Can a flat roof be replaced in winter?

The waterproofing on a flat roof depends on how well it grips what lies beneath. Moisture on the deck, or very cold surfaces, weakens adhesives, stops resins curing and traps water under the new layer. Trapped water later shows up as blisters, and a roof laid wet can start failing in its first year.

That is why roofers watch the forecast. The aim is to strip, dry, lay and seal in a single window, so that the deck is never left open to rain. There is no ridge to shed the weather, so an opened deck is exposed at once. Winter simply makes the windows shorter and the checks stricter, and it rewards a contractor who will wait for the right day.

Condensation is part of the picture. In a cold, damp spell the underside of a deck can sweat, and a new warm-roof build-up must be laid on boards that are properly dry, with the vapour layer sealed. If water gets trapped at that stage, it stays there for years. A good installer would rather postpone a day than seal moisture in.

Single-ply sheets such as EPDM rubber and PVC are made in a factory, so the material arrives ready to use. They can be laid in cooler conditions, provided the deck is dry and any adhesive is suited to the temperature. Some are fixed mechanically, which avoids adhesive altogether. Our guides to EPDM rubber roofing and PVC single-ply for a house describe how they are fixed.

Felt is mixed. Torch-on felt is heated with a flame, which helps in cold, but the deck must still be dry. Cold-applied and self-adhesive layers rely on grip from the adhesive, and each product states its own minimum temperature, so the data sheet for the one in use is the thing to read.

Hot-melt and glued built-up systems also depend on the temperature of the day, because the bitumen has to flow and grip the layer below. Mechanically fixed single-ply is the most tolerant of cold, as the fixings and seam welds do the work. Ask whoever quotes which method is planned and what conditions it needs, and be wary of anyone who offers to lay any system in any weather.

GRP, glass-reinforced plastic, is a liquid resin laid over glass fibre matting that sets hard on the roof. The chemical reaction that cures it depends on temperature and on a dry surface. In the cold, resin sets slowly or incompletely. In damp conditions, moisture on the deck or in the air can ruin the grip between layers.

A resin that has not cured properly can stay soft, craze or lift later. For that reason a GRP job is booked for a spell of dry, mild weather, and is postponed if the forecast turns. It is far better to wait a few days than to lay a poor skin, because a skin that never cured properly has to be stripped and done again. Dew and fog count as damp too, so a still, bright morning is not enough if the surface has not dried out. Our guide to GRP fibreglass roofing covers the system in more detail.

Winter days are short, and a roof that is stripped but not finished by dusk needs to be made watertight overnight. A careful crew plans the day around that, working in sections and sealing each one as they go. They strip no more than can be covered, and keep temporary protection to hand.

This also affects the cost and time of a winter job. Delays for weather are normal, and the written quote reflects the work, not a promise of a particular start day, because a roof laid in poor conditions costs you later. If your flat roof is leaking now, call or message us. Water coming in jumps the queue, and the opening job is stopping it from doing more harm.

Frost on a deck is a hidden problem. A board that looks dry can carry a film of ice or moisture that melts under the new covering. Roofers check the deck with a meter or by eye, and wait for it to dry. Wet insulation is replaced rather than covered, as it holds moisture against the deck and no longer works.

Winter is also when many leaks show up, because the roof is under the most stress. Our page on flat roof leak signs explains what to look for. If a leak is slow and the roof is structurally sound, a temporary repair through the cold months, with replacement planned for spring, can be a sensible plan.

If your house sits in a conservation area, check the position before you plan work, because the rules on materials can affect which system is allowed. Our page on flat roofs in conservation areas covers this. If water damage is involved and you are thinking about your policy, see flat roof insurance.

Once you book a survey, we photograph the roof, explain which systems suit it and what weather each needs, and send the written quote with the photographs. Our 10-year workmanship guarantee is in writing. Once the work is finished we sweep up and haul the rubbish off ourselves. The wider service is on the flat roofing page.