Masonry in cold weather: what winter means for mortar

Mortar does not dry the way paint dries. It cures. The water mixed into it reacts with the lime and cement and slowly turns the paste into something hard. That takes time, and it needs the right conditions. Too hot and dry, and the mortar loses its water before it can cure. Too cold, and the reaction slows down or stops.

Fresh mortar is full of water. If it freezes before it has cured enough, that water turns to ice and expands inside the joint. The mortar ends up weak and crumbly, it does not bond well to the brick, and it may fail in the first year.

The common rule of thumb is that mortar needs to stay above about 40 degrees while it cures. Masons working in cold weather use ways to keep it there, like heated enclosures and blankets, but that costs time and money and there are limits to what makes sense on a house.

In an East Central Illinois winter there are long stretches where nights drop well below freezing. Outdoor brick and mortar work during those stretches is a risk. That is why a lot of masonry gets done from spring through fall, and why it is smart to get on the list for exterior tuckpointing or chimney work before cold weather sets in.

Late fall is a tricky time. Days can be mild while nights freeze hard, so a wall that seems fine at noon can still get frost in fresh joints by morning.

• Spring to fall: the main season for outdoor masonry
• Late fall: watch the overnight lows, not the daytime high
• Winter: outdoor mortar work is limited by the cold
• Inside a basement: temperatures are steadier

It is easy to look at a 50 degree afternoon in November and think it is fine to lay brick. The trouble is the night. Mortar cures over days, not hours, and a joint that went in at noon is still soft and full of water at three in the morning when the temperature drops below freezing.

So the question is not only how warm it is when the work is done. It is how cold it will get over the next few days while the mortar is still curing. That is why masons in this part of Illinois keep one eye on the forecast all fall.

Summer heat is the opposite problem. On a hot, windy day, mortar in a sun baked wall can dry out before it cures. Masons deal with that by wetting the brick, working in shade where they can and keeping the fresh joints damp. It is one more reason the best masonry weather is the mild part of the year.

If you have a project that is not urgent, like tuckpointing a wall that is worn but holding, the best plan is often to line it up for good weather. Get it looked at, know what it needs, and plan the work for the season that suits it.

If something is failing fast, like a foundation wall that is bowing in or a chimney with loose brick at the top, do not sit on it until summer. Call and talk about it now. There may be ways to make it safe in the meantime, and some work can still happen depending on the job and the weather.

Frost matters for foundations in another way too. New footings go below the frost line so the ground freezing and heaving in winter does not move them. And fresh concrete, like fresh mortar, should not freeze while it is curing. So foundation work has to be planned around the weather as well, especially the digging and the concrete.

Every job and every winter is different. Brian Lee Smith can tell you what makes sense for your job and the time of year. Text a photo and your town to (217) 549-2230.