We're asked this most in September and October, usually by a client deciding whether to commission work now or wait for spring. The short version: cleaning and surveying can happen in almost any month; lime mortar work cannot, and that's the constraint that actually shapes a sensible schedule. Treating "restoration" as one activity with one weather window is where the confusion starts.
What the weather genuinely stops
Non-hydraulic lime mortar, the specification used on the great majority of London's pre-1900 stone and brick, sets by carbonation: it absorbs carbon dioxide from the air as it dries, slowly, over several weeks. That process needs the mortar to stay damp and to remain above roughly 5°C. Below that, the water in the mix can freeze before carbonation gets underway, and frozen mortar doesn't just pause. It fails: the joint pushes apart as the ice expands, and the work has to be cut out and done again.
This is the one genuinely hard constraint in the calendar. We do not repoint or carry out lime-based stone repairs once overnight frost becomes a realistic risk, typically from November through to March in London, and we tell clients that plainly rather than quote a date we can't keep.
What doesn't stop for winter
DOFF steam cleaning is a different story. It's carried out above 4°C with no frost forecast, which rules out relatively few days even in a London winter, and the heat of the steam works in the method's favour rather than against it. Site visits, surveys, trial panels and, for listed buildings, the consent application all proceed regardless of temperature.
If anything, autumn and winter are the better months to get a survey booked. Scaffolders and access contractors are less stretched than in the June-to-September peak, and a trial panel agreed now means work can start the moment spring conditions allow, rather than joining a queue that forms every April.
What summer changes, and it isn't nothing either
The assumption is usually that summer is the easy season and everything else is the problem. It isn't quite that simple. Fresh lime mortar cures by absorbing carbon dioxide slowly while staying damp, and direct summer sun on a south-facing facade can evaporate that moisture faster than the carbonation can keep up with. If the joint dries out before it has properly set, it can crack rather than cure, which is a different failure from frost damage but just as avoidable. On south and west elevations in June, July and August, we protect fresh pointing with damp hessian and, where access allows, keep it out of direct sun for the first few days. It's a small extra step, but skipping it is how a summer repointing job ends up needing revisiting.
Peak season also means peak demand for scaffold and access contractors, which is the other reason autumn and winter surveys are worth having: a trial panel and method statement agreed in November puts a project ahead of the queue that forms every spring, rather than behind it.
Why "wait until spring" often costs more time than it saves
On a listed building, Listed Building Consent typically takes six to eight weeks from a complete application, and the application itself depends on a trial panel result that needs 48–72 hours to assess properly. None of that clock depends on the weather. A client who waits until March to start the conversation is often still waiting on consent in May, having lost the whole early part of the working season to paperwork that could have run through the winter in parallel.
We'd rather have the survey and trial panel done in November, consent granted over the winter, and scaffold going up in the first dry week of March.
Staging a combined project
Most projects on period buildings combine cleaning with some repointing or stone repair, and the two don't need to happen in the same window. Cleaning can run through autumn or even a mild winter; the mortar work waits for the temperature to hold reliably above 5°C, which in practice means starting no earlier than April in most years. We sequence projects this way routinely: clean first, assess what the cleaned surface actually needs, then schedule the lime work for when it will cure properly, rather than forcing it into a date that suits the diary rather than the mortar.
A typical example: a Victorian stock brick terrace booked in for a survey in October gets its DOFF clean in November, once soiling and the state of the existing pointing are properly visible. The clean reveals which joints have failed and need raking out and repointing, which is added to the specification. That mortar work is then scheduled for the following April, as soon as overnight temperatures hold reliably above 5°C, with the scaffold already booked from the winter survey rather than chased fresh in spring. The building spends the winter looking better than it did, and the client isn't starting the repointing conversation from scratch six months later.
What we'd recommend, if you're weighing it up now
If a project needs Listed Building Consent, start the survey and trial panel now, whatever the month. If it's cleaning only, on an unlisted building, book it whenever suits: the method tolerates the season better than most clients expect. If lime mortar repair or repointing is part of the scope, plan for that part of the work to fall between April and October, and use the months before that for everything that doesn't depend on ground temperature: the visit, the trial, the paperwork, the scaffold booking.