Lime or Cement Flaunching? Choosing for a Soft-Brick Stack

What flaunching has to do

Flaunching is the mortar crown on top of a stack that holds the pots in place. It has three jobs. It beds the pots so they cannot move, it seals the gap between pot and brick so rain cannot run down the flue walls, and it forms a slope from the pot outwards so water falls away, ideally clear of the stack face by way of a small overhang.

That makes it the part of the stack that takes the weather first. Wind, rain, frost and sun all strike it directly, and there is nothing above it to shelter it. Its failure shows up first as hairline cracks, then as open gaps at the pot bases, and finally as pieces dropping away. Once that has happened, rain runs into the stack and the brickwork below begins to suffer.

For this reason the mix is not a minor detail. It has to be tough enough to survive exposure, yet compatible with the soft masonry under it. Getting that balance right is most of the skill.

The case for cement

A cement-based mix has real advantages at the head. It sets quickly, resists weather early, and holds a sharp shape. For a stack in sound, hard brick, a well-made sand and cement flaunching is a sensible, widely used answer, and it is straightforward to work with when conditions are poor.

On a soft-brick stack the position is less comfortable. Cement flaunching is stronger and less permeable than the brick beneath it, and the bond at the edge becomes a hard line where movement concentrates. Thermal movement and freezing then open a crack at the edge, which allows water into the very joint between flaunching and brick. Water gets trapped under the cement, and the soft brick under it spalls.

That does not mean cement is always wrong. A weaker, well-graded cement mix, perhaps with a plasticiser, can be successful, but it has to be chosen with care, and beyond that the joint with the brick must be detailed so that it does not trap moisture.

The case for lime and gauged mixes

Lime-based flaunching behaves much more like old brick and sandstone. It is more flexible, it allows moisture to move through it, and it tends to develop many fine cracks rather than a single wide one. Those hairline cracks usually self-heal to a degree as the lime carbonates. That is a good fit with the breathing masonry that features in the stacks of chimney repairs in Neston.

The drawback is time. Pure lime sets slowly and is vulnerable in its early days to rain and frost, so it must be protected and the work booked for settled weather. On the very exposed top of a chimney, many repairers choose a gauged mix, which is lime with a measured addition of cement to give it early strength. The proportions are kept low enough that the flaunching stays weaker than the brick.

The mix used for flaunching does not have to be the same as the repointing mortar, though they should be compatible. For the repointing side, Repointing a Chimney: Why the Mortar Mix Matters More Than the Finish sets out the thinking, and the question of whether to repoint at all is covered in Repoint, Rebuild or Remove? How the Decision Gets Made. Whichever mix is chosen, it should be written into the quotation.

Cracks, movement and when to revisit

Even a well-made flaunching will show a few hairline cracks in time, and in lime these are not a worry. Cracks wider than a hairline, or a gap opening between flaunching and pot, are a sign that water is getting in and the crown needs attention soon. A crack that runs around the whole edge of the crown suggests it has debonded from the brick.

From the ground you can often see the larger cracks with binoculars or a zoomed phone photograph, and loose pieces on the roof below are an obvious warning. Please do not climb up to check. If the flaunching is cracked and the stack also shows leaning or open joints, the problem is bigger than the crown and the options in What to Look For on a Chimney From the Ground are a useful next step.

As a rule, flaunching deserves a look every few winters, and sooner after a severe storm. Catching a crack early means a patch can be enough, where leaving it often means cutting the whole crown out and starting again. Keeping a dated note of when the crown was last worked on, and what mix was used, helps the next person to look at it make a sound judgement.

Bond, shape and the pots themselves

Whatever the mix, the flaunching only works if it bonds to clean, damp, sound surfaces. Loose old material has to be cleared off first, the brick top wetted so it does not pull water out of fresh mortar, and the pot bases brushed free of dust. A flaunching laid over crumbling old mortar will not last, whatever it is made of.

The shape is equally important. The surface should rise to the pot and fall outwards, with a small drip at the edge, so that water leaves the stack and does not run down its face. Flat or dished flaunching holds water and cracks sooner. Old clay pots are worth keeping and rebedding where they are sound, and cowls and guards can be fitted at the same time; Chimney Pots, Cowls and Bird Guards: Which Do You Actually Need? covers those options.

This is a job for the outside of the stack, done from a proper working platform. Anything concerning the flue or an appliance is for a HETAS or Gas Safe registered installer or a sweep. We offer free written quotations, are fully insured, and will tell you in writing which mix we propose and why.