A chimney stack is a thin tower of masonry with weather on every side. Rain hits it from all directions, the sun dries it, frost works into it, and the only thing holding the whole structure together is a few millimetres of mortar in each joint. Repointing is the job of cutting out the decayed mortar and filling the joints with fresh mortar. It is one of the most common pieces of chimney maintenance, and it is also easy to do badly.
Here we describe the trade method: the signs that point to repointing, what raking out involves, how the choice between lime and cement is made, and how a roofer reaches a stack safely. If you wonder whether to try it yourself, our separate answer on repointing your own chimney deals with that. Costs are in our chimney repair cost guide, and the wider service is on the chimney repairs page.
Signs that a stack is ready for repointing
A fair amount can be read off a chimney by standing back in the street with a pair of binoculars or a zoomed phone photograph. You are looking at the joints first and the bricks second.
What the mortar tells you
Healthy mortar sits close to the face of the brick or stone, slightly recessed at most, and is firm to the touch. Tired mortar is recessed deeply, so that a dark shadow runs along every course. It may be sandy and crumbling, with grains falling out when rubbed, or it may have cracked away from the edges of the bricks, leaving hairline gaps. Moss and small plants rooting in a joint are a sign that the mortar is soft enough to hold seed and water.
Clues elsewhere
Grit in the gutter below the stack, or sand-coloured grains on a flat roof or in a valley, comes from somewhere above. A loose brick that rocks, or a stack that has started to lean, goes beyond repointing, and our leaning chimney guide covers what to do. Brown stains on the chimney breast indoors may point to water getting in through the stack. The cause is not always the mortar, and we trace it properly in the guide to why chimneys leak.
Why it matters
Open joints let rain into the body of the stack. Water that sits in the masonry freezes, expands and cracks the brick face, a fault called spalling. Water running down the flue lining can reach the chimney breast and the ceiling beside it. And a stack with soft mortar is losing the very thing that keeps its bricks in line, so a repair left too long becomes a rebuild.
Looking from the ground is a starting point only. Joints on the side facing away from you, and the flaunching at the very top, are invisible from below.
Raking out: where good repointing is won or lost
Most of the skill in repointing is in the preparation. Fresh mortar pushed over crumbling mortar will not hold, so the old material has to come out first. This is called raking out.
How deep the old mortar comes out
The aim is to reach sound mortar behind the weathered surface, and to cut a clean, square-edged channel for the new filling to grip. Depth varies from joint to joint, because the decay does too. We do not rake to a set number in every joint. We rake until the mortar left behind is firm, and then check that the channel has straight sides and a flat back. Shallow, rounded or greasy slots give new mortar nothing to hold onto.
Tools, and care for the masonry
The old mortar is cut out by hand with a chisel and hammer or a thin rake. Power grinders are fast but they are also the quickest way to widen a joint and chip the edges of soft brick or stone, and on a historic stack the brick is more valuable than the mortar. On a fragile stack we stay with hand tools even though they are slower. Everything that comes out is bagged, because stray lime or cement dust on the roof below stains the covering and clogs gutters.
Brushing and wetting
Once the channels are clean, they are brushed out and dampened. A dry brick takes the water out of fresh mortar so quickly that it shrinks, cracks and fails to bond. Dampening stops that. Good practice is to wet the masonry thoroughly beforehand, but never so much that it is running with water when the mortar goes in.
Filling and finishing the joints
With the joints prepared, the new mortar goes in from the back of the channel forward, in layers if the joint is deep, so that no hollow is left behind it. Each layer is pressed firmly so that it bonds to the brick, then left to stiffen before the next goes on.
Matching the joint profile
The finish of the joint is as important as the mortar. An old stack has a particular joint profile: flush, slightly recessed, bucket-handle or weathered. We match what was there, so the repair blends into the stack and does not stand out as a patch. Joints are also finished to shed water, not hold it, which is why a deeply recessed joint is rarely right on an exposed chimney.
Colour and texture
Mortar is made from sand and a binder, and the sand decides the colour and the texture. A mortar that is too yellow, too grey or too fine will look wrong against old brick or weathered stone from the street. We usually mix a small sample, let it dry and compare it with the original before we start the whole stack. A match is easier to achieve than to fix afterwards.
