31°C, New York
Notification products, updates
Chimney stacks take a battering from every direction. Rain, frost, wind, and summer heat all work away at the mortar joints between bricks or stone. Mortar is deliberately softer than the masonry it holds, so it acts as a sacrificial layer. Over time, that sacrifice shows up as crumbling, cracked, or missing pointing. Once water gets into those open joints, it can travel down through the stack, soak into the surrounding walls, and appear as a damp patch on a chimney breast, ceiling, or even a stone floor below.
Frost is the usual culprit. Water seeps into a hairline crack, freezes, expands, and prises the joint open a little more. Repeat that cycle over a few winters and you have a clear route for rainwater. The leak is rarely dramatic. It often shows as a tide mark, a musty smell, or efflorescence – that white, salty bloom on brickwork or stone. By the time you notice damp inside, the mortar on the stack has usually been failing for years.
Before you reach for a pointing trowel, inspect the entire stack and its junctions with the roof. Failed mortar is a common cause of leaks, but it is not the only one. Work through this list from a safe position – scaffold, tower, or roof ladder with a harness:
If you have stone flooring near the chimney breast, damp from the stack can wick down and cause salts to crystallise under the stone, leading to staining or lifting. Fixing the source above is the only real cure.
Repointing a chimney is not a job for a windy day. Safety comes first: never lean out of a roof window with a trowel. Use proper access equipment, and if you are unsure, call in a roofer or steeplejack. For the mortar itself, traditional UK properties need a breathable lime mix. A strong cement mortar will trap moisture and damage old bricks. A good starting mix is 1 part natural hydraulic lime (NHL 3.5) to 3 parts sharp sand, or 1 part hydrated lime to 3 parts sand for pure lime work. Add water gradually – you want a putty-like consistency, not a sloppy soup.
You will need a plugging chisel, a lump hammer, a pointing trowel, a mortar gun or hawk and trowel, a wire brush, and a hose with a fine spray. Rake out the old mortar to a depth of at least 15–20mm, or two and a half times the width of the joint. Brush out all dust, then dampen the joints so the new mortar bonds properly. Do not soak the wall – just a mist is enough.
Work in small sections, starting at the top and moving down. Pack the lime mortar firmly into the joints in layers, not all at once. For deep joints, fill halfway, let it firm up for an hour or so, then finish. Use a pointing iron or trowel to shape the joint – a slightly recessed or weather-struck finish sheds water best. Avoid smearing mortar onto the face of the bricks; if you do, let it dry slightly and brush it off with a stiff brush.
Protect slate roofing below with a board or tarpaulin while you work. Mortar droppings can stain slates and block gutters. If the stack has a flaunching cap, rebuild it with the same lime mix, sloping it away from the pots. Cure the new mortar slowly. In hot weather, mist it with water for a few days. Never point in frost or direct hot sun. And do not repoint just the front face – the back and sides take the worst weather.
Repointing alone will not stop a leak if the flashings are shot. Check every lead detail: the apron at the front, the step flashings up the side, and the soakers under the slates. Lead should be dressed neatly over the slate or tile, not lifted or cracked. Code 4 or 5 lead is standard for most UK roofs. If the lead is split, replace that section – do not just squirt sealant into it. Where a slate roof meets the stack, make sure the soakers are intact and the slates are not broken. A common mistake is to repoint the stack and ignore the lead, only to find the damp patch returns after the next storm.
Inside, if you have stone flooring near the chimney, hold off sealing it until the masonry has dried out. Trapped damp under a non-breathable sealant will worsen the problem. Once the stack is watertight, let the wall dry naturally – this can take months in an old solid-walled property. Use breathable finishes and avoid cement renders or plastic paints on the chimney breast.
An annual check in late summer, before the autumn rains, saves a lot of grief. Look at the mortar, the flaunching, and the lead. Clear any moss or debris from the flashing and gutter. After a storm, glance up at the stack – missing mortar and slipped slates are easier to spot from the ground with binoculars. If you have stone flooring, watch for new damp patches or white salt blooms; they are early warnings. Traditional homes reward a little regular attention. Repointing a chimney is satisfying work, and doing it with lime mortar keeps the building breathing as it was designed to. If in doubt about access or the extent of the leak, a reputable roofer or conservation-aware builder is worth every penny.
Slate & Stone shares practical, down-to-earth guidance on slate roofing, stone flooring and traditional home maintenance for uk properties for readers across the UK.
Stay Tuned With Updates
Leave A Comment