Can a Flat Roof Be Fitted in Winter?

What winter changes on a flat roof job

A flat roof is only open to the weather while the old covering is off and the new one is not yet sealed. The risk in winter is not the cold itself. It is moisture. Timber that is damp when it is covered stays damp, and a membrane laid over it traps that water against the deck, where it can rot the boards and joists from above. So the first test on any cold-season job is how wet the deck is, not how low the thermometer reads.

Short daylight is the second factor. A crew has fewer hours to strip, dry, lay and seal, so the work is planned in smaller sections, each one made watertight before everyone leaves. On a small extension roof that is easy to do. On a long roof it means taking more care over how the job is phased.

How each covering copes

EPDM is a rubber sheet that is bonded down with adhesive or laid loose and ballasted. It stays flexible in low temperatures and goes down in one large piece, so there are few seams to cure. Adhesive still needs a dry, clean surface, and some adhesives are fussy about cold, so installers pick a product suited to the day. This is why rubber is the first system we reach for when the forecast is poor.

Torch-on felt is melted onto the deck with a gas torch. Cold slows the work and a damp deck prevents a good bond, so it is laid only on dry days. GRP is liquid resin that is brushed onto glass matting and left to harden. Low temperatures and high humidity slow the cure, and a resin that has not hardened properly can stay weak and tacky. Cold nights also mean condensation can settle on the surface before it sets. Our page on EPDM, GRP and felt compared sets the three materials side by side, and liquid roofing covers the coating option.

What a roofer checks before opening the roof

Before any covering comes off, we look at the weather for the days ahead, not just the morning. A settled spell is better than a clear day sandwiched between storms. We prepare enough dry insulation and boards to close up the same day, we keep tarpaulins and temporary sheeting on site, and we plan the strip so that the part we open can be sealed before dusk.

If frost is lying on the deck, it has to go before anything is bonded. Frost melts into water, and a sheet laid over it simply seals that water in. Insulation boards must be dry when they are fitted too, which is why a roof in a wet week is often better left alone until conditions improve than hurried. Boards stored on site are kept covered and off the ground for the same reason, since a pack that has sat in a puddle overnight is no use on a roof.

When it is wiser to wait

Some jobs are better held over. A roof that is already leaking should be made safe at once, and we do that first with a temporary repair, but a full replacement over a saturated deck gains nothing by being rushed. Sustained rain, fog that keeps surfaces wet, and a hard frost that persists through the day are all reasons to postpone and re-book.

A short delay costs very little compared with a covering that blisters or fails to bond, and it is a decision we make together with you, not one we make quietly. Our guide on the best time of year for roof work weighs the seasons against one another, and the article on how long a flat roof takes to replace shows what the programme usually involves.

How we handle it

The free survey comes with photographs of your own deck, so you can judge its condition for yourself, and a fixed written quote follows. If the deck is dry and sound we say so, and if we think a repair is enough we say that too. A winter start is agreed with you around the forecast rather than fixed to a date that the weather may overrule. Every finished roof carries our 10-year workmanship guarantee in writing, and the site is cleared and the waste taken away. For the wider picture on materials and cost, see flat roofing in Basingstoke.