distance about 2 diameters.
- Seams must face away from prevailing weather; lap direction follows water flow.
- Isolate dissimilar metals with a barrier to prevent galvanic corrosion.
Supports and movement
- Set out from a fixed datum and check line, level and fall.
- Space supports to prevent sag; allow expansion at joints and penetrations.
- Sealant must be compatible with the coating and applied to a clean, dry surface.
Worked example
A 7.2 m duct run has a maximum support spacing of 1.8 m. Number of supports = (7.2 ÷ 1.8) + 1 = 5. Supports are placed at 0, 1.8, 3.6, 5.4 and 7.2 m. If the run passes through a wall, add a sleeve and seal the annular gap, not the joint.
Traps
- Fastening before checking diagonals.
- Rivet pitch too wide, causing oil-canning.
- Seams facing the weather.
- Direct contact between aluminium and steel.
- Forgetting expansion allowance.