Newmarket Roofers carries out chimney rebuilds in Mildenhall, using the right materials for the job and explaining the options honestly before any work starts.
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A chimney stack rebuilt with the wrong materials will fail again sooner than it should. Getting the material choices right is not complicated, but it does require someone who understands how different bricks, mortars and capping systems behave on an exposed rooftop over time. Here is an honest breakdown of the main choices involved in a chimney rebuild, and what to ask your roofer about each one.
When a chimney stack is rebuilt from the roof line upward, the new brickwork needs to match the existing stack closely enough that it does not look jarring, and closely enough that the two sections move in a similar way over time. A mismatch in brick density or porosity can create a weak point at the joint between old and new.
The choice is generally between a standard facing brick and a reclaimed or salvaged brick that more closely matches the original. Reclaimed bricks cost more and take longer to source, but on a visible chimney on a period property they are usually worth it. On a less visible rear stack, a close-match new brick is often the practical answer.
The key question to ask is: what type of brick was used originally, and what is being used for the new courses? A good contractor will be able to show you the replacement brick before work starts, not just describe it.
Mortar mix is where more chimney rebuilds go wrong than almost anywhere else. The choice is broadly between a lime-based mortar and a cement-based mortar, and the wrong choice can cause more damage than the original problem.
Older properties - broadly, anything built before the mid-twentieth century - were typically built with softer bricks and lime mortar. Lime mortar is slightly flexible, which allows the very small seasonal movements in a chimney stack to be absorbed without cracking. If a contractor repoints or rebuilds such a stack with a strong cement mortar, the rigidity of the cement means that when movement occurs, the brick face cracks rather than the joint. Bricks are harder and more expensive to replace than mortar, so this is a bad trade.
Modern bricks are denser and harder, and a cement mortar is generally appropriate for stacks built in the latter half of the twentieth century. The specification should be matched to the age and condition of the existing brickwork, not chosen out of habit or because it is what the contractor normally uses.
The flaunching is the sloped mortar bed that sits at the top of the stack and holds the chimney pot in place. It is one of the first parts of a chimney to fail because it sits fully exposed to rain, frost and sun, is unsupported at the edges, and is often finished with a mix that is too weak or too thin.
When a chimney is rebuilt, the flaunching is nearly always replaced at the same time - even if the old pot is being reused. Trying to preserve failing flaunching around new brickwork makes no sense.
Standard flaunching is built up in mortar and finished with a smooth slope to shed water away from the pot. Some contractors use a proprietary pre-mixed product for this rather than site-mixed mortar. Both can work well if applied correctly and at a sufficient thickness - a thin application will crack in the first hard frost. The key is that the flaunching must be properly bedded and sloped, with no horizontal surfaces where water can pond.
A clay chimney pot in good condition can be reseated on new flaunching and will last many more years. Pots that are cracked, chipped at the base, or have had the top broken off should be replaced rather than reused, as a damaged pot admits water directly into the flue.
Replacement pots are available in a range of standard sizes and profiles. On a period property where the original profile is distinctive, matching it matters more than the price difference between pot types. On a standard post-war property, a plain roll-top or can-style pot in a matching terracotta colour is usually the right choice.
For chimneys that are no longer connected to an active fireplace, a pot cover or purpose-made capping unit can be fitted instead. This keeps rain and birds out of the flue while still allowing some ventilation, which helps prevent damp building up inside an unused stack.
The lead flashings at the base of the chimney - where the stack meets the roof slope - are a separate system from the stack itself, but they are always assessed when chimney work is carried out. Worn or cracked lead is a common source of water entry that is sometimes blamed on the stack when the stack itself is sound.
Lead is the standard material for chimney flashings in the UK because it is durable, easily shaped, and capable of accommodating movement between the stack and the roof. Properly installed lead work, bedded into the mortar joints as a stepped flashing on the sides and dressed as a back gutter at the top, will outlast several roof coverings.
Cheaper alternatives exist - lead-effect flashing tapes, for instance - but these are not a like-for-like replacement and are not appropriate for a permanent repair on a full chimney rebuild. If a contractor proposes a flashing tape as the permanent solution on a rebuilt stack, ask why they are not using lead.
Newmarket Roofers carries out chimney rebuilds in Mildenhall and across the wider area including Newmarket, Isleham, Wicken, Chippenham and neighbouring villages, as well as further afield to Bury St Edmunds, Ely and Cambridge. All chimney work is carried out from purpose-built scaffold. If you want a clear explanation of the options and materials before any work starts, call us to arrange a chimney survey.
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