Protection while it cures
Fresh mortar needs time to stiffen without being washed out by rain or scorched by sun. Where the weather is unsettled, we cover the work with hessian or sheeting at the end of the working day. If the forecast is harsh we postpone the pointing rather than risk the joints. Our how long chimney repairs take answer explains how weather and curing shape the programme.
Lime or cement mortar: choosing what goes back
Mortar is not a neutral filler. It has to behave properly alongside the brick or stone around it, and that is the main reason to choose between lime and cement with care.
The principle: mortar should be the softer partner
In a traditional wall the mortar is meant to be slightly softer and more porous than the masonry. Moisture then leaves the wall through the joints, and any movement is taken up by the mortar, which can be repointed in future. Repoint soft old brick or limestone in a hard, dense cement mortar and the roles reverse. Water that cannot escape through the joints leaves through the face of the stone, which then flakes and decays. The joint outlasts the brick, which is the opposite of what is wanted.
Where lime belongs
Older stacks built in lime, soft brick or Cotswold limestone are best repointed in a lime mortar. It is more flexible, lets the masonry breathe and can be matched in colour with local sands. Our guide to repairing stone chimneys with lime goes further into stone, and our answer on the best mortar for chimney repairs compares the mixes side by side.
Where a cement mix is reasonable
A stack built in the last century from hard, dense brick, in a cement mortar, is usually repointed in a similar mix. Matching the original mortar to the original masonry is the rule, not a belief that one binder is better than another. On such stacks the sand and the mix proportions still matter. Chimneys are exposed to more weather than any other part of a house, so a mix that is too weak for the situation will wash out sooner than it should.
Listed houses
Repointing a listed house usually calls for lime and may need consent for a change of mortar or joint finish. The council treats every part of a listed house as protected, so ask its conservation team before any mortar change goes ahead.
Reaching the stack safely
Repointing a chimney is slow, careful work done with both hands, which means the roofer has to be on a stable platform and not hanging from a ladder. A stack on a pitched roof is also at the edge of a slope, so the first question on any repointing job is how to get to it.
Options for access
A chimney on a gable end can often be reached from a scaffold tower or a scaffold lift standing on the ground beside the wall. A stack set well in from the eaves may need full scaffold carrying a working platform, or a roof ladder and a proper chimney platform on the slope. Each has a cost, covered in our page on scaffolding for chimney work. We decide which suits the house at the survey.
Streets and gardens
Stroud is built on hills, and many houses stand on terraces or lanes with no space at the front. A scaffold might have to be built on a slope, rest on a boarded base or be reached over a neighbour's garden. We work out these questions before the quote, so the cost of access is on the page and does not appear halfway through the job.
Working at height
Anyone cutting out joints at the top of a stack is working at height, with mortar dust and loose brick falling around them. A proper platform, a safe edge and a covered roof below are part of doing the job, not extras. If the survey shows a stack too fragile to rake from a platform, we tell you plainly and recommend a rebuild.
Where bats or nesting birds are present in or around a stack, work has to wait, and that is discussed with you before we begin.
Repointing, flaunching and flashing in one visit
Repointing is rarely the only repair a stack needs. The scaffold is up, the roofer is at the top and the other weak points are within reach, so it makes sense to look at them in the same visit.
The flaunching
The sloping mortar on top of the stack, around the base of the pots, is the part most exposed to rain and frost. If it is cracked or crumbling, repointing the joints below does little for a stack that still lets water in from above. Our guide to chimney flaunching describes how it is renewed, and it is normally renewed on the same visit as the joints.
The flashing
Lead flashing closes the gap where the chimney rises through the slope. Lifted or split lead is a likely cause of a stain below, even when the mortar above is sound. We check it during the survey, and where it has failed it is part of the same quote.
Pots and capping
Loose pots, and flues left open to rain, are other points to check while we are there. They are separate repairs, and each is covered in the quote.
What the survey records
At the survey we photograph the stack and the roof around it, record the condition of the joints and the lead, and note what the repointing is likely to include. The written quote then follows for the whole job, with the access plan noted. Because the work is done from scaffold or a platform, we can add anything found once the old mortar is out; we agree it with you first. A 10-year workmanship guarantee backs the finished work, issued on paper, and we remove the waste afterwards.