What weather is too cold, too wet or too hot to lay bricks or point?

Mortar, render and concrete all harden by a chemical reaction with water. Anything that takes that water away too soon, or freezes it before the reaction has produced any strength, damages the work permanently. That is why the weather is not an inconvenience in masonry but a genuine limit, and why the forecast that counts is the one for the next few nights rather than the current temperature. The chart sets out what each material tolerates in each band of conditions.

Safety

Step by step

1. Look at the overnight forecast, not the daytime one.Fresh work is most vulnerable in the hours after it is laid, which are usually the coldest. A mild afternoon followed by a clear frosty night is exactly the pattern that destroys new pointing.
2. Stop at or below freezing.Water expanding as it freezes disrupts the structure of the mix before it has any strength to resist. The material may look right, set later, and then crumble or blow off within a season or two.
3. Check the masonry and the ground, not just the air.Frosted bricks, frozen blocks and frozen ground count the same as freezing air. A north-facing stack of bricks left out overnight can be well below the air temperature in the morning. Bring materials under cover and keep them there.
4. Work with protection in cold but above-freezing weather.Everything is slower. Mortar takes far longer to stiffen and much longer to reach frost resistance, so plan less work per day, and cover the whole of it each evening with insulating material — not just a sheet, which does very little on its own.
5. Do not rely on frostproofer or antifreeze.A frostproofer is an accelerator: it shortens the time until the mix is frost resistant. It does not lower the freezing point and it does not protect work that freezes before that point. Never put antifreeze, salt or anything domestic into a mix.
6. Treat hot, dry and windy weather as a hazard.Evaporation competes with hydration for the water in the mix. Mortar stiffens on the board before you can use it, render skins and crazes, and concrete crusts while still soft underneath. Wind is worse than heat on its own.
7. Adapt to heat rather than adding water.Work in the shade or in the cool part of the day, dampen absorbent bricks and backgrounds before laying, mix smaller batches, keep the sand and the mixed material covered, and cover finished work as soon as it will take it. Retempering a stiffening mix with more water weakens it.
8. Stop in heavy rain and sheet fresh work.Rain washes cement out of a green joint, leaves pale streaks and blotches down the face, and saturates the wall. Sheet fresh work but hold the sheet off the face on battens — polythene lying against new pointing or new render leaves marks that never come out.
9. Let lime and render dictate a longer season.Lime mortar and lime render have essentially no frost resistance until they carbonate, which takes weeks. In the UK that makes them a late-spring-to-early-autumn job, not something to start in October.

Tips

• Damp, mild, still and overcast is the best masonry weather there is. It is not a day to stay indoors.
• Hessian kept damp under polythene is the traditional and still the best protection, in both hot and cold weather, because it keeps the work damp rather than just covered.
• In a cold spell, keeping the sand heap covered and the water tepid makes a real difference to how fast a mortar stiffens and how workable it stays.
• New concrete has the same rules but slightly more tolerance, because a thick pour generates its own heat as it sets.

Common mistakes

• Judging by how it looks the next morning — Frost damage in a fresh mix is not visible at the time. The work sets, looks correct, and fails in a later winter when the weakened material takes its first real frost.
• Covering fresh work with polythene laid straight onto the face — It marks new pointing and new render permanently, and it traps condensation against the surface. Hold sheeting clear on battens.
• Adding water to mortar that has stiffened in the heat — Retempering after the cement has begun to hydrate breaks the structure that is forming. Mix smaller batches instead and keep them shaded.

Questions people ask