Wind-Driven Rain and Exposed Chimney Faces

How rain arrives sideways

Rain that falls straight down wets a roof and the top of a chimney. Rain that is driven by wind wets the sides. On open ground with a long run of water in front of it, the wind arrives with real weight behind it, and the droplets are carried almost horizontally into whatever stands in the way. A chimney stands in the way more than anything else on the house, because it is the tallest part of the roof and has no eaves, no neighbouring wall and no gutter above it to take the first of the impact.

The windward face takes the load. Water strikes the brick or render, is soaked up by the surface, and is pushed a little way in by the gust pressure behind it. Brickwork is porous by design; it takes up water in a shower and gives it back once the wind drops and the air dries it. That is a perfectly healthy cycle, so long as the face gets time to dry between soakings. The difficulty on an exposed site is that the next front often arrives before the last one has gone.

Joints are the weak point of any face. Mortar is generally softer and more absorbent than the brick, and a joint that has been wetted again and again loses its sharp edge, then its depth, then its grip. Once a joint has washed back, the next dose of rain reaches the bed of the brick directly, and the face starts taking water from two directions at once.

Why one stack wears unevenly

Walk round a weathered stack and the four faces rarely match. The side the weather comes from is darker when wet, greener with algae, more worn at the joints, and often has lost more of its surface. The sheltered side may look almost as it did when it was built. This is not a sign of two different standards of work; it is simply a measure of how much water each face has had to deal with.

Orientation is only part of the story. A face shaded by a taller neighbouring building stays damp for longer, even when it is not the windward one. A face that catches afternoon sun dries quickly and so wears less. Corners are worse than flat faces, because wind accelerates round an edge and the arris of a brick, or the corner of a render panel, gets the sharpest of the impact. The top few courses suffer most of all, since they are exposed on every side with nothing above them.

Exposed stacks also tend to move more. A tall, slender element flexes very slightly with every gust, and that movement opens hairline gaps at the junctions: where the stack meets the roof, where the pots are bedded, and where render stops. Water finds those gaps first, which is why wear so often shows up as a line, a band or a stain pattern tied to a junction rather than an even loss across the whole face.

Reading the pattern from the ground

The pattern of wear is a useful record, and you can read much of it without leaving the garden. Stand back far enough to see the whole stack and compare the faces. If one is clearly more weathered than the others, exposure is the likely cause and the repair can be concentrated there. If all four are equally poor, age or a general failure of the mortar or render is more probable, and the work has to be wider.

Look next at where the wear begins. A band of dark, damp-looking brick just below the head suggests water entering at the top and running down. A stain that starts at the roofline points to the flashing or the back gutter. Patches of render that have blown, sitting on the weather side only, suggest water has been driven in behind the coat. A white powdery bloom at the edge of a crack means moisture has come in and is leaving again. The full checklist, including what is safe to judge from below, is in What to Look For on a Chimney From the Ground.

Binoculars or a phone zoomed in will help, and so will low sunlight, because light raking across a face shows bulges and loose areas that look flat in flat light. Please do not climb to see more. A clear photograph taken from below and sent to us tells a roofer more than most people expect, and costs you nothing.

How the pattern shapes the repair

Knowing which face takes the weather changes how a job is planned. On a brick stack the windward face usually needs deeper repointing, and a few bricks may have lost enough of their face to be cut out and replaced, while the sheltered faces may need only light work. The mortar still has to suit the brick; Repointing a Chimney: Why the Mortar Mix Matters More Than the Finish sets out the reasoning, and it applies to all four faces even when only one is in poor condition.

On a rendered stack the windward panel is the one to test most carefully by tapping, because loss of bond often begins there. It is also the face where the quality of the finish matters most: a coat that is too dense or too thin is the first to be beaten. Details that throw water clear, such as a drip at the head or a bell at the bottom of the render, do far more on an exposed site than on a sheltered one.

None of this removes the need to look at the whole stack. Chimneys on exposed sites are often wetter inside than they appear, and the sheltered faces can hide trouble that the weather has not yet revealed. Whether the work is a repoint, a patch or a rebuild is a question for someone who has seen the stack up close, and Repoint, Rebuild or Remove? How the Decision Gets Made explains how that decision is usually reached. We work on the outside of chimneys only, are fully insured and give free written quotations; our local page for chimney repairs in Flint says more about how we approach exposed rendered stacks.