Why a wet deck is the real problem
The weather on the day matters less than the state of the surface the membrane lands on. Rain, dew and frost all leave moisture on the deck or in the top of the boards. If a covering goes over that, the water has nowhere to go. It sits between the boards and the membrane, and later turns into blisters, a poor bond, or rot in the timber below.
That is why a flat roof is never laid in the rain, and why a sheet of frost or a heavy morning dew has to clear before work starts. A roof can be well made and still fail early if it was laid onto a damp deck on a day that looked fine from the ground.
A roofer checks the surface before each layer goes down, and a dry deck does not always look dry. Boards that were wet the night before can hold moisture below the surface, and a covering sealed over them will keep that moisture in. This is one reason a winter flat roof job is often planned around a settled spell and not the first free date in the diary.
Cold and the three kinds of covering
How much cold matters depends on how the roof is waterproofed. The rules below are general; every product has a data sheet that names its lowest working temperature, and the installer follows it.
- Adhesive-bonded sheets such as EPDM. The adhesive and the primer have a working range, and a sheet bonded when it is too cold or damp may not grip properly.
- Resin systems such as GRP. The resin cures by chemical reaction, which slows in the cold and can fail to cure properly on a damp or cold surface.
- Torch-on felt. The bitumen is melted with a flame, so it bonds in cooler air, but the deck still has to be dry and frost free, and it stays a hot-work job that needs care.
None of these is a case of simply waiting for summer. Winter work can be done well when the conditions on the day suit the product, and badly in a warm spell with a wet deck. The point is to match the covering and the day, not to follow the calendar. If the installer cannot tell you the product's lowest working temperature and how they will check it, ask again before agreeing a date.
What Manston's averages say about planning
Thanet's Met Office station at Manston has published averages for the thirty years from 1991 to 2020. They are averages, not forecasts. Manston records about 613 mm of rain a year, 110.05 days with 1 mm or more of rain and 21.43 days of air frost.
With that many wet days on the record, a flat roof job in any season has to expect to lose days, and frosty mornings that delay a start are an ordinary part of winter. A programme that allows for them is a sensible one. A roofer who promises a fixed date months ahead for a winter flat roof is promising something the weather will not honour.
For you, that means two things. Ask what the plan is if a layer cannot go down on a given day, and expect the strip and the new covering to be sequenced so that no part of the deck is left uncovered overnight. Spring and autumn spells are not magic either; the same deck checks apply in every season, and a bright April morning can follow a soaking night.
If the roof is leaking now
A leaking flat roof in a cold wet spell is a different problem from planning a new one. The first job is to stop more water coming in: a temporary cover, a patch or clearing a blocked outlet. That is a make-safe, and it is meant to hold for a short time while the deck dries out.
Then comes the dry window. Once the surface is dry and the forecast is settled, the permanent covering can go on. If the make-safe has kept the boards from soaking further, that wait does little harm. Put out buckets and photograph any stains with the date, because photos are useful for a survey and, if you plan to claim, for your insurer, who should hear about the leak first.
When a patch or a new roof is the question, the repair or replace guide sets out the choice, and the torch-on felt guide explains the covering where hot work is involved. Our flat roofing page lists the full range of flat roof work.
