Pitched roofing in Basingstoke

What we find

Most tiles on a pitched roof are held by gravity, their own weight and a nail or two. On plain clay tiles, a nail through a small hole near the top of each tile hooks it onto a batten. Interlocking concrete tiles are usually nailed only at certain rows and held in line by the lugs under each tile. When tiles slip, the usual cause is that the fixing has gone, not the tile itself.

Roofers call the condition nail sickness. The steel nails corrode over decades, particularly in a loft that gets damp or condensation-prone, and the shank thins until the head can no longer hold the tile. Sometimes the nail has simply pulled out of a batten that has itself softened. The tile then drifts down the slope a few millimetres at a time with each gust and freeze, and leaves a gap above it where rain can get in. On houses built in the 1960s and 1970s on estates such as Kempshott and Popley, we often see a row of tiles that has crept down together, with the gap at the top hidden from the ground. Looking up from inside the loft, you can sometimes see the rusted nail ends lying on the underlay.

What we find

A single cracked tile is one of the most common things a roofer is asked to fix. Frost splits tiles that have soaked up water, a loose branch or aerial can crack one, and someone walking on the slope in the wrong place will snap the edge off a tile. Concrete tiles are brittle across their width, and old clay tiles can delaminate and flake from the surface, so a tile that looks sound from a ladder may be crumbling underneath.

The question we are asked next is whether the damage is a symptom. One broken tile after a storm is bad luck. Several broken tiles scattered across a slope, or breaks concentrated in one area, point to something else: a roof that has lost its tile fixings, a batten that has rotted below, or tiles that have reached the end of their life and are absorbing water. Underneath a broken tile, the underlay takes the rain until someone notices, and it is the underlay and the timber that suffer first. A ceiling stain that appears weeks after a gale usually traces to a tile that cracked on the day and was never seen.

What we find

Underlay is the sheet that lies over the rafters and under the battens. It is the second line of defence: it catches wind-blown rain and snow dust that gets under the tiles, and carries it down to the gutter. On roofs built before the 1990s, the underlay is usually bitumen-coated hessian felt, laid in rolls with an overlap. Over the decades it dries out, goes brittle and cracks, especially along the lines where it was stapled over the rafters or dragged across a purlin.

When we look into a loft under a roof this age, we find felt that has split along the line of a rafter, felt that has torn where a batten nail pierced it, and in the worst cases felt that has rotted away at the eaves so that the batten ends are exposed. A sag between the rafters is common and not always a fault. Underlay is meant to drape slightly so that water can run beneath the battens. A sag that holds water, or a lower edge that hangs into the gutter, is different. Dust and cobwebs on the underside of the tiles, damp patches on the felt and rusty nail tips are the signs that wind-blown water is getting in.

What we find

Battens are the small timber strips nailed across the rafters that the tiles hang from. On a plain tile roof the batten spacing sets the tile gauge, the exposed length of each tile, and on an interlocking roof it fixes the position of every row. They are the part of the roof you never see unless you strip it, and the part that most often fails without anyone noticing.

Battens rot where the roof is wet. That means below a long-term leak, at the lowest rows beside the gutter, and around a valley or chimney where water collects. Old battens can also be an untreated softwood that has been in the loft since the 1950s, and the nails hold less well each year. Signs from inside include dark staining, a spongy feel where you can push a screwdriver in, and daylight at the nail line. Signs from outside include a row of tiles that has dipped, or a slope that has a visible wave along it. An under-gauge batten, too thin for the tile weight, can also bow between rafters, which opens joints between tiles.

What we find

Where two slopes meet at an inside corner, the valley collects the water from both and sends it down to the gutter. It takes more water than any other part of a pitched roof. Hip roofs, L-shaped houses and rear extensions with a pitched roof joining the main one all have at least one, and so do the many two-storey houses around Basingstoke with a gabled porch or bay.

Valleys fail in a few ways. On older houses the valley lining is lead, and lead can split along a crease after years of heating and cooling or wear a hole where moss has sat. On 1980s and 1990s houses, the valley may be a GRP trough, or cement-bonded tiles cut to the angle with mortar bedding, and the mortar has cracked away. Moss, leaves and silt blocking the narrow channel make water back up and run under the tile edges. We also find valleys where the underlay has not been dressed into the trough, so any water that gets under the tile cuts drips straight into the loft.

The stain on the ceiling tends to track the line of the valley, because the water follows the same path inside as outside.

What we find

Most owners need replacement tiles in small numbers, but sometimes a whole slope is tired and the choice of tile comes up. Concrete tiles became the standard on British houses from the mid twentieth century because they were cheaper and quicker to lay than clay. Clay plain tiles are still found on older houses, and clay interlocking tiles on some better-quality new builds. The material changes the look, the weight and how long the covering lasts.

A tile's weight matters because the roof structure under it was built to carry a specific load. Concrete tiles are heavier than clay plain tiles, and a roof originally covered with light tiles may not have been designed for a heavier one. Appearance matters too: on streets with a consistent character, a different tile colour or profile sticks out. In a conservation area or a listed house, changing the material can need consent, and you should speak to the council's conservation officer first, and we will wait for their answer. A mismatched slope also draws the eye on a semi-detached house, where your neighbour's roof sits beside yours with the old tiles.

What we find

Wind lifts roofs more than most people expect. A gale blowing across a roof slope creates suction on the leeward side, and on exposed sites such as the ridge lines on the edge of town, the north Hampshire downs and the new estates on open land, that suction can lift a tile that is poorly fixed. The sound of tiles rattling in the loft in a storm is a warning.

Tiles lift for several reasons. The fixings may have corroded, as we saw earlier. The roof may have been laid with fewer nails than the tile maker specified. Clips may be missing along the eaves and verge, where wind pressure is highest. And on older roofs, tiles are often fixed only by gravity and a single nail every few rows, which was the accepted practice before the current standards. When a tile lifts, it can crack on its neighbours, drop off the roof, or settle back loose and slide down the next time. A gap left behind lets rain in and wind pull at the tiles beside it, so a single lifted tile can lead to a patch of slipped ones.

What we find

A roof slope should look straight from the road. A dip in the line, a belly in the middle of a slope, or a ridge that is no longer level means something below the tiles has moved. It may be minor and long-standing, or it may be a fault that is getting worse.

The causes we find sit in the roof structure. Rafters can sag if they are too thin for their span or if the purlins, the horizontal beams that hold them midway, have deflected. Trussed roofs, common on houses from the 1960s onwards, can be altered by a later owner who cut a truss member to make room for a loft hatch or a water tank. Old roofs can also be overloaded when a lighter covering was replaced with heavy concrete tiles. Rot at the wall plate, where the rafters sit, lets the roof drop at the eaves, and woodworm or damp in old timbers weakens them. The tiles themselves can look in good order while the shape below changes, and the wave eventually opens gaps in the courses. Our page on roof timbers: rafters, purlins and trusses shows what each part does.