The short answer
A roof replacement is planned so that only the area that can be made weathertight in a day is opened up. Stripped sections are covered with underlay or heavy-duty sheeting before the crew leaves, and if heavy rain is forecast the strip is postponed. Light showers during the day are managed by covering the slope as the work goes.
Working in sections
The method that protects a house best is also the simplest: never uncover more slope than you can seal again by dusk. Instead of stripping the whole house at once, a roofer takes a slope, or part of one, and makes it watertight before moving on. The old covering and battens come off, the rafters are inspected, and fresh underlay is laid and battened down, so the section is sheltered even before the final slates or tiles go on.
On a terrace or a long roof the work is split along a natural line such as a party wall or a valley, keeping the open area small. On a hipped roof or one with complex junctions the sections are chosen with the same question in mind: which parts can be dried in during daylight?
Underlay is the membrane that runs across the rafters and sits under the battens. When it is properly lapped and supported it sheds water for a while without any covering above. It is not a substitute for the finished roof, but it is a real barrier, and a slope that has been stripped and re-underlaid can ride out a shower.
Covering the roof at day's end
Whatever the size of the section, the roof is made safe before the crew leaves. Once the old slates have gone and fresh underlay is down, the underlay is secured with battens. Where bare timber is exposed, heavy tarpaulins or sheeting are fixed over it, weighted and tied so wind cannot lift them. Scaffolding helps, since sheets can be roped back to it and on some jobs the whole roof is sheeted as a temporary cover.
Gaps near the ridge, chimneys and valleys deserve special attention. Water finds the lowest point, and a stripped valley or the junction beside a chimney can admit rain unless it is covered. Part-finished leadwork is covered as well. The crew checks the sheeting before they leave and again in the morning.
The loft below needs thought too. Stored belongings and loft floors can be splashed by drips, so move precious items out or cover them beforehand. Advice on what to clear is in preparing the loft before the roofers arrive.
Reading the forecast
Weather is watched closely, because the aim is to prevent trouble, not to manage it. Before each stage the forecast is checked. If heavy or prolonged rain is expected, the strip is held back and the crew turns to tasks that can be done under cover, such as preparing lead, cutting battens or working on a section already made watertight.
Wind matters as much as rain. A strong gust can lift sheeting, throw tiles and make work on the slope unsafe, so high wind stops work even on a dry day. Very cold weather affects some materials, notably mortar and sealants, and frost on slate makes the surface slippery.
In practice a sensible programme has slack built in. A wet week can add days, as the typical length of a replacement makes clear, and starting in a settled spell helps, though no month is risk-free. Whether a particular season is worth waiting for is argued in timing a replacement.
Customers sometimes ask whether a stripped slope will let rain through to the bedrooms. In normal practice it will not, because sealing each section is the whole point of the routine. What does happen is that a very wet spell can push the work back by a few days, and a good contractor tells you so as it happens, not afterwards.
What can still go wrong, and the response
No method is perfect. A sudden squall can catch a section half closed, or a tarpaulin can work loose overnight. The response is quick: re-cover the gap, dry the area and check the timber and the ceiling below. Stopping the water comes first, and finding the cause comes after.
Wet timber is not a disaster if it is handled properly. The structure is inspected as it is exposed, and rafters or battens that are damp but sound are allowed to dry before they are covered. Soft or rotten timber is a separate matter and is repaired, as the answer on whether roof timbers need replacing describes.
Inside the house, check the top-floor ceilings after heavy rain and tell the crew straight away if you see a stain. Early notice limits damage.
Roofs that cannot be stripped in sections
Some roofs are harder. A flat roof cannot be left open in the same way, because water pools on the deck, so its membrane is laid in one continuous operation on a dry day, over a deck that is dry. A roof with lead gutters or a parapet may be worked in stages around the gutter lines. A listed building may have conditions on how and when the roof is opened, and those conditions can shape the programme.
On complex roofs a temporary roof built above the structure on the scaffold is sometimes used, which lets the whole roof be stripped under cover. It adds cost but gives control. If yours has unusual features, ask how weather protection will be arranged before you accept a quote.
Once the work is finished, the regulatory position is explained in Building Regulations and a new roof, and consent questions are answered in whether planning permission applies. The wider service sits on the roof replacement page.
