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Know Your Roof – The Chimney Stack and Associated Terminology


chimney stack diagram

Chimney Stack Diagram

Renown here again to unravel the mysteries of chimney terminology so that you’ll know just what we’re talking about when we use weird and wonderful terms like flaunching, soakers and apron!

Simply put, the chimney is the exhaust system that safely carry noxious gases and smoke from your fire and out of your home. It is also probably the most vulnerable area of your roof as it offers an open route to rainwater if not properly protected and maintained.

1. The Chimney Stack

Starting with the basics, the section of the chimney protruding through the roof is the chimney stack. Of brick construction this is the most visible part of your chimney from ground level and it forms the final section of the flue that carries fumes away. Exposed to the harshest weather conditions, the mortar joints of the brickwork can become badly eroded causing damp to occur in rooms directly below. Perished mortar on the chimney stack should be raked out and the brickwork re-pointed. For extra protection we also recommend coating the stack with Thompson’s water seal when the mortar has dried.

2. Flaunching

Above the stack we have the flaunching. This is a mortar ‘crown’ that bonds the chimney pots and top course of bricks together. Like the stack, flaunching suffers the effects of weather and can crack and fall away, causing the pots to become unstable. Flaunching mortar should be mixed using grit sand, as this is less prone to cracking than building sand when thickly applied.

3. Pots

Back to basics. Pots sit on top of the brick chimney stack. They are relatively trouble free providing they are paired with the appropriate cowl. This is determined by whether you have a gas or real fire. If a chimney is unused the pots should be capped with a pepper pot cowl, which allows ventilation but prevents rainwater entering the flue.

4. Flashings


lead flashing diagram

Lead Flashing Diagram

Now back to the bottom of the chimney where the stack meets the roof and here we find the flashings. The majority of chimney leaks can be attributed to problems in this area as the flashings are normally comprised of five lead pieces which are subject to both wear and as we often find, poor or incorrect installation! For the most part unseen from ground level here they are:

1. The front elevation of the chimney stack is protected by the front apron flashing. The top edge should be let into the brickwork  two or three courses above the roof covering.

2. The step flashings resemble a set of steps climbing the side of the stack. The step shape is cut to allow fixing in the mortar courses of brickwork, avoiding cutting into the bricks themselves. The steps should be secured in the chase with lead pegs and pointed with new mortar.

3. The lead soaker is the ‘unsung hero’ of the system. Hidden beneath the tiles or slates, they team with the step flashings to form a watertight seal to the side elevation.

4. Back Gutter. As the name suggests this piece of lead acts as a gutter that then disperses rainwater down the sides of the stack. It is supported by a timber back board that is visible within the loft space.

5. The rear apron flashing is a strip of lead fixed into the stacks rear brickwork. It provides a watertight seal to the upstand of the back gutter.

We hope you’ve enjoyed our guided tour of the chimney and that in future you won’t be offended when we tell you that you’ve got a flashing problem!


March 29th, 2016

Category: Roof Terminology