How to repair a chimney stack

Inspired by a helpful YouTube guide. This article draws on the popular UK video "Save Thousands With This DIY Chimney Stack Repair" from the Skill Builder channel — Roger Bisby's long-running UK building and construction series. Roger walks through the full assessment and repair process in a way that makes it clear what is genuinely DIY-able and what needs a roofer. Very useful watching before you go anywhere near a roof ladder.

1. Assess the stack from the ground first

Grab a pair of binoculars and spend five minutes looking at the stack from the ground. You are checking for missing or crumbling mortar in the joints, cracks in the haunching (the sloped cement collar around the base of the chimney pots), any loose or leaning pots, and whether the brickwork itself looks sound. Take photos — they are useful when you get up close.

If the stack looks to be leaning or whole bricks are loose or missing, stop there. That is structural work that needs a roofer or structural engineer. If it is mortar joints and haunching only, that is typically DIY territory.

2. Set up safe access to the roof

You need a proper roof ladder — one with a wheel to run up the slope and a hook to go over the ridge. This is not negotiable. A standard ladder leant against the gutter is not a substitute. Roof ladders are available from hire shops for a day rate, and they make an enormous difference to how safe and stable the whole job feels.

Wear rubber-soled shoes with good grip. Never work alone on a roof. Tell someone what you are doing and when you expect to be back down. And do not go up in wet, icy, or windy conditions. The job will still be there on the next dry day.

3. Clear out loose and crumbling mortar

Once you are safely on the roof, use a cold chisel and club hammer to rake out any loose or crumbling mortar from the joints. You want to go back at least 15 mm, ideally 20 mm, so the new mortar has enough depth to bond properly. Brush out all the dust and debris with a stiff brush.

Check the haunching around the pots at the same time. If it is cracked through or lifting away from the brickwork, rake it back and plan to replace it in step six. Do not just apply new mortar over old cracked haunching — it will fail again quickly.

4. Mix a lime-based pointing mortar

On a chimney stack, use a lime mortar rather than a standard cement mix. Lime stays slightly flexible, allowing for the thermal movement that a chimney experiences every time it heats and cools. Ordinary Portland cement is too rigid and tends to crack away from the bricks rather than staying bonded. A mix of one part NHL 3.5 natural hydraulic lime to three parts sharp sand works well.

Add water slowly until the mix is stiff enough to hold its shape but still press cleanly into the joint. Too wet and it will slump; too dry and it will not bond. It should look and feel similar to a stiff peanut butter. Make small batches — lime mortar has a limited working time.

5. Repoint the joints

Work from the top of the stack downward, filling the raked-out joints with your lime mortar. Press the mix in firmly with a pointing trowel or a brick jointer, making sure there are no voids or air pockets. Build up in two layers if the joint is deep — let the first layer firm up for 20 minutes before adding the second.

Finish the joints to match the original profile as closely as you can. On most older UK brickwork that means a slightly recessed or flush weatherstruck finish. Avoid a very proud, rounded finish on a chimney — it catches rainwater and accelerates deterioration.

6. Repair or replace the haunching

If the haunching was sound, just clean it and fill any hairline cracks with a flexible exterior sealant. If it was crumbling or cracked through, form new haunching using a 3:1 sharp sand to cement mortar (ordinary cement is fine here, unlike for the joints). Slope it away from the pots at a gentle angle so rainwater runs off cleanly.

Press the mortar firmly around the base of each pot, then smooth it off with a wet trowel. You want a continuous, sloped collar — no gaps, no ridges that will pool water. Check that the pots themselves are still solid and not wobbly before you start — a loose pot needs to be re-bedded in fresh mortar before the haunching goes on.

7. Allow a slow cure

Lime mortar cures slowly and does not like drying out too fast. If the weather is warm and dry, dampen the work lightly with a fine mist of water after a few hours and again the next morning. In cold weather, cover the work with hessian if you can manage it — lime mortar should not be exposed to frost within the first three or four days of laying.

Leave the stack alone for at least a week before checking whether the work has held. Minor hairline shrinkage cracks in the mortar are normal and not a problem. Large cracks, or mortar that sounds hollow when tapped, suggest something went wrong with the mix or the preparation.

When to call a handyman

Call Richard if the stack is leaning or structurally compromised, if whole bricks are loose or missing, if the flashings are also failing, or if you are not comfortable on a roof. Chimney work at height is one of those jobs where a professional saves both time and worry.