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Chimney flashing is the waterproof barrier where your roof meets the chimney stack. Traditionally this is lead, although you will also see mortar fillets and, on newer builds, proprietary tapes. It takes a battering from driving rain, frost, summer heat and the constant slight movement of the building. Over time, the mortar can crack and crumble, lead can lift or split, and sealant can shrink and pull away. Once that happens, rainwater finds an easy path down the brickwork and into your loft or rooms below. The result is damp patches on the chimney breast, stained ceilings and that unmistakable musty smell.
Before you reach for a ladder, look for clues inside and out. Indoors, check the chimney breast and adjacent ceiling for brown water stains, peeling wallpaper or a damp, cold feel. Outdoors, use binoculars from the ground to inspect the flashing. You are looking for:
If you can see any of these, it is time for a closer look. But only if you can do so safely.
Working at height is the biggest risk in this job. Never lean a ladder against the gutter or climb onto a fragile roof. Use a properly secured ladder with a stand-off, or a roof ladder hooked over the ridge. If you are not confident, stop and call a professional. Assuming you can access the flashing safely, examine it close up. Run your hand along the lead – it should be smooth and firmly dressed into the brickwork. Check the mortar joints for softness. Look behind the chimney for the back gutter, which is a common leak point. Clear any leaves or twigs that are blocking the flow of water. Take photos so you can compare later.
Good preparation is everything. Start by removing moss, dirt and loose debris with a stiff brush and a scraper. Then rake out any cracked or crumbling mortar using a plugging chisel and a hammer, or a mortar rake. You want to create a clean, sound surface about 20mm deep for repointing. Brush out the dust and dampen the brickwork slightly if it is very dry – this helps new mortar bond. If you are using a sealant, the surface must be completely dry and free from grease. Wipe down lead with a clean cloth. Do not be tempted to seal over wet or dirty surfaces; it will fail within months.
For minor gaps where the lead has pulled away from the brickwork, a high-quality external sealant can work well. Choose a butyl, polyurethane or polysulphide sealant – not standard silicone, which does not adhere well to masonry and can shrink. Apply it with a sealant gun, pushing it firmly into the gap, then smooth it with a damp finger or tool. For larger failures, repoint the mortar. Mix three parts sharp sand to one part cement with a plasticiser, and press it into the joint with a pointing trowel. Finish with a neat profile. If the lead flashing has lifted, gently dress it back into the brickwork using a lead dresser or a piece of hardwood. Secure it with lead clips if needed. For a temporary fix, self-adhesive flashing tape can bridge a split, but it is not a long-term solution. Always work from the bottom of the flashing upwards, and check the back gutter is clear and watertight.
If the lead is badly torn, the chimney stack itself is crumbling, or you simply do not have the head for heights, it is time to bring in a roofer or damp-proofing specialist. They can replace the flashing entirely, which is often the only permanent answer when lead has reached the end of its life. To prevent future problems, keep the flashing clear of moss and debris, check it twice a year – ideally in spring and autumn – and repoint any mortar as soon as you spot a crack. After a storm, do a quick visual check. A little maintenance now will save you from expensive damp repairs later. And remember, if water is already coming in, drying out the inside is just as important as stopping the leak outside.
SiteSeal shares practical, down-to-earth guidance on uk property damp-proofing, waterproofing and sealant advice for readers across the UK.
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