Why Does a Chimney Leak During Driving Rain?

Two Different Tests

A roof faces two quite different challenges and a chimney fails each of them differently.

Driving rain is wind-blown, arriving more or less horizontally from one direction. It tests whether the covering and the details on that particular face can keep water out when it is being pushed at them rather than falling on them. Masonry that would shed vertical rain perfectly well can absorb driving rain, and a lap or an upstand that is adequate for one is not necessarily adequate for the other.

Prolonged heavy rain is vertical and it is about volume. It tests whether the water can get away fast enough - the back gutter, the valley, the outlet, the gutter itself.

A chimney can pass one and fail the other, and which one it fails narrows the search enormously.

If It Leaks in Driving Rain

Look at one face of the stack: the one the weather comes from. On the Frodsham hill that is usually the west or south-west.

The likely causes, in order: eroded pointing, letting wind-driven rain straight into the joints; spalled brickwork, where frost has blown the hard outer face off and left a soft absorbent surface; and a side flashing that has pulled out of its chase or lifted along its top edge on that side.

The mechanism with masonry is worth understanding, because it explains the delay people notice. Wind-driven rain saturates the outer skin of the stack. If the rain lasts long enough, the moisture front works through to the inner face and appears on the breast - which is why the damp often arrives hours after the weather did, and why a short sharp squall does nothing while a long westerly gale produces a patch.

It also explains why it is one-sided. The sheltered face of the same stack, of the same age, in the same mortar, stays dry.

If It Leaks in Heavy Rain

Look above the stack, not at it.

The back gutter is the horizontal lead channel on the uphill side of a chimney. It has to catch everything running down the slope above and take it round both sides. On a wide stack on a shallow pitch that is a serious volume of water through a narrow channel.

If it has been formed too shallow, or if it has silted up, it overflows backwards into the roof - every single time it rains hard, and never when it does not.

This is the Frodsham one. The town sits under a wooded hill and a great many properties have mature trees close to them. Leaves, twigs and seed wash down the slope and the back gutter is the first thing that stops them. A back gutter with half an inch of compacted leaf mould in it is not a gutter any more.

The good news is that this is the cheapest chimney fault there is to put right, because nothing has failed. It needs clearing, not replacing.

What to Notice Before You Call

Was it windy, or was it simply raining hard for a long time? That one answer sends us to opposite ends of the same chimney.

Which direction was the wind in? If the damp only follows a westerly, that is the face to look at.

How long after the weather did the damp appear? Immediately points at a route straight through - a lifted flashing, an overflowing gutter. Hours later points at saturation through masonry.

And has the damp ever appeared when it was not raining at all? If so, this is not the page you need - that is condensation or salts rather than water coming in.

The Preventable Half

If you have trees over or near the house, the back gutter and the valleys are worth clearing once a year after leaf fall, and it is the single cheapest preventive job on a Frodsham roof.

It is not a job to do off a ladder. Reaching a back gutter means getting to the uphill side of a stack, which is the middle of a slope, and that means being on the roof properly.

What is worth doing yourself is looking. If you can see the chimney from an upstairs window of a neighbouring property, or from further up the hill, a back gutter full of debris is usually visible.