Moss and Lichen on a Chimney: Clean It or Leave It?

Three kinds of green, three different behaviours

People tend to describe anything coloured on a chimney as moss, but a stack can carry three quite different things at once. Algae is the thin dark or greenish bloom that stains a face without building any thickness. Lichen is the flat, crusty patch in grey, yellow or pale green that sits tight on the surface and grows very slowly. Moss is the soft, three-dimensional cushion, and it is the one that causes trouble.

The difference comes down to what each organism holds onto. Algae and lichen live on the surface of the brick and mortar and take moisture from the air. Moss anchors itself with fine threads and builds up a sponge of its own dead material underneath, so it hangs on to rainwater long after the sky has cleared.

That is why the same stack can have a harmless crust of lichen on one face and a worrying bank of moss on a ledge only a few bricks away. Treating them as a single problem is how people end up either ignoring something serious or scrubbing something that was doing no harm.

What they actually do to brick and mortar

Lichen does not dig into sound, hard-fired brick in any way that matters over a few decades. On softer brick or weathered mortar it can roughen the surface slightly, and when it dies back it leaves pale scars, but the damage is mostly to appearance. A stack with a scattering of lichen and joints that are still full and firm can usually be left alone.

Moss is different because of the moisture it holds. Masonry that stays damp is masonry that freezes while it is damp, and frost working inside a wet brick face is what makes the face flake away. Mortar that sits saturated softens and crumbles, and the threads that anchor the moss will follow any hairline in the joint and widen it. The plant is not eating the stack; it is keeping the stack wet enough for the weather to do the damage.

Moss also hides things. A cushion over the bed joints, around the base of a pot or along a ledge can conceal open joints, a cracked head or a pot that has begun to move. That is a good reason to look closely at any moss you can see from the garden before deciding it is only cosmetic. The guide to What to Look For on a Chimney From the Ground lists what is worth noticing without leaving the ground.

When to act and when to leave well alone

Act when the growth is on horizontal or near-horizontal surfaces, in the joints, or around the pots, because those are the places where water sits. Act too if the moss is thick enough to lift in a mat, if the stack stays dark for days after rain, or if bits of sandy mortar are turning up in the gutter alongside it.

Leave things be when the growth is thin lichen on a vertical face of firm brick, the joints are tight and the stack dries out quickly in a spell of dry weather. Removing lichen from a sound face brings little benefit and carries the risk of damaging the surface in the process, so a quiet stack with a little character is not a reason to book work.

If you are unsure, the useful question is not how much green there is but what is underneath it. Moss can be removed and the masonry inspected in the same visit, and that is often the cheapest way to find out whether you have a cosmetic matter or the early stage of a repair. Our chimney repairs in Frodsham page explains how shade and falling debris encourage it.

Cleaning without making things worse

The safest method is the plainest one. Dry growth is lifted by hand and brushed away with a stiff, non-metallic brush, working gently so that the face of the brick and the surface of the mortar stay intact. Wire brushes score brick and leave bright tracks in the mortar, and they should be avoided. Where lichen is stubborn on a sound face it is usually better to leave a trace than to scour it to nothing.

Pressure washing is the method to refuse. A jet strong enough to move moss is strong enough to blast soft mortar out of the joints and strip the hard, fired skin from the face of older brick, which then absorbs water faster than before. It also forces water deep into the stack, which is the opposite of the aim. Chemical washes can have a place on a wall, but on a stack head they run onto the roof below and do nothing for mortar that has already broken down.

Cleaning is also work done at the top of the stack, which means proper access such as scaffold or a safe tower, not a ladder propped by the owner. If you can see growth from the garden, please describe it to us rather than going up to deal with it. After cleaning, any joints that have opened are repointed, and if the head itself is the problem the answer is in Repointing a Chimney: Why the Mortar Mix Matters More Than the Finish and the flaunching guide in this section. The inside of a flue, including sweeping, is a job for a sweep or a HETAS registered installer.

Looking after a stack once it is clean

A stack that has been cleaned and repaired still benefits from a little attention. A look from the garden after the first wet autumn will show whether water is now leaving the head quickly or still lingering, and whether the dark patches have returned. Sooner than you might expect, the answer tells you whether the cause was the growth or the conditions around the stack.

Overhanging branches, a gutter full of leaf and a shaded roof slope all feed the problem, and each can be dealt with separately. If you want the stack to stay clean, reducing the debris that lands on it matters more than any treatment. The cost of leaving things is slow rather than sudden, which is why it is so easy to put off. A free written quotation will set out what is needed, and nothing more.