Why Cement Mortar Ruins Older Brick Chimneys
I’ve been climbing up onto roofs around Sittingbourne, Faversham, and the Medway Towns for more than thirty years. One of the most common problems I come across on Victorian terraces and period cottages isn't just old age—it's well-meaning repairs done ten or fifteen years ago where somebody smeared hard sand-and-cement mortar over soft, historic brickwork.
To understand why that causes so much damage, you have to look at how old bricks behave. Up until the 1920s, bricks in Kent—like traditional yellow stock bricks or soft red clay bricks—were fired at much lower temperatures than modern bricks. They act like a firm sponge. When driving rain hits your chimney stack, the bricks soak up moisture, and when the wind and sun come out, that moisture needs to evaporate back out into the air.
Originally, those bricks were laid in soft lime mortar. Lime is intentionally softer and more porous than the brick itself. It acts as a sacrificial wick: moisture is drawn out through the mortar joints rather than through the face of the brick.
What Happens When You Trap Moisture Behind Cement
Ordinary Portland cement is almost completely waterproof and sets harder than the old brick around it. When a period chimney is repointed with hard cement, two things happen:
- Freeze-thaw spalling (blown brick faces): Because water can no longer escape through the hard cement joints, it gets trapped inside the softer brick. When we get a hard winter frost, that trapped water freezes and expands, blowing the front face clean off the brick. Once the baked outer crust of a Victorian brick is gone, the powdery core crumbles rapidly.
- Cracked joints and damp lofts: Chimneys take the full force of high winds—especially near the Swale estuary, Sheerness, and Whitstable. Old stacks move slightly in the wind and as temperatures change. Cement is brittle; it shrinks slightly as it cures and cracks away from the brick, opening up thin hairline gaps that suck rainwater straight down into your loft or bedroom chimney breast.
Lime vs Cement: How They Compare on the Roof
Customers often ask if lime mortar is sturdy enough to survive high up on an exposed roofline. The short answer is yes—provided the right grade of hydraulic lime is used and properly tended while it cures. For chimney stacks, we use Natural Hydraulic Lime (typically NHL 3.5), which sets chemically when mixed with water as well as absorbing carbon dioxide from the air.
| Feature | Hydraulic Lime Mortar (NHL 3.5) | Sand & Cement Mortar |
|---|---|---|
| Best Suited For | Pre-1919 properties, Victorian terraces, Georgian cottages, soft red & yellow stock bricks. | Post-1930s semi-detached houses, modern estates, hard machine-made bricks. |
| Breathability | High — draws moisture out of the brickwork so the stack dries out quickly after rain. | Low — seals the joint, forcing moisture to evaporate through the brick face instead. |
| Flexibility | accommodates slight thermal movement and wind sway without cracking away from the brick. | Rigid — prone to hairline shrinkage cracks if the underlying masonry shifts. |
| Weather Conditions Needed | Cannot be laid in freezing weather (needs 5°C and rising) and requires damp-curing protection in hot sun. | Sets faster, though still must not be laid during frost or heavy rain. |
When Is Cement or a Gauged Mix the Right Choice?
Lime isn't a magic bullet for every single roof. If you live in a 1950s semi-detached house in Rainham, a 1970s bungalow in Bapchild, or a newer home in Ashford, your chimney was built using dense, kiln-fired bricks and Portland cement mortar from day one.
On modern brickwork, using a well-proportioned sand-and-cement mortar (often plasticised or gauged with a small amount of lime to improve workability and reduce shrinkage) is completely appropriate. The bricks themselves are hard enough to resist frost, and a strong mortar bed is needed on exposed flaunching—the sloped mortar cap at the very top of the stack that holds your chimney pots in place.
When I inspect your chimney, the first thing I look at is the brick type, the original bedding mortar behind the outer face, and how exposed the stack is to the weather. I’ll tell you plainly which mix belongs on your roof and why.
How I Repoint a Chimney Properly (And Why "Butter Pointing" Fails)
Whether your chimney needs hydraulic lime or cement, the mortar is only as good as the preparation underneath it. A common shortcut in the trade is "butter pointing" or "strap pointing"—where someone scrapes off a few millimetres of loose dust and smears a thin ribbon of fresh mortar over the front of the joint. Within two winters, frost gets behind that thin crust and pushes it straight off onto your roof tiles.
