Bedding a Row of Chimney Pots in One Flaunching

One bed, many pots

Flaunching is the sloping collar of mortar that fixes a pot to the top of a stack and throws rain away from it. On a single-pot chimney the collar is a neat ring. On a stack carrying a row of pots it is better described as a shared plateau: one continuous layer of mortar that runs from pot to pot and out to the edges of the brickwork, with each pot standing in it like a post in a concrete footing.

That sharing is a strength and a weakness. Each pot is held by its own mortar and also by the pots on either side, so a row is usually steadier than a lone can on the same roof. The weakness is that a fault in the bed rarely stays local. Water that finds a way in under one pot travels along the bed, freezes, lifts the mortar and loosens the next pot along. By the time a leaning can is noticed from the pavement, the bed around its neighbours is often already tired.

It also means a row should be judged as a row. Before anyone talks about replacing one pot, the whole head needs reading: the line of the rims, the height of each pot above the bed, and whether the mortar between them is solid or hollow. The pointers on What to Look For on a Chimney From the Ground help with the first part of that check.

Why cracks open between the pots

The narrow strip of mortar between two neighbouring pots is the most awkward part of the whole head. It is thin, it is hemmed in on both sides by clay that moves differently from the mortar, and it has to shed water from two directions at once. Mortar shrinks a little as it cures, and a hairline gap tends to form where it meets the glazed or fired surface of a pot, because clay does not grip as well as brick.

Once that hairline exists, rain is drawn into it. In a cold snap the water expands, the gap widens, and each following wet spell gets further in. A crack running across the bed from one pot to the next is more serious than a hairline at a single pot base, because it suggests the bed has split in two and the pieces are moving against each other. A crack that is open enough to see daylight through, or one with dark staining below it, means the bed is no longer doing its job.

Do not be misled by a head that looks complete. A bed can sit intact on top and be hollow or crumbling underneath, particularly where a patch of new mortar has been laid over a failing layer. Patching over a cracked bed hides the fault for a season or two and then lifts away in sheets. If the repair has to last, the old bed has to come out, not be buttered over.

Getting the falls right

A flaunching is only as good as its fall. The surface should slope from the base of each pot down and outwards, so that rain runs off the head in the quickest direction instead of sitting against the clay. Between pots, the mortar should rise slightly into a low saddle, so water is divided and sent to either side rather than gathered in a trough. The edge should finish with a small overhang, a drip, which lets water fall clear of the brickwork instead of running back under the bed and down the face of the stack.

Flat spots are the usual fault. They tend to appear where a repair was rushed, where the mortar was trowelled out thin to cover an uneven area, or where a pot was reset at a slightly different height from its neighbours and the bed was levelled to hide it. Standing water on a flat patch breeds moss within a season, and moss holds more water, so the problem feeds itself. A feathered edge that tapers to nothing over the brick is no better: it breaks away and leaves the bed with a ragged margin that water walks straight under.

Fall also depends on the stack beneath. On a wide head the bed needs to be thick enough at its centre to give a proper slope, which means enough depth of mortar around the pot bases and enough of the pot sitting clear above it. That is another reason a row has to be set out together: the height of every rim, and the depth of mortar around it, is decided by the row, not by any single pot.

Re-forming the head in sequence

A proper re-form starts with taking the pots off. They are lifted one at a time, and the old bed is cleared back to sound brick, which is also the moment to look at the top courses and the internal divisions beneath. A top course that has gone soft needs rebuilding before anything is bedded on it. Our Repoint, Rebuild or Remove? How the Decision Gets Made guide explains how that call is made when the damage goes deeper than the mortar.

The pots then go back in as a set. They are dry-fitted first, so the row can be levelled, lined up by eye from the front and checked for height against each other. Sound pots are cleaned and reused; a cracked one is replaced with a match, which is covered in Chimney Pots, Cowls and Bird Guards: Which Do You Actually Need?. Each is then bedded on fresh mortar, pushed down so no hollows are left under the base, and the bed is built up around the whole row in one working session so it cures as a single piece with no cold joints between pots.

Mortar choice matters as much as workmanship. The mix should suit the age and softness of the brick and be strong enough to survive the exposure, without being harder than the stack beneath; the thinking behind that is set out in Repointing a Chimney: Why the Mortar Mix Matters More Than the Finish. After laying, the bed needs a dry spell to set and should not be left open to heavy rain or hard frost before it has taken, which on exposed roofs can mean waiting for a better day. On the outside of the stack that is the work we do at Roofline Solutions, and our chimney repairs in Wallasey page shows how it applies to the tall multi-pot stacks in Wallasey.