How to repoint a chimney stack

Video by Skill Builder. This walk-through draws on the video "Repointing Old Brickwork - Best Mix & Tools" from the Skill Builder channel, presented by Roger Bisby — building journalist, former BBC Watchdog expert, and someone who has been getting into the detail of brickwork repairs for decades. His section on mortar mix ratios for older brickwork is particularly worth watching before you start.

1. Assess the chimney from the ground first

Use binoculars or a camera with a zoom lens to inspect the mortar joints from the ground before you commit to getting up on the roof. Look for joints that are visibly recessed, crumbly, or where the mortar has dropped out entirely. Check the flaunching — that is the sloped cement bed around the base of the chimney pot — as this cracks and lifts away over time and is a common entry point for water.

If the brickwork itself is spalling or the stack looks like it is leaning, that is beyond a repoint and needs a builder or structural assessment. Repointing is for joints that have eroded or softened; it will not fix bricks that are already damaged. Be honest about what you are looking at before you drag a ladder up there.

2. Set up safe access

Working on a chimney stack requires either a roof ladder hooked over the ridge or scaffolding, depending on the pitch of your roof and the height of the stack. A standard domestic extension ladder leaned against guttering is not adequate — it is unstable and leaves you without both hands free to work. On most two-storey houses in Sandwich and the surrounding villages, hiring a scaffold tower for a day is the sensible option.

Wear a hard hat, non-slip boots, and do not work in wet or windy conditions. That sounds like standard advice you can ignore, but a chimney stack on a damp October morning in East Kent with a bit of breeze is a different proposition to a dry July day. Plan accordingly.

3. Rake out the old mortar

Use a plugging chisel and club hammer to rake out the existing mortar to a depth of at least 15 mm — 20 mm is better. You need enough depth for the new mortar to key in properly. Shallow repointing — just smearing new mortar over the face of the joint — will crack and fall away within a couple of winters. Take your time and work methodically across the chimney face.

A small angle grinder with a mortar-raking disc makes this faster on large areas, but on a chimney stack where space is tight and the bricks may be older, hand tools give you more control. Older properties in this part of Kent often have lime mortar joints — if the mortar is soft and pale, that is a sign you are dealing with a lime mix, which matters for the next step.

4. Choose the right mortar mix

This is where a lot of DIY repointing goes wrong. Modern cement-heavy mortars are too rigid for older brickwork. If you fill Victorian or Edwardian lime mortar joints with a hard cement mix, the bricks become the weakest link and start to spall as the wall moves with temperature changes. For any chimney on a pre-1950s property, use a mix of one part cement, one part lime putty, and five or six parts sharp sand. It gives a slightly softer, more flexible joint that works with the brickwork rather than against it.

On a more modern stack — post-1960s engineering brick, for example — a standard 1:4 cement and sand mix is fine. If you are not sure what you are dealing with, err on the side of more lime rather than less. To be fair, a slightly soft mix that bonds well will always outlast a hard mix that cracks at the edges.

5. Apply the new mortar and finish the joints

Dampen the raked joints with a brush dipped in water before you apply any mortar — dry brickwork will suck the moisture out of the mix too quickly and weaken the bond. Work the mortar in firmly using a pointing trowel or a small hawk and leaf trowel. Press it well into the back of the joint, not just the front face.

For the finish, a weatherstruck joint — angled slightly inward at the top so water drains away from the face — is the traditional and most practical choice for exposed chimney work. A bucket handle profile (a slightly concave finish) also works well. Avoid a flush or raised finish on a chimney; both collect water on the top edge of the joint and accelerate erosion. Once the mortar has gone off to a firm, thumbprint-resistant state, brush the face lightly with a dry stiff brush to remove any smears.

6. Repair the flaunching if needed

While you are up there, check the flaunching around the base of the pot. If it is cracked or has lifted away from the stack, rake it out and replace it with a fresh mix — roughly 1:3 cement and sharp sand, built up in a smooth dome shape that slopes away from the pot and overhangs the chimney face slightly. It does not need to be pretty, but it does need to be solid and to shed water.

Fit a chimney pot cowl or cap if the flue is no longer in use. An open pot on a disused flue is a fast track to damp problems inside the chimney breast, and a simple terracotta cap costs a few pounds and fits in minutes. Mind you, check with a flue specialist first if there is any doubt about whether the chimney is fully redundant — some older properties in East Kent have complex shared flue arrangements that are not obvious from outside.

When to call a handyman

Call Richard if you are not comfortable working at height, if the roof pitch is steep or the stack is tall, or if the flaunching needs a complete rebuild. Access and safety are the main reasons this job gets handed over — the repointing itself is not complicated, but getting up there safely, with the right tools and materials to hand, is where having someone experienced makes a real difference. Richard can also cast an eye over the rest of the stack and flag anything else that needs attention while he is up there.