Chimney guides

Lime Mortar vs Portland: The Mistake That Destroys Old Chimneys

The most damaging thing that happens to old Massachusetts masonry is not neglect. It is being repointed by someone who did not know there was a difference.

Soft handmade brick with wide lime mortar joints on an old chimney

Short answer

Mortar is meant to be softer than the brick around it, so that movement and moisture damage the joints first — joints can be raked out and replaced, brick cannot. Massachusetts masonry built before roughly 1900 was laid in lime mortar, which is very soft, because the handmade brick of the period is soft too. Repoint it with modern Portland cement mortar and the relationship inverts: the brick becomes the sacrificial part and starts spalling.

Why softer mortar is the whole point

A brick wall moves. It expands in heat, contracts in cold, settles, and takes up and gives off moisture. Something in the wall has to absorb that, and the design intent has always been that the mortar does.

Lime mortar is soft, slightly flexible and porous. It lets moisture pass through and evaporate out of the joint rather than through the brick face, and it crushes rather than cracks when the wall moves. When it eventually fails, you rake it out and replace it. That is repointing, and it is a maintenance cycle rather than a repair.

Portland cement mortar is harder than the soft handmade brick used in this state before the twentieth century. Put it into those joints and the joint no longer gives. Moisture is driven out through the brick face instead, and movement is taken by the brick instead. Both of those destroy the brick, and it happens over a few winters rather than a few decades.

How to tell what you have

Age is the strongest signal — pre-1900 is almost certainly lime. Lime mortar is noticeably lighter in color, often with visible lumps of unburnt lime, and it can be scratched with a coin where Portland cannot. If the joints have already been repointed in gray cement, you will usually see it: harder, darker, sharper-edged, and frequently smeared over the brick faces.

How mortar should be specified

The answer is not "lime mortar" as a category. Lime mortars vary considerably, and matching matters.

By analysis, not by eye

A sample of the existing mortar is tested for binder type and aggregate, and the replacement is matched to it — including the sand, which is what determines color. On a property in a historic district this is frequently a requirement rather than a courtesy.

Joint profile matters too

The way a joint is finished affects how water runs off it. Matching the original profile is part of doing the job properly and is often what a district commission is actually looking at.

It costs more and takes longer

Lime mortar cures slowly and cannot be rushed. A contractor who tells you the job needs a longer weather window is telling you the truth, not padding the schedule.

If it has already been done wrong

This is the common case, and the honest answer is that it is not always worth undoing.

Cutting out hard cement pointing without damaging soft brick is slow, skilled and expensive, and on a chimney it may not be justified. What is usually justified is stopping the water: fixing the crown, sorting the flashing, capping the flue. Most of the damage attributed to the wrong mortar is actually the wrong mortar plus water, and you can remove half of that equation for a fraction of the cost.

Need this done?

Chimney Masonry Repair across Massachusetts

Free to use, no obligation — we never charge you.

See chimney masonry repair →

Frequently asked questions

Is lime mortar repointing more expensive?

Usually, because the material costs more, the analysis is an extra step, and the work is slower. It is considerably cheaper than replacing brick that hard mortar destroyed.

Can I tell from a photograph?

Sometimes. Wide, pale, slightly irregular joints on soft-looking brick with variable color suggest lime. Narrow, gray, sharply tooled joints suggest a later repointing. A scratch test settles it.

Does this apply to the whole chimney?

It applies to whatever was originally laid in lime. Where the top few courses were rebuilt in the 1970s in cement and the rest is original, those are two different repairs on one stack.