Chimney leaking when it rains

The short answer

A chimney is a brick tower stuck through the roof, and water can get in at the join, at the top, through the sides, or down the flue.

In order of how often we find them: the lead flashing at the base, the flaunching and pots at the top, the pointing on the stack, and an open flue with no cap on an unused chimney.

Each one leaves a different pattern inside. Read the pattern, look at the stack from the ground, and you will usually know which it is before a roofer goes up.

One: the flashing at the base

Where the stack comes through the slates, the joint between brick and roof is sealed with lead: an apron across the front, soakers and step flashings up the sides, and a back gutter behind, each chased into a mortar joint in the brickwork. It is the commonest leak on a chimney because it is the most complicated bit of lead on the roof, and on older houses it has been up there a long time. The lead cracks along the folds, the mortar it is chased into falls out and the flashing drops away from the brick, or, on some houses, the back gutter was only ever cemented and the cement cracked the first winter.

Inside, a flashing leak shows as a stain on the ceiling beside the chimney breast, or damp low on the chimney breast wall in the room below, and it comes with rain from any direction. From the ground you may see the flashing lifted away from the brick or a line of missing mortar above it.

Two: the top of the stack

The flaunching is the sloped mortar cap on top of the stack that holds the pots and sheds water off the brickwork. It cracks with age and frost, and once cracked it lets rain into the top of the stack, where it soaks down through the brick core. A cracked or missing pot does the same. Inside, this shows as damp high on the chimney breast wall in the top room, often in a band across the width of the breast, and it is worse after long rain than after a short heavy shower because the brick has to fill up first.

From the ground, look for a crumbling or dished top to the stack, a pot leaning, or green growth on the flaunching where water sits.

Three: the pointing on the stack

Brick is porous and the mortar between the bricks is the weak point. When the pointing has weathered back, rain soaks into the joints, and on a stack that faces the weather it goes right through. In a frost the water in the joints freezes and pushes the face off the bricks, which is the flaking you see on old stacks. Inside, it shows as general dampness on the chimney breast on the side that faces the wind, worse in winter, and it tends to leave a tide mark of salts on the plaster as it dries.

From the ground, missing mortar between the bricks and spalled brick faces are the giveaway. A stack that has been rendered can hide it until the render cracks.

Four: an open flue on an unused chimney

When a fireplace is blocked up and the chimney is left open at the top, rain falls straight down the flue. With no fire below to dry it, the flue stays cold and damp, the water soaks into the flue lining and the brick, and the damp comes through into the room as a patch on the chimney breast, often at the level of the old fireplace or where the flue bends. There is usually a soot smell after rain. This is the easiest of the four to fix: a cap on the pot that keeps rain out and lets the flue breathe.

The mistake is to seal the flue completely. A sealed flue sweats, and the damp turns up anyway.

Telling them apart

The pattern inside points to the cause. It is not exact, but it is a good start.

• Stain on the ceiling beside the stack, or damp low on the breast, any wind: flashing
• Damp high on the breast in the top room, worse after long rain: flaunching or pots
• Damp across the breast on the windward side, salts on the plaster, worse in winter: pointing
• Damp at fireplace height on a blocked-up chimney, soot smell after rain: open flue, no cap
• Damp that is there all year and does not follow the rain: possibly not a leak at all but salts in the old flue, which is a different conversation

What the repair involves

All four are repairs, and most chimneys on a two-storey house need a scaffold to do them properly: a tower against the gable or a chimney scaffold over the ridge. Flashing is cut out and renewed in Code 4 or Code 5 milled lead, chased into raked joints and pointed. Flaunching is broken off and recast. Pointing is raked out and redone in a mortar that suits the brick, with any spalled bricks cut out and replaced. An open flue gets a cap or cowl. If the top of the stack is loose, the top courses come down and go back up. If the stack is unused and in poor condition all the way down, taking it down below the roofline and slating over is sometimes the better answer, and it ends the leak for good.

Whatever is done, ask for photos from the scaffold before and after. The top of a chimney is the one part of a house the owner never sees.

How it goes

1. Send a photo of the stack and the damp. One from the road showing the stack and the roof around it, one of the patch inside. Between them they usually say which of the four it is.
2. We look from the roof. Same-week visit on most calls. The flashing, flaunching and pointing are photographed close up before a quote goes out.
3. Written quote, access included. Within 48 hours, naming the cause and the fix, with the scaffold in the price so nothing appears on the day.
4. Repair and photos. Flashing and pointing on a scaffold are normally one to two days. Close-up photos of the finished work the same day.

What we do not do

This guide covers leaks at the chimney stack and the roof around it, which is what we repair across Dublin, Wicklow and Kildare. We do not line flues, sweep chimneys or fit stoves, and we do not build new chimneys. If a stack is loose enough to be dangerous we will say so at once and make it safe before anything else.

Common questions

Why does my chimney only leak in heavy rain?

Because the brick has to fill up first. A stack with worn pointing or cracked flaunching soaks up a short shower and dries out; a long wet spell fills the brick and the water comes through. A flashing leak, by contrast, shows with almost any rain because it is an open path.

Can I fix a leaking chimney with sealant?

No. Sealant over a cracked flashing or flaunching lasts a season and then fails, and it has to be cleaned off before the proper repair can be done. The failed lead or mortar has to come out and new go in.

Do I need scaffolding for a chimney repair?

For most stacks on a two-storey house, yes. Nobody can rake out and repoint a chimney properly standing on a wet ridge. A tower or chimney scaffold is included in the quote. Low stacks on bungalows can sometimes be reached from a roof ladder.

The damp is there in dry weather too. Is it still the chimney?

Possibly not a leak. Old flues leave salts in the brick that pull moisture from the air, and that shows as a damp patch on the breast that does not follow the rain. It is worth having the stack checked first, because the two often go together, but the fix for salts is inside, not on the roof.

Get a free quote

Call 085 781 4603 or request a quote. Free site visit, written quote within 48 hours.