The Frost Doesn't Negotiate: Winter Notes on Concrete, Mortar and Render

November in the yard has a particular sound. The gate scrapes, the kettle goes on, and somebody asks whether the sand is frozen. At McPhail & Sons we have been loading vans and serving builders since 1952, and the first proper cold snap still catches people out. Concrete and mortar are not complicated materials, but they are sensitive to temperature. A few small decisions at the counter and on site can be the difference between a job that lasts and a surface that crumbles by spring.

The frost line, without the jargon

The frost line is simply the depth to which the ground freezes during a cold spell. In the UK it is usually shallow compared with places that have hard continental winters, but it varies with location, soil and exposure. A sheltered garden in Cornwall is not the same as an exposed site in the Borders. For everyday work, the practical version matters more than the technical one: if the ground is frozen, it is not a surface to pour onto.

A frozen sub-base can thaw after the concrete has gone down, leaving a void underneath. That is how you get cracking, rocking slabs and a repair job that nobody wanted. The same applies to materials. Frozen sand, frozen ballast or half-frozen water in the mixer will not make good concrete or mortar, no matter how carefully it is gauged. If the pile has been sitting out overnight and has a crust on it, break it up, check it and keep it covered.

Concrete: cold slows the clock

Fresh concrete does not 'dry' in the way a painted wall dries. It sets by hydration, and that chemical reaction slows down as the temperature falls. Below about 5°C, strength gain is much slower. At or below freezing, water in the mix can freeze, expand and damage the concrete before it has had a chance to gain strength. The usual guidance is to avoid placing concrete when the temperature is at or falling below 5°C unless you have proper precautions in place.

That does not mean every winter pour is doomed. It means the pour needs a bit of thought. Tell your ready-mix supplier what the site conditions are and ask whether the mix needs adjusting for cold weather. Do not start adding your own accelerators or extra cement at the back of the van. More cement does not automatically mean more strength, and the wrong admixture can cause problems that show up months later.

On site, the basics still win. Do not pour onto frozen ground. Have the access clear so the wagon is not sitting at the gate while the mix waits. Plan the pour for the part of the day when the temperature is rising, not falling. Protect fresh concrete after placing with insulating blankets, hessian and polythene, or proper curing blankets. A slab left exposed on a clear frosty night is asking for trouble.

Mortar and render: thin layers lose heat fast

Mortar in a brick bed is a thin layer, so it chills quickly. Small batches mixed with frozen sand or icy water will never behave properly. Keep sand under cover and off the ground on pallets. Bring cement bags inside or store them dry and raised. Mix only what you can use while the temperature is workable, and avoid mixing on frozen boards or a frozen slab.

Render is more exposed again. Do not apply it to a frozen substrate, and do not carry on when the temperature is falling towards freezing. New render needs to be protected from frost for the first day or two, usually with hessian or sheeting, but it also needs to be kept from drying too quickly. A north-facing wall in the shade at three in the afternoon is not the place to be finishing a render coat in December.

One warning worth repeating: never add automotive antifreeze to mortar or concrete. It is not the same as a purpose-made cold-weather admixture, and it can do real damage. If you are unsure, ask before you tip anything into the mixer.

Ordering and storing through November and December 2026

Buy for the week, not the quarter. Cold weather is a poor reason to fill the yard with bags that will sit under a tarpaulin for a month. Keep cement dry, keep sand covered and keep bags off a damp floor. If you are ordering ready-mix, book the slot and make sure the site is ready. If you are buying small bags for a DIY job, store them in a shed or garage that is not damp, and bring them into a warmer space a day before mixing.

For trade customers, a thermometer in the van is a cheap piece of kit. Check the overnight low, not just the morning air temperature. A sunny lunchtime does not undo a frozen night, and the forecast is not a guarantee. If the job is marginal, wait. A day lost now is better than a slab or a rendered wall that has to be taken down later.

A short cold-weather checklist

• Check the ground and sub-base for frost before you pour or lay.
• Avoid placing concrete, mortar or render when the temperature is at or falling below 5°C, unless you have proper protection planned.
• Keep sand, ballast and cement covered, dry and off the ground.
• Never use frozen materials or add extra water to loosen a stiff mix.
• Do not use automotive antifreeze in concrete or mortar.
• Protect fresh work overnight with insulating covers, and check it the next morning.
• Order what you need for the week, not a stockpile for the season.
• If in doubt, ask your supplier. A quick phone call is cheaper than a failed pour.

Cold weather does not stop UK building work, but it does change the rules. The sites that get through winter well are usually the ones that plan the pour, cover the materials and accept that the frost does not negotiate. If you are unsure what you need for a job this month, call into the yard. We have seen a few Novembers since 1952, and we would rather help you get it right first time.