Sealants and Waterproofers on a Chimney: Why Faces Must Breathe

What these products are meant to do

Masonry treatments fall into two broad groups. Some are penetrating water repellents, usually based on silicone or silane compounds, which soak into the surface and line the pores so rain beads off. Others are film-forming sealants and paints, which sit on top and make a continuous skin. Both are sold on the same promise: spray it on and the stack stays dry.

On a hard, dense, modern brick wall with sound joints, a good penetrating repellent can have a place, and it is not always wrong. The trouble is that a chimney is among the least suitable places for it. The stack is exposed on all four sides, warmed from within, and made up of many different materials, from old brick to mortar to flaunching.

The question to ask of any coating is not whether it keeps rain out, but where the water that is already in the wall will go afterwards. A repellent that cannot be reversed deserves more thought than one that can, since the wall carries it for as long as it stands.

How trapped moisture does the damage

Every wall takes in water. Some comes as rain, some as vapour from the flue, some from the roof junction or from a gap in the lead. A breathing wall deals with this by drying outwards through the face and joints whenever the weather allows.

A film or a deep repellent layer reduces that outward drying, even if it claims to be vapour-permeable. Water can still get in through cracks, open joints and the top of the stack, but it now leaves far more slowly. It sits behind the treated face, carrying salts with it, and when frost arrives it freezes there. The result is a face that blisters, flakes or peels in sheets, often within a few winters, with the sealant still attached to the loose material.

This is especially likely on the soft handmade brick and sandstone found in older parts of chimney repairs in Neston, where the material draws in water readily and relies on being able to give it up again. A treatment that works reasonably on a dense engineering brick can be quite wrong here.

Defects that a coating cannot fix

Most chimney damp has a specific cause. Joints that have eroded back, flaunching that has cracked and let rain run down beside the pots, lead that has lifted at an apron, or a missing cover under a pot are all common. A coating applied over any of these does not close the opening. It hides the symptom for a short time and then fails, usually taking some of the face with it.

Clear sealant squeezed into a crack at a joint is a related habit. It looks like a repair, yet it is rarely sound, since the movement that opened the gap will open it again, and the sealant then holds water in rather than keeping it out. Lead flashings behave the same way: a bead of mastic along the top edge of an apron is a stopgap at best, and the better detail is explained in Lead Flashing or a Cement Fillet? How to Tell Which You Have.

If you are not sure what is causing the dampness inside, Is It the Chimney? Working Out Where the Water Is Getting In helps you tell a roof or stack problem apart from condensation or something else entirely.

Spotting a coating that is already on

Many stacks have been treated by a previous owner, and nobody remembers. Clues include a faint sheen on the brick that stays after rain, water that beads on one face while the rest of the wall soaks it in, and patches where the surface is peeling in thin, glassy flakes. Brick that was a warm red may now look slightly darker or glossy.

A flaking coating on a stack is not a reason for alarm, but it is a reason for a closer look at the face behind it. Often the brick underneath is damp and soft, and a quick check of the joints, flaunching and lead shows why. Do not scrub or power-wash it off, as that can drive water in and strip the weakened face along with the coating.

If you plan to buy a house with an obviously treated stack, it is worth having the head looked at before you commit. A stack that has been coated and then repaired well can still be a sound purchase, but you should know what you are taking on before you go ahead with the sale.

What is done instead

The usual plan is simple and not glamorous. Find where the water is getting in and stop it. That might mean raking out and repointing in a lime mix that suits the brick, re-forming the flaunching so it sheds water clear of the pots, renewing a flashing, or fitting a cowl or cap to keep the weather out of an unused flue. Once the entry points are shut, the wall dries out by itself.

There are occasional cases where a breathable water repellent is discussed, for example on a particularly exposed face of a hard brick stack that is otherwise sound. It is a decision made after the defects have been mended, not instead of mending them, and it should be tested on a small area first. If a stack has already been coated, removing the coating is rarely practical; the better course is to repair the joints and heads, then let the wall show whether it settles.

We carry out the outside work on chimneys only, and offer free written quotations. Anything involving the flue itself or an appliance is for a HETAS or Gas Safe registered installer or a sweep. Our quotation will describe what is to be repaired and what is not, which is also worth checking against What Should Be on a Chimney Repair Quotation?.