What Roof Damage Can't Be Seen From Outside?

Damage under the tiles

The tiles are only the outer layer. Beneath them sit the underlay, which catches wind-blown rain and condensation, and the battens, the small timber strips that the tiles hook onto. Old felt underlay becomes brittle, tears at the laps and sags between rafters, collecting water that rots the battens from below. A tile can look perfect while the batten beneath it has softened and lost its grip on the nail.

Tile fixings fail too. Nails corrode, and the nibs that hold a tile to a batten can snap, so a tile that looks in place is held by little. In a gale it lifts. You would not know without lifting a few tiles or looking at the underside from the loft.

Sarking boards, the boards laid under the covering on some older roofs, and the timber under a flat roof deck can swell and rot in just the same way. A soft patch underfoot is the usual sign, though by then the damage is already well advanced.

Damage at the details

Flashings are the biggest hidden weakness. Lead soakers, the small pieces folded under each course at a wall, and the wider flashings round a chimney can lift, split or corrode where they meet the masonry. The mortar fillets that hold them can crumble. The result is a slow leak that appears only in heavy rain from a certain direction.

Valleys behave the same way. A lead or fibreglass valley may carry a pinhole or a hairline crack that does not show from the ground. Ridge and hip tiles bedded in old mortar can look sound yet be loose, and the dry-fix parts that replace mortar may have lost screws. BS 5534 asks for mechanical fixing of ridge, hip and verge parts, so mortar alone should not be relied on.

Soakers deserve a closer note. They are small, hidden and numerous, and they corrode where the lead meets the brick. A bad run can leak at a dozen points along a wall without a single drip appearing in the same place twice. That makes diagnosis from the inside alone slow and uncertain.

Lifting a few tiles at the suspect point is often the quickest way to see what a flashing or an underlay is really doing.

Damage in the roof space

Condensation causes some of the most overlooked damage. When warm air from the house reaches a cold roof void it condenses on the timbers. Left alone it feeds mould and rot, and over time it can weaken rafters and joists. Poor ventilation at the eaves and ridge makes it worse, and insulation pushed hard against the eaves is a common cause.

Chimney breasts can be hidden problems as well. A leaning stack, cracked flaunching or failed pointing lets water into the brickwork and the loft side of the breast. Our page on roof damp and condensation surveys looks at how these are traced. Where water has got in, the effect is usually a ceiling stain that appears a long way from the real source.

Wet insulation is another warning sign. Mineral wool that is damp loses its value and holds water against the timbers. If it is flattened or stained in one area, the roof above it is a good place to start looking for the way in.

How it is found

Hidden damage is found by combining views. A look from outside records the visible state, a close look at the flashings and valleys reads the details and the loft shows the underside. Thermal imaging and damp meters help in some cases, and we explain them in our guide to thermal imaging roof surveys.

Pictures of each fault go into the report for you to keep, and we say where the damage is minor and where it needs work. Repair is preferred wherever it works. The fixed written quote follows the free survey, and you can find the whole service described under roof inspections in Basingstoke.

Do not rely on a quick look from a ladder to rule hidden damage out. Some faults only show under a damp meter, or after a long wet spell, or with the tiles lifted. A proper inspection is slower for that reason, and the report should say how far it went.

If a survey does turn up hidden damage, we show you the evidence first and explain what it means for the timbers, the covering and the cost, and then agree what to do. Often the repair is small once the cause is known.