The short answer
White marks on chimney brickwork are usually mineral salts or lime carried out of the wall by water. Efflorescence is a fluffy white or grey bloom of salts that dries on the surface. Lime run is a hard white streak where lime from the mortar has washed down the brick. Neither is harmful alone, but both show that water is moving through the stack, and the cause is worth finding.
How to tell salts from lime run, why white deposits appear on a stack, and what they say about the damp inside the masonry.
Efflorescence: salts on the surface
Bricks and mortar contain small amounts of soluble salts. When water soaks in, it dissolves some of them. As the wall dries, the water travels to the surface and evaporates, leaving the salts behind as a powdery white or greyish crust. That crust is efflorescence. It brushes off dry, and it tends to come back when the weather turns wet and then dries again.
On a chimney it is common, because the stack is so exposed. Rain hits all four sides and the flue adds moisture from inside, so a lot of water is moving through the brick. A patch that appears after a wet spell and fades in summer is the classic pattern.
Efflorescence itself does no harm. It is the sign of the problem, not the problem. The same salts that stay at the surface can, in the wrong conditions, crystallise inside the brick and push the face off, which is spalling. Our answer on why chimney bricks spall explains the link.
A dry stiff brush is the way to remove it. Washing it off with water only puts the salts back into solution, and they return to the surface.
Lime run and mortar streaks
Lime run looks different. It is a hard, whitish streak or crust running down from a joint, a cap or a cracked section of flaunching. Lime is a part of many mortars, and when rain gets in behind a joint it dissolves some of the lime and carries it out and down the brick. As the water dries, the lime hardens again as a pale, sometimes stalactite-like deposit. It is not brushed off easily.
Lime run on a stack almost always points to a defect higher up. It might be a cracked mortar cap, an open joint, a gap in the flashing or a missing section of flaunching. The streaks mark the path of the water. Follow them up from the stain and the fault is often at the top of the run.
A roofer will look for where the path begins. The flaunching around the pots, the lead at the roofline and the top courses are the first places to check.
White staining from a fresh repoint is a different thing and fades as the mortar cures.
What the marks tell you about moisture
Both kinds of white mark tell the same story: water is passing through the wall in quantity. A little is normal in a stack that stands in the weather. A lot, especially if it comes back year after year, tells you the brick is staying wet for too long, which makes it more likely to flake in frost.
Several faults can cause it. Joints that have worn away let rain straight into the stack. A cap that has cracked sends water down the inside. A flue sealed at both ends and left unventilated builds condensation. Hard cement mortar can block the brick from drying at the joints. Our answer on venting a capped chimney explains the flue side, and the one on lime mortar chimney repointing covers the mortar side.
Check indoors too. If the breast inside the house shows damp patches or a white bloom on the plaster, the moisture is coming through the stack and the flue, not only the outside face.
Taking a photo every season helps, because the pattern over the year is what a roofer wants to see.
What to do and what it may cost
Start by looking, not scrubbing. Find where the water enters, repair that, and the white marks will stop coming back. Cleaning the deposit off before the cause is cured only gives you a clean wall for a few months. A roofer can survey the stack from above, photograph the joints, cap and flashing, and say which fault is feeding the stain.
Avoid acid washes and strong chemicals on old brick, and avoid sealing the surface, because a coating over wet masonry can trap the water and make the flaking worse.
As a national guide, Checkatrade's April 2026 chimney repair table gives repointing at £300 to £1,000, flashing at £250 to £1,000 and a cap at £180 to £520. Which of those applies to you comes down to the fault behind the stain and the height of the stack, and tables like this often leave VAT out.
After the survey you get one written price, and the chimney repairs hub lists the stack jobs we take on. A photo of the staining plus the stack height lets us decide where to start looking, whether that is the cap, the joints or the lead.
More on this in Chimney Repairs in Crawley
Two white deposit questions
Can I wash white salts off the bricks with water?
Water only dissolves the salts and soaks them back into the wall, so they reappear when it dries. Use a dry stiff brush instead, and wear a mask and eye protection. Do this from a safe position at ground level or have a roofer deal with it, and then fix whatever is letting the water in.
Do white marks mean the chimney is about to fail?
Not by themselves. They show moisture is moving through the brick, which is worth acting on, but a stack can carry them for a long time. Worry more if bricks are flaking, joints are open or the stack leans. A survey with photos will show which applies to yours.