How to Repair Chimney Flaunching – DIY Guide
Repair chimney flaunching by chipping out all cracked mortar, cleaning the stack top, then applying a fresh 3:1 sharp sand and cement mix, shaped to slope away from the pots. Allow 24–48 hours to cure before rain. A sound repair lasts 10–15 years.
- Club hammer
- Cold chisel
- Pointing trowel
- Bucket and mixing board
- Wire brush
- Safety rope or roof anchor
- Safety goggles
- Knee pads
- Sharp sand
- Ordinary Portland cement
- Waterproof bonding additive
- Masonry primer or PVA bonding agent
- Clean water
- Plastic sheeting or damp cloth (for curing)
- Fibreglass reinforcing mesh (optional for large repairs)
Step-by-Step Guide
Access the chimney stack safely
Before touching the flaunching, set up safe access — a roof ladder hooked over the ridge and a safety rope anchored to a secure ridge bracket are the minimum requirements. The HSE is clear that working at height carries serious risk; never lean out from a window or stand on unsupported tiles. Check the roof surface is dry and not frosty before you climb. Wearing a hard hat protects against any loose debris you dislodge while working.
Inspect and mark all failed flaunching
Tap across the entire flaunching — the sloped mortar bed that holds the chimney pots to the stack — with the handle of your club hammer. A hollow sound indicates debonded sections that must come out. Mark every suspect area with chalk. While you are up there, check the condition of the chimney pots themselves and inspect the lead or flashing around the chimney base for cracking or lift.
Remove all cracked and hollow mortar
Use a cold chisel and club hammer to cut back every failed section to a firm edge — do not just patch over loose material. Remove the full depth of any cracked flaunching rather than undercutting and hoping the new mortar will hold. Brush the entire surface thoroughly with a wire brush to remove dust, moss, and friable material. The substrate must be clean and mechanically sound before any new mortar is applied, otherwise the repair will fail within one winter. If you find the chimney mortar joints are also deteriorating, repoint those at the same time.
Prime the substrate and mix the mortar
Dampen the cleaned masonry thoroughly — dry brick will suck moisture out of fresh mortar too quickly, causing it to shrink and crack. Apply a diluted bonding agent (PVA or a proprietary masonry primer) to the damp surface and allow it to become tacky before laying mortar. Mix your flaunching mortar to a 3:1 ratio of sharp sand to ordinary Portland cement, adding a waterproof bonding additive to the gauging water. The finished mix should be stiff enough to hold its shape on a trowel without slumping — a wet mix will slump and crack as it dries.
Apply and shape the new flaunching
Work the mortar firmly onto the primed surface, pressing it down to eliminate air pockets. Build the flaunching up in one coat if the repair is under 50 mm thick; use two coats with fibreglass mesh between them for deeper sections. The critical detail is the slope: the finished surface must fall away from each chimney pot at a consistent angle toward the outer edge of the stack, so rainwater sheds clear of the joint. Tool the edge where the flaunching meets the pot with a damp trowel to create a neat, slightly concave joint — this is the point most likely to crack on an old chimney, so get it tight. If you are also dealing with water getting in elsewhere, our guide to fixing a roof leak covers the broader diagnosis process.
Cure and protect the repair
Cover the finished flaunching loosely with damp hessian or polythene sheeting for at least 24 hours — longer in warm or windy conditions. Rapid drying causes surface shrinkage cracks that allow water in and undo the repair. Do not allow frost to reach fresh mortar within 48 hours of application; if cold weather is forecast, reschedule or use a frost-resistant admixture. Once cured, check the surface for hairline cracks and fill any that appear with a flexible exterior sealant before descending. Consistent monitoring as part of your annual roofing maintenance routine will extend the life of the repair significantly.
Frequently Asked Questions
What is chimney flaunching?
Flaunching is the sloped mortar bed on top of a chimney stack that seals the joint between the chimney pots and the brickwork. It is shaped to shed rainwater away from the stack and prevents water from entering around the base of the pots.
Can I repair chimney flaunching myself or do I need a professional?
A competent DIYer can carry out a flaunching repair if they can access the stack safely with proper roof ladders and a safety line. The mortar work itself is straightforward — the risk is entirely in safe access at height. If you are not confident working on a pitched roof, instruct a roofing contractor.
What mortar mix should I use for chimney flaunching?
Use a 3:1 mix of sharp sand to ordinary Portland cement, with a waterproof bonding additive mixed into the gauging water. This produces a mortar strong enough to resist roof-level exposure without being so rigid that it cracks with thermal movement.
How long will a chimney flaunching repair last?
A properly prepared and mixed repair — with all failed material removed, the surface primed, and the mortar correctly cured — should last 10–15 years. Skipping surface preparation is the main reason repairs fail prematurely. Regular inspection as part of your annual roof check catches any new cracking before it becomes a bigger job.
Can I repair flaunching in cold weather?
Avoid applying mortar when temperatures are at or below 3 °C or if frost is forecast within 48 hours of completion. Fresh mortar that freezes before it has cured will fail completely. Use a frost-resistant admixture if conditions are borderline, and always cover the repair overnight.
Rather than building the flaunching in one thick layer, slightly score the surface of a first thin scratch coat, let it stiffen for an hour, then apply the final shaped coat. This two-coat approach dramatically reduces shrinkage cracking and gives the mortar a far stronger mechanical bond — the same principle used in external render.
Sources
- HSE — Working at Height Regulations 2005: guidance for those who work at height — hse.gov.uk
- Which? — Chimney repairs: what you need to know — which.co.uk
- Historic England — Chimneys: maintenance and repair guidance — historicengland.org.uk
This guide is for general information only. Always work safely and follow manufacturer instructions. DIYnut accepts no liability for injury or damage arising from DIY work.
