Flaunching Cracks, From Hairline to Failed

What the flaunching is there to do

The flaunching is the mound of mortar on top of the stack that bridges the gap between the pots and the brickwork around them. It does two jobs at once. It holds each pot in place, and it slopes outward so that rain running down the pots and across the head is thrown clear of the stack instead of sinking in.

Because it sits at the top of a chimney it takes the worst of the weather: driving rain, frost, hot sun and wind, with nothing to shelter it. It also moves a little, as the brickwork, the pots and the mortar itself expand and contract at slightly different rates. A crack is therefore not unusual, and the useful skill is telling a harmless one from a failing one.

Most of what can be judged is visible from the garden with a pair of binoculars or a phone zoomed in. Never climb up for a closer look. The wider picture is covered in What to Look For on a Chimney From the Ground.

Grading the cracks you can see

Start at the lightest end. A fine hairline crack, no wider than a pencil line, that runs short and straight across the surface is commonly shrinkage from when the mortar first cured. If the surrounding mortar is firm and the crack is not growing, it can be noted and watched over a season or two.

Move up a grade when cracks are wider, run all the way round the base of a pot, or form a network across the head like dried mud. Cracks that follow the edge of a pot suggest the pot and the mortar are no longer working as one, and water can now run in at the gap. Map cracking across the surface suggests the mortar is breaking up as a layer. Both are worth a proper look soon.

At the failed end the signs are plain: pieces missing, edges crumbling away, a gap you can see daylight or shadow through, or a pot that leans or has shifted. By this stage rain is entering the head every time it falls, and the mortar is unlikely to be saved by a skim of filler. Our chimney repairs in Frodsham page describes how a head under heavy shade and debris tends to arrive at this point sooner.

Repair or re-form: how the choice is made

Repair is the right answer when the mortar is mostly sound and the trouble is limited, for example a small area of cracking, or a short gap where one pot has lost its bedding. The loose material is cut out, the area is cleaned, and new mortar is worked in and shaped to match the fall of the rest of the head.

Re-forming means taking the old flaunching off and building a new one. That is the better choice when the mortar sounds hollow or comes away in the hand, when cracking is widespread, when the head is flat or dished, or when the pots have worked loose. Patching on top of a bad bed hides the problem for a winter, and the repair cracks in the same places.

A good re-formed head is shaped with a clear fall away from each pot and a drip edge so that water leaves the stack rather than running back under it. The same visit is the moment to check the pointing below it, which is discussed in Repoint, Rebuild or Remove? How the Decision Gets Made, so you do not pay for access twice.

Bedding the pots properly

A pot is only as secure as the bed it stands in. The pot needs to sit level on the top course of the brickwork, surrounded by a solid mortar bed, with the flaunching built up around it so that it is gripped on all sides. A pot that has been dropped on a thin dab of mortar will rock in a gale and open its own cracks.

Where the existing pots are sound, they are usually lifted, cleaned and bedded back, which keeps the look of the stack and avoids matching problems. A pot that is cracked or fretting badly is replaced, and the choice of a pot, cowl or guard to suit the use of the flue is discussed in Chimney Pots, Cowls and Bird Guards: Which Do You Actually Need?.

One caution on scope. Our work covers the outside of the chimney: flaunching, pots and the pointing around them. If you have a live fire or stove below, the flue lining, its condition and any testing belong to a HETAS or Gas Safe registered installer, and sweeping to a sweep. If a pot has come loose, please keep clear of the area beneath it and call us for a free written quotation.

Keeping an eye on the head between visits

After a repair or a new head, little is needed beyond looking now and then. A glance from the garden in spring and autumn is enough to catch a pot that has shifted, fresh cracks, or growth beginning to take hold on the surface. Early signs are easy to deal with; a head left for several winters is much harder to save, and the repair grows with every wet season it is left.

It also helps to know what is not a concern. Slight colour differences between new and old mortar fade as the new work weathers, and a fine line where the flaunching meets the pot as it settles is normal on fresh work. What deserves a phone call is anything that changes quickly: a widening gap, a leaning pot, or mortar on the roof below. We are fully insured and offer free quotations, and for a stack that cannot wait there are 24-hour emergency call-outs Monday to Friday.