How do you know if roof battens are rotten?

Battens are the narrow timber strips fixed across the rafters, running level from eaves to ridge. Each tile or slate hangs from a batten by a nib or a nail, so the whole covering depends on them. Their spacing sets how much of each tile is exposed, which is why they must be laid accurately.

Timber rots when it stays damp. On a roof that happens where water gets past the covering and sits on the batten, or where moist household air rises into the loft, meets cold timber and turns to droplets with nowhere to go. Torn or perished underlay lets water reach the wood, and the nail holes are the first place rot starts, because they hold moisture against the grain. A blocked valley or a broken gutter that soaks the lowest courses can destroy battens in one area while the rest of the roof stays dry.

Where the roof has no ventilation at the eaves or ridge, damp air lingers in the roof space, and the timber suffers from the inside as well.

Go up with a torch and stay on the joists or on boards. Do not step on the plasterboard ceiling between them. If the roof has an underlay, you may only see the rafters and the underside of that sheet, with the battens hidden above it. Older roofs often have no underlay, and then the battens are in plain view with the backs of the tiles showing.

Look for timber that has turned dark brown or black, for patches of white or grey fungal growth, and for wood that looks cracked into small cubes. Press a blunt screwdriver into a suspect batten with light pressure. Sound softwood resists. Rotten wood gives way, and may crumble to a damp powder.

Other loft clues include daylight where there should be none, nail points rusted through, tiles whose nibs have broken off lying in the insulation, and a musty smell after rain. Our page on torn roof felt in the loft covers the underlay side of the same problem.

From the pavement or garden, look along each slope at a shallow angle. A healthy roof reads as straight lines. A rotten batten tends to sag or snap, so the courses of tiles dip, ripple or step out of line. Tiles may sit proud or drop below their neighbours.

Slipped tiles are a common prompt. One or two can come from wind or a poor repair, but a cluster in one area suggests the battens beneath no longer hold the nails. If tiles keep slipping after being refixed, the problem is probably the timber rather than the tile. Our page on whether roof tiles need clips explains how tiles are held, and clips cannot grip a batten that has lost its strength.

Leaks and stains on the ceilings below are late signs. By the time water shows inside, the timber above has often been damp for a while. Moss thick enough to hold water against the tiles, or a gutter that overflows onto one patch of the roof, tells you where to look first.

If rot is confined to a small patch, for example beneath a failed valley or a slipped area near the eaves, the tiles can be lifted, the bad battens replaced and the tiles fixed back. Where the evidence supports it, that limited job is the one we put in the quote, because it cures the actual fault without replacing sound timber.

If battens are soft across much of a slope, patching only shifts the weak point a few courses along. New tiles on tired timber will slip again, and the underlay below is likely to be past its best too. At that point the slope, or the whole roof, is better stripped and re-battened with new treated timber, new underlay and mechanically fixed tiles.

Re-battening has a regulatory side. The Planning Portal's tests are under 25 per cent of the total building envelope and under 50 per cent of the roof touched. A larger strip can bring Building Regulations approval, and sometimes a thermal upgrade, into play; we talk you through where your roof stands at survey. Our page on changing from tiles to slate covers what changes if you want a different covering at the same time.

Keep off any roof that may have weak battens. The timber might not take your weight, and the tiles will crack under your feet. A roofer working from a ladder or tower, with boards to spread the load, can lift a few tiles and test the battens directly.

On older or protected homes the covering may be part of the building's character, and tiles reclaimed from the roof are often reused. Our page on tiled roofs in conservation areas covers the consent side.

We survey with a camera: pictures of the slopes and of the loft timbers come back to you, and a written quote follows. Workmanship is covered by a 10-year guarantee, written down. Before you ring, a few clear loft photographs of the darkest or softest timbers will help us plan what to bring, including spare battens cut to the right section. Our other pages on tiled roofs are gathered under pitched roofing.