Because I do the work myself without passing jobs off to subcontractors, every chimney repointing job I take on follows the proper method:
- Proper Safe Access: You cannot rake out and repoint a four-sided chimney stack properly off the top of a ladder. We put up proper scaffolding so every face of the stack, the pot flaunching, and the stepped lead flashings at the base can be reached safely.
- Grinding & Raking Out to 20mm–25mm: All old, crumbly mortar is raked out to a depth of at least 20mm (about three-quarters of an inch) using dust-controlled tools, taking care not to nick the edges of soft period bricks.
- Flushing and Dampening the Joints: All loose dust is brushed and washed out. Crucially, the brickwork is dampened down with clean water before pointing. If you push fresh mortar into bone-dry Victorian brick, the brick sucks all the moisture out of the mortar before it can set properly, leaving it weak and powdery.
- Full-Depth Ironing & Brushing: The new lime or cement mortar is pressed firmly into the back of the joint with a pointing iron so there are no air pockets. With lime mortar, once it hits the "green" leather-hard stage, we Stipple-brush it with a churn brush to expose the coarse sand grit and help the wall breathe.
- Checking the Lead Flashings & Pots: While the scaffold is up, I always check the mortar flaunching around the chimney pots and the Code 4 or Code 5 lead flashings where the stack meets the tiles. Half the time, a "leaking chimney" is actually a split lead back-gutter or loose flashing joint that needs dressing back in.
Conservation Areas, Shared Stacks & Highway Permits
If you live in a period property around Faversham’s conservation zone, the historic streets of Rochester, or near Canterbury, using traditional lime mortar and matching sand aggregate isn't just good building practice—it's expected under local heritage guidance to preserve the character of the brickwork.
Two other practical things to bear in mind before work starts:
- Shared Party Wall Stacks: On terraced and semi-detached houses across Sittingbourne, Chatham, and Gillingham, the chimney stack usually straddles the boundary line between you and your neighbour. For simple repointing you rarely need formal paperwork, though it is always best to chat with your neighbour first—frequently they want their half of the stack repointed at the same time to share the scaffolding cost. If a stack is so badly decayed that it needs chimney stack removal or partial rebuilding, the Party Wall etc. Act 1996 applies.
- Scaffolding on Pavements: If your front door opens straight onto the pavement on a tight terraced street, any scaffolding erected on the public footpath requires a highway licence from Kent County Council or Medway Council. I sort out all of that logistics ahead of the job.
Common Questions About Lime & Cement Chimney Pointing
How can I tell if my chimney currently has cement or lime mortar?
Lime mortar is usually a paler, creamy-buff or chalky off-white colour with visible grains of coarse sharp sand in it. Cement mortar is typically dark grey, very smooth, and rock hard. Often on older houses, you will see grey cement ribbons sitting proud of the bricks where a previous repair was done over the original cream-coloured lime underneath.
Can you remove old hard cement mortar without breaking my Victorian bricks?
It depends on how deeply the cement was put in. If the previous person only "butter pointed" the outer 5mm, it usually taps out cleanly. If rock-hard cement was packed deep into soft yellow stock bricks, trying to chisel every speck out can tear the brick faces off. On my initial roof inspection, I test a couple of joints to see whether safe removal is possible or if individual spalled bricks need cutting out and replacing with reclaimed matching stocks.
Why does lime mortar cost slightly more than standard sand and cement?
Natural Hydraulic Lime (NHL) and graded sharp sands cost more from the merchant than ordinary builders' sand and Portland cement, and the labour takes a bit longer. Lime has to be mixed in specific batches, pressed in carefully, brushed back once it firms up, and protected from drying out too fast in the wind or sun. However, doing it once with the right material saves you having to rebuild the entire stack down the line.
What time of year is best for lime mortar repointing?
Spring through to mid-autumn is ideal for lime work. Lime cures much more slowly than cement, and if night-time temperatures drop below 5°C while the mortar is still wet, frost can spoil the set. If you have an urgent chimney leak in the middle of January, I will inspect the stack, secure the lead flashings or weatherproof the area to stop water coming in, and give you honest advice on timing the permanent mortar work.