Where Render Meets the Pots: Collar Cracks and Water Entry

Three materials in one small ring

At the top of a rendered stack, three different materials come together in a space only a few inches wide. There is the render on the walls of the stack, the mortar bed that slopes up to the pots, which is called flaunching, and the clay of the pot bases themselves. They are all fixed together, but they are not alike. Clay barely moves with temperature, and it takes up little water. Mortar and render move more and take up more. The ring where they join is the one place on the chimney where all three have to stay sealed against each other.

The ring takes more punishment than any other part of the head. It is the highest point of the stack, wet from every side, and the pots above it heat and cool with every fire or sunny day, with smoke warming the clay from inside. Frost, wind and sun work on it daily, and the surfaces slowly separate. A gap of less than a millimetre is enough to start the job.

Because it is so small, the junction is easy to overlook. People notice a missing pot or a loose brick long before they notice a ring of hairline cracks near the base of a pot. That is a pity, because this is where the trouble starts, and the repair is cheapest in effort and disruption when the cracks are still fine.

How the cracks form

The first cracks usually run in a circle round the pot, along the line where the mortar has parted from the clay. The cause is nearly always movement: the pot expands and contracts differently from the mortar, wind rocks tall pots slightly, and the mortar loses its grip as it ages. A second pattern is a crack radiating from the pot towards the edge of the stack, which often comes from shrinkage in a mix that was too strong or too wet when it was laid.

A third kind appears where the render finishes against the flaunching. If the coat was taken hard up to the bed and bonded across the joint, the two materials pull against each other and the crack follows the line of the join. The same thing happens when render is carried over the edge of a bed that has since softened and crumbled.

Once a crack is open it gets worse faster than it started. Water enters, freezes and widens it, and fragments of mortar fall out, leaving a gap that is easy for the next shower to enter. The pots, which depend on the bed for their grip, begin to loosen as well. A pot that moves under your eye, or a visible lean, needs attention soon.

How water gets in and where it goes

Rain that runs down a pot meets the collar first. If the junction is sound, it runs over the surface of the bed and off the edge, to the face below. If there is a gap, it enters it. From there it can go three ways: down into the void behind the render, along the back of the bed into the brickwork, or into the top of the flue alongside the pot base. All three are harmful, and the first is the quietest.

Water behind the render cannot easily leave. It saturates the top courses of brick, spalls their faces in frost, softens the joints and travels down the stack until it finds the lowest opening. That is often the roof junction, so staining on a bedroom ceiling or chimney breast can have its origin in a cracked ring at the very top. Damp on a Chimney Breast: Leak, Condensation or Salts? explains how to tell this from other causes of damp.

Water entering the flue is a separate concern. Some rain always enters an open pot, and flues are built to cope with that, but a persistent leak at the pot base wets the brickwork around the flue and can deposit soot and tar staining on the inside wall. Anything to do with the flue lining, or with an appliance that is in use, belongs with a HETAS or Gas Safe registered installer and a sweep, and not with the roofer.

Re-forming the junction

A hairline crack in an otherwise firm bed can sometimes be raked out and filled, but that is a stopgap, and honest advice is to treat it as one. When the bed is loose, hollow or widely cracked, the right repair is to take it off completely, clean the pot bases, make good any damaged render at the edge, and form a new bed. The new bed is laid to a fall so that water leaves it quickly, with each pot bedded firmly and the mortar finished in a way that throws water off rather than collecting it.

The render below is dealt with at the same time. Hollow areas near the head are cut back to a firm edge, and the coat is finished clear of the new bed with a small, clean margin, rather than being carried into it. The mortar used should suit the brick and the render; stronger is not better, and a bed that is much harder than what is beneath it will crack away from it again. The same thinking underlies Chimney Pots, Cowls and Bird Guards: Which Do You Actually Need?, where the fit of a pot or cowl depends on the bed beneath it.

We carry out this work from the outside, are fully insured and give free written quotations. If you are in or around chimney repairs in Flint and notice cracking at the base of your pots, take a photograph from the ground and send it over; we can usually tell from that whether it is a repair or a re-form.