What Are the White Marks on My Chimney?

What efflorescence is

Bricks and mortar contain small amounts of soluble salts. They come from the clay, the sand, the cement and sometimes from the flue gases. When water enters the masonry, it dissolves some of those salts and carries them along as it travels. When the water reaches the surface and evaporates, the salts stay behind as a fine white deposit, often fluffy, powdery or crystalline. It may look like frost, a white bloom, or a streaky stain running down from a joint.

Efflorescence is common on new brickwork in its first year or two, as the building dries out. On a chimney that has stood for decades, it points to something more current. The brickwork is taking in water regularly, and it is drying out through the surface, which leaves the salts on show.

Why chimneys show it so readily

Stacks are the most exposed masonry on the house. They are wetted from all sides by rain and spend long periods damp in winter. Water can enter in several ways, and each leaves its own pattern.

• Failed mortar joints let rain soak straight into the brick. The marks follow the joints and look patchy.
• A cracked flaunching, the mortar collar around the pots, lets water pour down inside the stack. Marks may appear low on the shaft or even on the chimney breast inside.
• Failed lead flashing at the base lets water behind the brickwork. Staining shows low on the stack, close to the roof.
• Flue condensation, particularly from gas boilers and unvented flues, adds moisture and salts from inside. Marks can appear all the way up.
• Missing or cracked pots and caps leave the top of the flue open to rain.

Salts from burnt fuel add to the problem in chimneys that have been used. This is why a chimney can show staining that a garden wall does not.

Is it harmful?

The white deposit itself is not dangerous to the house. It brushes off and does not eat into the brick directly. The trouble starts when the salts crystallise inside the brick instead of on its surface. In that case, the growing crystals push the face of the brick apart in a process known as spalling, which causes it to flake and crumble. Our page on why chimney bricks crumble covers that stage. So efflorescence is a warning light. It tells you water is getting in, and it may be a sign that damage is coming if the cause is not found.

It can also reach inside. Salts moving through the breast into the room show up as white powder on wallpaper, patches that reappear after redecorating, and damp rings. Our guide to damp on chimney breasts explains what to do about those.

What to do about it

1. Dry-brush the deposit off with a stiff brush, and do not wash it with water, which only dissolves the salts and carries them back in.
2. Find the source of the water. A roofer should check the pointing, flaunching, pots and flashings for the failure that lets water in.
3. Fix that fault: repoint the joints, repair the flaunching, renew the flashing or replace the cap.
4. Let the stack dry. Brickwork can take weeks or months to dry out once the source is stopped.
5. Brush off any further bloom as it appears. It should fade and then stop.

Avoid sealing the brickwork with a waterproof coating. A film on the outside traps moisture inside, and the salts then crystallise behind the surface, which accelerates spalling. If protection is needed, a breathable treatment applied by a specialist is a different matter, but fixing the leak is the better answer.

Where repointing is the answer, take care over the mortar. Hard cement used on soft brick makes the problem worse, as our page on lime or cement mortar for chimneys explains.

Marks that are not efflorescence

Not every pale mark on a stack is salt. Lichen and algae form grey, green or yellowish patches on the shaded sides of the brickwork, and they feel crusty rather than powdery. Lime run-off from mortar or from old pointing can leave white streaks below a joint. Soot and tar staining, usually brown or black, comes from flue gases seeping through a leaking joint. A drop of water on the white deposit tells you something: efflorescence dissolves and disappears while the surface is wet, then returns when it dries. Lichen does not. If you are unsure, photographs taken from a few angles help a roofer identify it.

When to call a roofer

Call one if the marks keep coming back after brushing, if they are spreading, if bricks are starting to flake, or if you have damp inside. We photograph all faces of the stack and the roof around it, show you where water is entering and give a fixed written price for the repair. Efflorescence alone often means a modest repointing or flaunching job, though the figure is set after we have seen it. For the wider range of services, see chimney repairs in Basingstoke.