Chimney flaunching repair

Flaunching is the sloped mortar cap on top of a chimney stack that beds the pots and throws water off. It is the smallest part of a chimney, the first part to fail, and the cause of a surprising share of expensive chimney damage.

If you have been told your flaunching has gone, or you can see a cracked or crumbling mortar cap around your chimney pots, this page explains what it is, why it matters more than its size suggests, and what putting it right actually involves.

What flaunching actually is

Look at the top of any traditional chimney stack. The pots do not simply sit on the brickwork — they are bedded into a wedge of mortar that slopes away from each pot down to the outside edges of the stack. That wedge is the flaunching.

It does two jobs at once:

• It holds the pots. Chimney pots are heavy, tall and catch wind. The flaunching is what keeps them upright and secure.
• It sheds water. The slope carries rain off the top of the stack and over the edge, rather than letting it stand on the brickwork or run into the flue.

People sometimes call it haunching, or simply the mortar cap or crown. All the same thing.

Why flaunching fails

It is a thin, fully exposed layer of mortar sitting on the highest, most weather-beaten point of the building, with no protection whatsoever. Failure is a matter of when, not if.

• Thermal movement. Clay pots and mortar expand and contract at different rates. Over years of heating and cooling, the bond between pot and flaunching works loose and a hairline crack opens around the base of each pot.
• Frost. Once that hairline crack takes on water, freezing expands it. Each winter widens it.
• Wrong mix. Flaunching cast in too strong a cement mix is brittle and cracks; too weak and it erodes. It needs to be durable but able to tolerate movement.
• Insufficient thickness or fall. A flat or shallow cap holds water instead of shedding it. A cap that is too thin has no life in it.
• Age. Even well-made flaunching is a wearing part. It is not intended to last the life of the stack.

What failed flaunching looks like

• Visible cracks radiating from the base of a pot
• Chunks of the mortar cap missing, or the pot's base exposed
• A pot that looks tilted, or that has visibly dropped on one side
• Mortar debris in the gutter after wind or heavy rain
• Vegetation or moss growing on top of the stack
• Damp appearing on the chimney breast in the room below
• The cap looking flat rather than sloped

Most of this is visible from the ground with binoculars, or from an upstairs window of a neighbouring property. Please do not go up to look.

Repair, or recast?

Patching a crack in flaunching is almost always a false economy. The crack is a symptom of the bond having failed between mortar and pot, and filling the gap from above does not restore that bond — it bridges it, briefly. The patch then cracks along the same line, usually within a season or two, and in the meantime it hides what is happening underneath.

The durable answer is to cut out the old flaunching entirely and recast it. That means:

1. Removing the failed mortar back to sound brickwork at the top of the stack
2. Lifting the pots out, checking each one, and setting aside any that are cracked
3. Checking the top courses of brickwork underneath — this is where hidden damage is usually found, because it is what the failed cap has been letting water into
4. Re-bedding the pots plumb and secure
5. Casting a new cap with a proper fall away from each pot to the edge of the stack, and a clean drip edge so water leaves rather than tracking back down the brickwork

Chimney pots

Because pots and flaunching are physically bound together, they are effectively one job. A pot cannot be replaced without disturbing the flaunching, and flaunching cannot be recast without lifting the pots. If either needs attention, do both.

Common pot problems:

• Cracked or split pots, usually starting at the rim
• Frost damage to the top lip, where water sits
• Loose pots where the flaunching around them has gone
• Missing pots, leaving the flue open to rain and birds

On a period property in Lewisham, the pots are often part of the street's character — matching the existing profile matters, and salvaged or reclaimed pots are frequently a better visual match than modern equivalents. If a flue is genuinely redundant, capping it is worth considering instead of replacing the pot; see capping and disused chimneys.

Access and timing

Flaunching sits at the very top of the stack, so it needs proper access — in most cases scaffolding. That access cost is usually larger than the flaunching work itself, which is exactly why it is worth checking the pointing, the flashing and the brickwork at the same time. Doing flaunching this year and pointing next year means paying for the scaffold twice. See what chimney work costs.

Timing is weather-dependent rather than seasonal: fresh mortar must not freeze before it cures, so a hard frost or a frost forecast will delay the work. Otherwise it can be done through most of the year.

Flaunching questions