Wind Load on Tall Chimney Stacks: Where It Cracks and Leans

How wind actually acts on a slender stack

Most people picture wind as a shove from one side. On a chimney it is more complicated. The windward face takes a push, the two side faces and the leeward face experience suction as air accelerates around the corners, and the pattern flickers from moment to moment as gusts arrive and die away. A tall stack is slim for its height, so each of those changes is felt as a small flexing of the whole column.

The size of the load rises quickly with wind speed and with the area presented to it. A pot with a cowl on top, a television aerial, or a stack widened by render all add area at the very top, which is the point with the longest lever back to the roof. That is why the same gale can leave a low, broad stack untouched and leave a tall, narrow one with a hairline you had not seen before.

What matters most is repetition. A stack on an open slope is not tested once by a storm; it is flexed a very large number of times by ordinary weather. Mortar that survives a single hard push can still loosen after many small cycles, because each one opens the bond between brick and joint by a fraction and lets water in to do the rest.

Where it cracks, and which way it leans

Bending is greatest at the base, so the first sign is usually a thin horizontal crack in a bed joint a few courses above the tiles. It often shows on only one face at first, the face that goes into tension when the wind is from the prevailing direction. Over time the crack widens, the joint opens a little at one end and closes at the other, and the stack takes a slight set in the direction the wind has been pushing it.

Lean is easier to judge than people expect. Stand square on to one face, well back, and compare the stack with a vertical edge such as a door frame, a downpipe or the corner of the house. Then do the same from the adjacent side. A stack that looks straight from one angle and tilted from another has rotated along one axis, and a change from one season to the next is more telling than any single look.

Pots add their own clues. On a multi-pot stack, a pot that sits out of line with its neighbours has moved in its bed, and a ring of cracking in the flaunching around its base shows that the head is moving independently of the pot. Neither tells you the whole stack is unsafe, but both tell you the top is no longer behaving as one piece.

Why bedding matters more as height increases

Mortar does two jobs in a stack: it spreads load evenly between bricks and it holds them against sideways movement. In a low stack the second job is rarely tested. In a tall one, the joints close to the roof are asked to resist a bending force that grows with every extra course above them, and a joint that has eroded back from the face, or that was laid too dry, has far less contact area to do it with.

This is why repointing a tall stack is not the same job as repointing a wall. The old mortar is raked out to a proper depth, the joints are filled in layers and compacted, and the mix is chosen to be compatible with the brick rather than simply as hard as possible. Our guide to Repointing a Chimney: Why the Mortar Mix Matters More Than the Finish covers the mix; the point here is that joint depth and compaction are doing structural work on a tall stack, not just keeping the rain out.

The head needs the same attention. Flaunching that has cracked lets rain into the top courses, where it saturates the mortar and, in cold weather, freezes and pushes the joints apart. Pots that are bedded into sound flaunching share some of the wind load with the stack; pots rocking in a failed bed hammer on it.

Why the lead at the base has to cope with movement

Where the stack leaves the roof, lead flashings are chased into the brickwork and dressed down over the tiles or slates. If the stack is flexing in the wind, the top edge of that lead is being worked in and out of its chase, and the folds at the corners are being bent repeatedly. On a sheltered house this barely registers; on a tall stack in the open it is a real source of wear.

The lead needs to be heavy enough for its size and exposure, wedged and pointed in the chase so it cannot be drummed loose by the wind, and fixed so that it can still expand and contract without tearing. It is also worth understanding the difference between lead and a cement fillet, which cannot move at all and tends to crack off a stack that does; Lead Flashing or a Cement Fillet? How to Tell Which You Have explains that.

If the flashing on the windward side has lifted while the sheltered side sits tight, that is the wind telling you something about the stack as well as the lead. A flashing renewal on its own will not fix a stack that is moving, so it is worth having the brickwork looked at at the same visit.

What to do when you notice a change

Start from the ground and keep a simple record. Note the date, the wind conditions, and what you can see: a crack, a lean, a pot out of line, lumps of mortar in the gutter. Our guide to What to Look For on a Chimney From the Ground lists the signs, and a few photographs taken in the same spot over time will show movement more reliably than memory. Please do not climb up to check.

Then ask for a survey. We work on the outside of chimneys in and around chimney repairs in Heswall, and a free written quotation will set out whether the answer is repointing, re-forming the head, a part rebuild or taking the stack down. Anything that touches the flue itself, or the structure below roof level, belongs to a HETAS or Gas Safe registered installer, a sweep or a structural engineer, and we will say so when it applies.