Old Flue Liners and Pot Bedding at the Top of a Stack

What is actually at the top of a stack

A chimney head looks like a single lump of masonry from the garden, but it is made of several parts that each do a different job. The brick shaft carries the load. Inside it, in many stacks, runs the flue itself, which may be nothing more than the brick opening, or may be lined with clay pipes, or with a later metal or concrete liner. On top of the brickwork sits a sloping bed of mortar called flaunching, and set into that is the pot, which gives the flue a finished outlet above the roof.

Older stacks were often built with no liner at all, the smoke simply passing up a parged brick channel. Later work, and anything altered after a boiler or fire was changed, may carry liners that stop short of the top, finish flush with the brick, or stand proud inside the pot. None of this is visible in detail from below, and it is one reason that a head can look sound while behaving badly.

Short stacks like those on many of the newer estates in Runcorn have even less on show: often a single flue, a single terminal and a modest patch of mortar. The same logic applies, only with fewer parts to go wrong.

How a pot should be bedded

A clay pot is a loose item that has to be fixed to a structure that moves slightly with temperature and moisture. It should sit on a bed of mortar, with its base pushed firmly into the mix so there is no void under the rim, and the surrounding flaunching should rise to meet it in a smooth fall away from the pot on every side. Water landing on the head then runs off the mortar and clear of the stack, rather than into the joint at the pot base.

The common faults are easy to describe. The flaunching is too thin at the pot, so rain finds the edge. It is a flat, wide shelf rather than a slope, so water sits. Or it was done in a strong mix that has shrunk away from the clay and left a hairline gap all round. Any of these lets water into the top of the stack, where it can soak the upper brickwork for months before anything shows indoors.

The mix matters because the pot and the brick have different appetites for water. The general principles are set out in our guide to Repointing a Chimney: Why the Mortar Mix Matters More Than the Finish, and the same thinking carries up to the flaunching.

Reading liner ends and pot bases from the ground

With binoculars on a dry day, you can learn a fair amount without going near the roof. Look at the line where the pot meets the mortar. A clean, unbroken junction suggests the bedding is doing its job. A ring of dark shadow, or a visible gap, suggests the pot has lifted slightly or the mortar has shrunk back. A crack running in a circle round the base is the classic sign of a pot working loose in its bed.

Check the pot itself for tilt. A pot that leans, even slightly, is rarely sitting in a full bed, and a leaning pot on a windy ridge deserves prompt attention. Look at the mortar for missing chunks, green growth that suggests it is staying wet, or stains running down the brickwork below, which show where water is escaping the head.

You will not be able to see a liner from the ground, and you should not try to look down a flue. Whether a liner is cracked, broken or missing is a question for a sweep or a HETAS or Gas Safe registered installer, who can inspect the inside properly. Our wider checklist for what to notice from below is in What to Look For on a Chimney From the Ground.

Where the roofer stops and the specialist starts

The dividing line is simple. Anything on the outside of the stack, such as the pot bedding, the flaunching, the pointing, the lead and the cowl, is roofing and masonry work. Anything inside the flue, such as lining, relining, sweeping, smoke testing and checking an appliance, belongs to a registered installer or a sweep. Roofline Solutions does not carry out flue lining or appliance work and will say so plainly.

In practice the two often meet. If a pot has to come off to re-form the flaunching and there is a damaged liner end underneath, the right course is to make the head safe and weathertight, flag what has been seen, and leave the internal repair to the right trade. If the flue serves a working appliance, that conversation should happen before the work, not after it.

Where the flue is no longer used, the picture changes again. A redundant flue still needs a sound pot or cap and some ventilation, as explained in Capping a Flue You No Longer Use, and Why Sealing It Solid Causes Damp. For a stack that is past repair, Repoint, Rebuild or Remove? How the Decision Gets Made sets out the options, and chimney repairs in Runcorn covers how those choices tend to play out on local roofs.

What a sound re-bedding involves

Re-bedding a pot is a short job when it is the only thing wrong, but it is done in a set order. The pot is lifted clear, the old mortar is cleaned off the top course of brick, and the bed is checked for loose or frost-damaged units. A fresh bed is laid, the pot is set level and plumb, and the flaunching is formed in a smooth slope to its rim before it is finished and left to cure under protection.

The work is done from scaffold or a proper platform, not from a ladder, because both hands and a board are needed. It is also the point at which a cracked pot is often replaced, since lifting an old one can finish it off. A written quotation should name the work, say what will happen if a hidden fault turns up, and say who handles the flue side; see What Should Be on a Chimney Repair Quotation? for what to expect.