When your chimney stack needs rebuilding, the materials you choose and how they are put together will determine how long the work lasts - North Walsham Roofers handles chimney rebuilds in Cromer and the surrounding area.
A chimney rebuild is not simply a matter of stacking bricks back up and filling the joints. The materials used - the bricks themselves, the mortar mix, the flashing, the capping and the pot - each play a specific role, and the wrong choice in any one of them will shorten the life of the finished stack. This page covers the main options honestly, including the trade-offs, so you can have an informed conversation with whoever does the work.
Where a chimney is being rebuilt above roof level, the choice of brick affects both the appearance of the finished stack and how well it holds up over time. There are three broad options.
Reclaimed bricks are the best choice for matching the look of an older property. A good reclaimed brick from a salvage yard will often be harder and more durable than a modern equivalent, because older bricks were fired at higher temperatures. The drawback is availability - finding the right colour, size and texture to match the existing lower courses of your stack takes time, and for some brick types there simply is not a good match available. Reclaimed bricks also need checking individually before use, since a spalled or soft brick is no better than a failed one.
New bricks are consistent in quality and easier to source. A good quality engineering brick is very hard and frost-resistant, making it well suited to exposed chimney work. The disadvantage is that new bricks often look obviously new against older brickwork, and engineering bricks in particular can be very different in colour and texture from the softer handmade bricks used in older stacks. Where appearance matters - and on a period property it usually does - this is worth thinking about carefully.
Matching bricks from the same manufacturer is possible if the existing brickwork is relatively modern and the manufacturer is still producing the same product. Your roofer should be able to identify the brick type from the size and markings and advise whether a close match is available.
Mortar choice is critical and frequently got wrong. The general principle is that mortar should be slightly softer than the bricks it bonds, so that any movement in the stack is absorbed by the joint rather than cracking the brick face. A joint can be repointed; a cracked brick face cannot be repaired without replacing the brick.
For older chimneys built with soft handmade bricks, a lime-based mortar is usually the correct choice. Lime mortar is flexible, breathable and can be repointed without damaging the surrounding brickwork. Portland cement mortar is too hard for this type of brick - it bonds more strongly than the brick itself, so when movement occurs the brick face spalls off instead of the joint cracking. This is a common and unnecessary cause of accelerated chimney deterioration.
For more modern brickwork, a sand-and-cement mix in the right ratio is generally appropriate. The ratio matters - too much cement produces a mortar that is too hard; too little and the joint is weak and washes out quickly. A 1:4 or 1:5 cement-to-sand ratio is typical for chimney work on modern bricks. The mix should also be matched to the colour of the existing mortar as closely as possible, particularly if only part of the stack is being rebuilt.
The lead flashing where the chimney meets the roof slope is where most chimney-related damp problems begin. Lead is the standard material for this junction and has been used for centuries because it is malleable, long-lasting and can be shaped to follow complex profiles. Code 4 lead (the thickness designation) is the minimum for most chimney flashing applications; Code 5 is heavier and is preferred for step and cover flashings on larger stacks.
Lead flashing has a long service life when properly installed - well-laid lead can last many decades. The weak points are the raglets (the grooves cut into the brick mortar joint to hold the flashing in place) and the joints between sections. Lead expands and contracts with temperature, and flashing that is fixed in long, unbroken runs will pull loose at the raglets or crack at mid-point. Correctly installed flashing is laid in short sections, overlapping at joints, with lead wedges driven into the raglets and pointed over with mortar or a flexible sealant.
Some roofers use aluminium or lead-effect flashing tape as a cheaper alternative. These products are not equivalent to lead and should not be used on a permanent chimney rebuild. They have a much shorter service life and are more likely to lift or crack. Ask specifically whether the flashing will be Code 4 or Code 5 lead before agreeing to any chimney work.
The chimney pot and its bedding are the topmost part of the stack and take the worst of the weather. Clay pots are the traditional choice and are still widely available in a range of profiles. They are durable and look right on almost any property. The pot needs to be properly bedded in mortar at the top of the stack - a pot that is simply sat loose will rock in wind and eventually topple. The bedding mortar is haunched (sloped outward) so that water runs off rather than pooling around the base of the pot.
For redundant chimneys - those that are no longer connected to a working fireplace - the correct approach is to cap the flue rather than remove the pot. An air brick or purpose-made ventilating cap allows the flue to breathe, preventing condensation inside the stack. Simply mortaring over the top is not good practice because it traps moisture.
Concrete chimney caps (the flat section of mortar at the top of the stack, also called the flaunching) are a weak point. Flaunching cracks as it ages, allowing water under the pot base. Replacing flaunching at the same time as any chimney rebuild is good practice - patching cracked flaunching rarely lasts more than a couple of seasons.
A chimney rebuild does not always mean replacing the entire stack from roof line to pot. Where only the upper section has deteriorated, it is reasonable to take down to the point where the brickwork is sound and rebuild from there. The junction between old and new brickwork needs careful preparation - the top course of old bricks must be cleaned back to a solid, level surface before new work begins.
A full rebuild from the roof line is necessary where the deterioration extends down to or below the roof structure, where sulphate attack has deformed the lower courses, or where the original construction was poor throughout. On some older properties, the entire stack has been built with inadequate foundations or with a flue arrangement that never met the requirements for good draw - a rebuild is an opportunity to address these issues properly.
If the chimney serves a working fireplace or stove, the flue should be lined. An unlined flue in an older property allows combustion gases to permeate the brickwork and can allow embers to reach the roof space. The options are a rigid ceramic liner (the most durable, fitted section by section from above), a flexible stainless steel liner (easier to install in a chimney that is not straight, and suitable for gas or oil appliances as well as solid fuel), or a pumped-in insulating lining system.
If a chimney rebuild is being carried out on a property with a working fireplace, it makes sense to assess the flue liner at the same time. The scaffold is already up and the stack is open, which makes liner installation far more straightforward than a separate job later.
We carry out chimney rebuilds in Cromer and across the surrounding area. If you want to discuss the materials that would be right for your particular chimney - whether that is matching the brickwork on an older property or choosing the right lead specification - call North Walsham Roofers and we can talk through the options before any work is agreed.
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