Chimney Repointing and Flaunching Explained

Your chimney stack takes more punishment than almost any other part of your property, fully exposed to wind, rain, frost and sun from every direction, so it's no surprise that the mortar holding it together eventually gives up. Once it does, water finds its way into the structure and causes damage that spreads well beyond the chimney itself. Repointing - replacing the mortar between bricks - and flaunching repair, the sloped mortar collar around the chimney pots, are essential maintenance that many homeowners overlook until the problem becomes serious and expensive.

This guide explains what these repairs involve, the signs that tell you they're overdue, and why acting on them promptly saves real money down the line.

Where Chimney Mortar Actually Does Its Job

Chimneys rely on mortar in several critical places. The bed joints and perpends - the horizontal and vertical lines between bricks - keep water out of the structure and hold the brickwork in position, so once this pointing fails, water penetrates the masonry and can eventually destabilise the whole stack. The flaunching, meanwhile, is the mortar slope around the base of the pots, sealing the gap between pot and stack and directing water away rather than letting it pool; when this fails, water gets straight into the flue and chimney interior. Mortar in the flashing chases also holds the lead flashings in place where roof meets chimney - see our guide to chimney lead flashing for more on that particular junction. All of this mortar is fully exposed, taking the full force of weather from every direction plus the thermal cycling that comes from the chimney heating and cooling.

Why Chimney Mortar Fails in the First Place

Frost damage is the primary culprit in the UK climate - water penetrates small cracks or porous mortar, freezes when temperatures drop, and expands, forcing cracks progressively wider with every cycle. Age and weathering take their own toll even on well-installed mortar, as decades of UV exposure and repeated wetting and drying gradually degrade the surface and leave it more vulnerable to frost. Sometimes the original mix itself is the problem - mortar that was too weak, too strong for the surrounding bricks, or poorly mixed will fail prematurely, and overly hard modern cement mortar can cause real problems when used to patch older chimneys originally built with softer lime mortar. Chimney movement, whether from general building settlement or sulphate attack from flue gases causing internal expansion, can also crack mortar that would otherwise have been perfectly sound.

Recognising When Repointing Is Overdue

Visible deterioration - crumbling, recessed or missing pointing seen from ground level, binoculars helping if the chimney is high - is the clearest indicator that water is entering the structure. Mortar debris turning up in gutters or on the roof around the chimney base tells the same story, since eroding mortar has to end up somewhere. Damp patches appearing on chimney breast walls inside the house, particularly after rain, often get misdiagnosed as other leak sources when the chimney is actually the cause, while white efflorescence staining on the brickwork signals that moisture is moving through the masonry and carrying salts to the surface. Cracked flaunching is usually visible from roof level and, once it appears, tends to worsen quickly.

What Proper Repointing Actually Involves

Quality repointing is a good deal more involved than simply pushing fresh mortar into visible gaps. Old mortar needs raking out to an adequate depth - typically 15-20mm minimum - since surface-filling doesn't create a joint that will last, and the exposed area then needs cleaning and dampening so the new mortar cures and bonds properly. Selecting the right mortar matters just as much as the technique: it needs to be compatible with, and generally softer than, the existing bricks, with lime mortar often the right choice on older Surrey properties, since overly hard cement mortar forces expansion and contraction stresses into the bricks themselves rather than the joints, cracking the masonry instead of protecting it. The new mortar is then packed firmly to eliminate voids and finished flush or slightly recessed - never a raised "ribbon" finish that looks tidy but sheds water poorly - and protected from rain and frost while it cures.

What Flaunching Repair Involves

Failed flaunching almost always needs complete replacement rather than a patch. The pots are carefully lifted clear - often already loose once the flaunching has failed - and the old, deteriorated mortar removed back to sound brickwork. A new mortar slope is then built up, typically to a 2:1 or 3:1 mix and a minimum 50mm thickness at the edges, sloping up towards the pot bases so water is directed outward rather than pooling. The pots are reset into the fresh flaunching, checked for vertical alignment, and the whole repair protected from rain during the several days it typically takes to cure fully.

Why This Is Rarely a DIY Job

Chimney work brings genuine access challenges and safety risks that shouldn't be underestimated. Proper scaffold access is typically required, since working from a ladder doesn't allow the stable working position quality pointing demands, and falls from chimney height are frequently fatal - professional roofers carry the training, equipment and insurance this work requires. Selecting the right mortar for your specific brickwork also takes real experience; the wrong choice causes either premature failure or, worse, damage to bricks that were previously sound. For most homeowners, the safety risk and scaffold cost mean DIY rarely saves money once everything is accounted for.

Good quality workmanship and materials extend the interval before the next repair is needed, while regular inspection - an annual look with binoculars from ground level costs nothing - catches deterioration early, before it becomes expensive. If your chimney is showing any of these signs, LT Leadwork & Roofing can assess its condition and recommend the right repair. Call us on 07566 234868 - we serve Reigate, Epsom, Dorking, Banstead, Leatherhead and the surrounding Surrey area.

If you'd like to know more about how we approach this kind of work, visit our Chimney Services page for full details, example photos and FAQs.