Can High Winds Damage Roof Flashing?

How Wind Actually Loads a Flashing

The picture most people have is wind pressing on a roof. On the windward slope it does, but that is not what causes the damage.

Air flowing over a building accelerates as it passes edges, ridges and anything projecting, and accelerated air is at lower pressure. So most of a roof in a gale is experiencing suction - being pulled upward - and the suction is strongest exactly where things stick up.

A chimney is the biggest projection on the building. The air going round and over it accelerates hard, and the flashings around it sit in the most aggressive part of the wind field there is on a house.

That is why chimney flashings on a West Kirby seafront property lift while the same detail on a sheltered inland house of the same age does not.

The Free Edge Is the Weak Point

A cover flashing is held two ways. Its top edge is turned into a chase raked out of a mortar joint, wedged with lead wedges and pointed - that part is genuinely held down.

Its bottom edge, lying over the tiles or slates, is the free edge. On a sheltered roof, dressing and the weight of the lead are enough. On an exposed one they are not, and the answer is clips - strips fixed under the covering and turned over the lead to hold the free edge.

Once wind gets under that free edge it has something to work at, and the same gust that would do nothing to a clipped flashing will lift an unclipped one a little further each time until it is standing away from the roof.

A lifted flashing does not necessarily leak in ordinary rain, which is why it can sit like that for a year with nobody noticing. It leaks in the first proper westerly.

Salt Is Doing Half the Work

The West Kirby-specific part, and it is about the fixings rather than the lead.

Airborne salt accelerates the corrosion of steel and iron considerably. The clips holding a free edge, and any nails or screws in the arrangement, corrode faster here than a few miles inland - so a specification that is entirely adequate in Chester gives up years earlier facing the Dee.

The lead itself is unaffected. Lead is one of the most corrosion-resistant materials in ordinary construction and salt does not trouble it. It is what holds it on that goes.

Which produces the classic coastal finding: sound lead, lifted along one edge, with nothing wrong with the material at all.

The Other Thing Wind Does

Fatigue. Every gust that lifts and drops a piece of lead flexes it, and lead that is flexed repeatedly in the same place work-hardens and eventually cracks.

That combines badly with a sheet that is too long. An oversized flashing is already fighting its own thermal movement; add repeated wind flexing at the free edge and it splits along a fold considerably sooner than it would inland.

So on this coast, sheet length matters more than it does elsewhere - correctly sized pieces with lapped joints rather than one continuous run.

What a Coastal Repair Should Include

Non-ferrous fixings and clips as standard rather than as an upgrade. This is the single most important difference and it costs very little.

Clips at the free edge wherever exposure demands them, which on the west-facing side of a West Kirby property it generally does.

Correctly sized pieces with proper laps.

And the chase raked out and wedged properly rather than the lead being bedded in mastic, because a mastic-bedded top edge on an exposed stack is a flashing waiting for a gale.

Put a flashing back exactly as the one that failed and it goes back into the conditions that failed it.