Toothing New Brickwork Into Old on a Chimney

What toothing means

Toothing is the practice of leaving alternate bricks projecting at the end of a wall, or cutting alternate bricks out, so that the new work can lock into the old in the same way that bricks lock into each other within a wall. The word comes from the way the edge looks, like a row of teeth. It is the opposite of a butt joint, where two faces are simply pressed together with mortar between.

On a chimney the principle is applied vertically as well as sideways. When the head is taken down to sound brickwork, the top course of what remains becomes the foundation for the new work. The new bricks are then bonded across the old joints, rather than laid directly over the same vertical lines, so the load and the weather are shared across the junction. A rebuild that ignores this may look tidy for a season and then open up along the join.

Think of it as the difference between a wall and a pile. A pile of bricks held by mortar can fall over in one piece; a wall with overlapping bricks is a single structure.

Cutting back to sound work

Before anything can be joined, the old work has to be sound. The bricklayer takes the stack down until the brickwork is firm, the mortar is holding and the bricks are not loose or crumbling. That may be lower than the owner expected. Building new brick on top of weak mortar is a poor use of money, because the new work is only as stable as what it sits on, and a stack can look fine from below while being tired inside.

The top surface that remains is cleaned and wetted if needed so it does not draw water out of the fresh mortar too fast. Any loose bricks at the edge are removed and re-bedded. If the stack has been previously patched, as What to Look For on a Chimney From the Ground suggests you might spot, the earlier repair may need to be taken apart too, because a join between two repairs is no stronger than the weakest of them.

Dust and loose grit are brushed away before the first course goes down. Mortar bonds to a clean, sound surface far better than to a powdery one, and this small step is often skipped when a job is rushed.

Stepped joints and keeping the bond

Where new brickwork meets old along a vertical edge, such as when one face of a stack is repaired and the next is left, the join is stepped rather than cut straight. Bricks are removed in a stair pattern, so every course in the new work overlaps the course below it. Each overlap is a mechanical key. A straight vertical cut, by contrast, is only held together by mortar, and mortar in tension is a weak thing.

The bond has to be continued across the join. Whether the original was laid as stretchers, as English bond or in some other pattern, the new work follows the same sequence, and the courses are set out to land at the same height as the old ones. If the gauge differs slightly because the new brick is a different size, the adjustment is made gradually and in a place where it will not show. The effect, when it works, is that you cannot find the join without being told where it is.

The same thinking applies if the old work is slightly out of plumb. The new courses are laid true and any difference is absorbed gradually, not forced out at the join, which would show as a kink in the line of the stack.

Why new and old move differently

Old brickwork has finished most of its settling. It has absorbed moisture, dried, been heated and cooled for decades, and the old lime mortar around it may be soft and slightly flexible. New brickwork and its mortar are still drying, and will shrink a little as they do. Bricks also expand with moisture and heat, and brick types differ in how much. The result is a small amount of differential movement at the junction.

That is the reason for choosing a mortar that is not stronger than the brick, as discussed in Repointing a Chimney: Why the Mortar Mix Matters More Than the Finish. A mortar that is too hard concentrates the movement into the brick, which then splits, whereas a slightly softer mix takes the strain in the joint. A hairline crack along the line of the join in the first season is not always a failure, but a widening crack, or a gap that lets water in, deserves a proper look.

Weather plays its part as well. Fresh work should be protected from hard frost and driving rain while it sets, because a join that freezes before it has gained strength will be weaker for good.

What to expect and what to ask

A well-made join is hard to photograph and hard to find. Poor ones show as a vertical crack at the edge of the new work, a step in the joints, or courses that visibly change height. If you are comparing quotations, ask how the new work will be tied into the old, how far down the stack will be taken, and what will be done with any loose or patched sections. A confident answer is more useful than a low figure.

Roofline Solutions carries out part and full rebuilds from the outside of the stack, and the chimney repairs in Wirral page explains how that works across the peninsula. We do not carry out work inside the house, on chimney breasts or on flues. Any lining or testing of a flue is for a HETAS or Gas Safe registered installer or a sweep, and the written quotation should set out the scope in plain terms.

Be wary of quotations that describe only the finished look and say nothing about preparation. How the old stack is prepared and where the new work starts is the part that decides how long the repair lasts.