Most metalwork does not fail in the monsoon because the metal was wrong. It fails because of a detail at an edge, a joint or a fixing — somewhere water was allowed to sit, or to get behind something and not get out again. These are the failures worth designing against before the rain arrives, not after.
Windows: water gets in at the sill and the gap
Two leak paths dominate. The first is the gap between the frame and the masonry. If it was not packed and sealed properly, driving rain tracks straight through it, and because the water appears on the inside wall below the window, people blame the window rather than the installation.
The second is the sill. A sill must slope outwards and have a drip detail on its underside, so water runs off and falls clear instead of curling back along the soffit to the wall. A flat sill with no drip is a reliable way to stain a facade and damp an internal wall.
Specify: external-grade sealant correctly backed, a sill with positive fall, and drainage slots in sliding tracks kept clear. On uPVC and aluminium sliders, those little weep holes in the bottom track are not optional — blocked, the track becomes a trough and water rises over the inner lip.
Roofs: edges, overlaps and fixings
A metal roof almost never fails in the middle of a sheet. It fails at the ridge, the side flashing, the overlap between sheets, and at every screw.
- Overlap. Too short an overlap on a low pitch lets wind drive water back up under the sheet. The flatter the roof, the longer the overlap needs to be.
- Fixings. Self-drilling screws with bonded EPDM washers, driven straight and to the right torque. Overtightened screws dish the sheet and hold a puddle; undertightened ones leak. Plain washers without the rubber bonded layer will leak within a season.
- Flashing. Where the roof meets a wall, a proper flashing tucked into a chase beats a bead of sealant every time. Sealant alone is a repair, not a detail.
Gates and railings: where water sits
Hollow sections are the problem. A box-section gate frame that is open at the bottom fills with water, holds it against bare internal steel, and rusts from the inside out where nobody can see it. By the time it shows at the surface the section is already thinned.
Specify sealed ends, or deliberate drain holes at the lowest point of every hollow member so water can leave. Both work. What does not work is leaving it to chance.
Base plates are the other trap. A plate sitting flat on a terrace or driveway holds a ring of water around its edge permanently. Either bed it properly so there is no gap for water to sit in, or lift it on a small plinth so it drains.
Dissimilar metals
Put stainless fixings into a plain steel member, or aluminium against untreated steel, and in persistent damp you get galvanic corrosion — one of the metals corrodes noticeably faster than it would alone. It is a slow problem, so it rarely gets blamed on the right cause.
Where two different metals must meet outdoors, ask for an isolating washer or gasket at the junction. It costs almost nothing at fabrication stage and is impossible to retrofit neatly.
Pergolas, canopies and anything that collects rain
A covered pergola gathers a startling volume of water in one storm. If the design has no fall and no gutter, all of it runs down the wall the structure is bolted to. That becomes internal damp within a season, and the damp is invariably blamed on the wall.
Any covered structure attached to a building needs a fall, a gutter and a downpipe discharging somewhere sensible. This is the single most frequently omitted line in a pergola quote.
A short pre-monsoon checklist
- Clear the weep holes in every sliding window track.
- Check the sealant line around frames for gaps and shrinkage.
- Look under the bottom rail of gates for standing water or soft rust.
- Clear roof gutters and check that downpipes actually run.
- Touch up paint chips down to bare metal now, not in August.
None of this is expensive. All of it is cheaper than the repair.