What Is a Chimney Stack?

The parts of a stack, from the roof upwards

Chimney terms get used loosely, and it helps to know what each one means before you talk to a roofer. Starting at the roof and working upwards, a typical brick stack has these parts.

• Flashings: strips of lead where the stack meets the roof, which seal the joint against rain. They include an apron at the front, step flashings up the sides and a back gutter behind. Our page on what chimney flashing is explains them.
• Shaft: the main brick column, with flues running up inside it. It may have one flue or several.
• Corbelling: the courses of brick that step outward near the top, forming a decorative and protective ledge.
• Cap or flaunching: the sloping mortar collar on the top of the stack that holds the pots in place and sheds water. See chimney flaunching.
• Pots: the clay or terracotta tubes on top of each flue, which improve the draught and keep rain out. Our page on chimney pots and cowls covers them and the covers that go on top.
• Damp-proof tray: in better-built stacks, a layer of lead or damp-proof course inside the brickwork near the roof, stopping water travelling down into the house.

Not every stack has every part. Many 1960s and 1970s houses have plain stacks without corbelling, while Victorian terraces often have tall, richly detailed ones. The names are the same in either case.

Stack, breast and flue: how they fit together

A chimney is three things working together. The flue is the hollow passage through which smoke and gases travel. The breast is the part inside the house, the projecting brickwork that surrounds the fireplace and the flue. The stack is the part that rises above the roof. The three form one continuous structure. The weight of the stack passes down through the breast to the foundations, which is why removing a breast without support for the stack above is dangerous. Our page on removing a chimney breast and keeping the stack explains how that is handled.

Stacks vary in position. Ridge stacks sit along the top of the roof, gable stacks rise from the end wall, and side stacks sit on the slope. Where a stack stands on the wall between two houses, it serves both, which makes it a shared structure with shared responsibilities.

What stacks are made of and why they wear

Most stacks are brick laid in lime or cement mortar. Some are stone, and some modern ones are concrete blocks or have a rendered finish. Whatever the material, a stack is among the most exposed parts of the house. It stands clear of the roof, takes rain from all sides, gets the full force of the wind, and freezes harder than the walls below. Flue gases add moisture and sometimes acids from within. Mortar joints wear back, bricks flake, flaunching cracks and flashings lift. A well-built stack may last a very long time, but it needs upkeep.

The problems that follow are covered across the hub. Weathered mortar leads to repointing, which our page on chimney repointing describes. Water entering at the base causes leaks, as in why a chimney leaks. A stack that has moved shows as a lean, covered in leaning chimney stacks.

Looking after a stack

You can learn a lot from the ground. Look up at the stack in good light, with binoculars if you have them. Check that the lines are straight, the mortar is flush rather than recessed, the pots are upright and the lead at the base is not lifting. Note any plants growing on it, stains running down the brickwork, or bricks that look flaky. If something looks wrong, do not go up to check it yourself. Ask a roofer to inspect. We take photographs of all four faces, the top and the flashings, and explain what we see in plain terms, before giving a fixed written quote for anything that needs doing.

Two more points are worth knowing. Some stacks carry more than one flue, and each flue needs its own pot and its own checks. And if the stack is shared with a neighbour, repairs may need to be coordinated. Costs vary widely with height and access, so the figure for your stack follows the survey. A stack that is sound and well detailed costs very little to maintain, while one left to weather for years can need part of the shaft rebuilt, which is why early checks pay. For a full overview of the services around chimneys, see chimney repairs in Basingstoke.