Roofers in Bath who put the roof right, as it was built

Walk along almost any Georgian street in Bath and you see a level line of stone at the top of the front wall. That line is the parapet: a low wall carried up past the eaves so that the roof sits out of sight behind it. Between the parapet and the bottom course of slates runs a parapet gutter, a lead-lined channel that collects everything the front slope sheds and carries it to an outlet, often straight through the wall to a hopper head and downpipe. Indoors it shows only as a brown mark on a top-floor ceiling or a damp patch high on the front wall.

The builders of the terraces wanted an unbroken stone skyline, and the hidden gutter was the price of it. It works well while the lead is sound and the outlet runs clear. The difficulty is that it holds water against the building by design. A blocked outlet, a crack in the lead or a joint that has opened will put water into the wall head, the timber wall plate and the ends of the joists, quietly, for months.

How the lead fails

Lead in a gutter is laid in lengths with small steps between them, called drips, so that each length can expand and contract on its own. When a length is too long, or has been nailed down too tightly, the daily movement gathers at one point and the sheet cracks. We also find splits where lead was dressed over a sharp arris, where an old repair used a bitumen patch that has since lifted, and where years of boots have worn the surface thin.

A box gutter is the same idea in a box-shaped trough, set between two roof slopes or behind a parapet on a terrace with a valley roof. It fails in the same ways. On some Bath terraces the front gutter and a central box gutter both drain through the house, so one blocked outlet can wet a ceiling at the back as well as the front.

What we do on a survey

We clear the gutter and its outlet first, because silt and leaves hide splits. Then we go along every length looking for cracks at the drips, lifted laps, soft boards under the lead, and places where the timber has dropped so water now sits in a hollow. Each defect is photographed beside a landmark such as the outlet or a chimney, so the pictures make sense later. Where the lead is sound and only a joint has opened, repair is the right answer. Where lengths are cracked across their width or the boards beneath have rotted, the gutter needs relaying on new boards with the lengths and drips set out properly.

Bath's protections matter here. The city was inscribed as a World Heritage Site in 1987, about half of it lies inside the city-wide conservation area, and thousands of its buildings are listed. B&NES advises talking to the council before any work to a listed building, so we tell you at the quote stage whether the job is likely to raise a consent question.

Figures: UNESCO and B&NES (inscription); Bath Preservation Trust (conservation area); Bath Record Office (listed buildings)

If you live in a house with a parapet, the simplest check is from indoors. After heavy rain, look at the ceiling along the front wall of the top floor and at the wall just below the cornice. A stain there, even a faint one, is worth a look from above. Our lead roofing page explains how gutters are relaid, and the guide to Georgian terrace roofs shows how these roofs were put together.

The ridge is the line where the two main slopes meet at the top of the roof. It is covered by a row of ridge tiles or, on some slate roofs, by a lead-covered timber roll. Hips, the sloping ridges that run down the corners of a hipped roof, are covered in the same way. Because the ridge is the highest and most exposed part of the house, it takes the wind from every direction, and it is often one of the first things to show damage after a storm.

For generations ridge tiles were simply bedded on mortar. The tile sat in a bed of sand and lime, later sand and cement, with mortar packed into the joints between tiles. That holds for as long as the mortar keeps its grip. Over the years the bed shrinks, frost opens hairline cracks and the roof moves a little with the seasons. The joints drop out first. Then the bedding lets go on one side, and a tile can lift in a gust and slide down the slope, breaking slates as it goes.

What the standard says now

British Standard BS 5534:2014 changed the position. Under it, the tensile strength of mortar on its own is no longer relied on to keep ridge and hip tiles in place, and bedded tiles and fittings should also carry mechanical fixings: a screw, clip or wire into a ridge batten or the timber below. A second standard, BS 8612:2018, is the test specification for dry verge, hip and ridge systems. Dry-fix means holding the ridge with clips, screws and a flexible roll under the tiles instead of mortar, so there is no bedding to crack or wash out.

On a newer house a tested dry ridge is often the neatest answer. On a Bath stone terrace or a listed house the appearance of the ridge counts for a lot, and a bedded ridge with hidden mechanical fixings keeps the traditional line while meeting the standard. Where an old ridge was bedded in hard cement over soft stone or old slate, we generally rebed it in a lime-based mix.

The verge and the gable

The verge is the edge of the roof where it finishes over a gable wall. On older roofs it was sealed with mortar over a strip of slate or tile, the undercloak, laid along the top of the wall. When that mortar fails, the wind gets under the edge slates and lifts them. On Bath terraces the gable is often hidden behind a parapet or a party wall, so the verge may be a lead flashing against stone instead. We look at both ends of the roof for that reason.

When we survey a ridge we test the tiles for movement, look for open joints and cracked bedding, check that the ridge batten is sound and note whether the existing fixing meets current practice. A ridge that has only lost some pointing can be repointed. One with loose tiles along a run needs lifting, a new batten where the old one has rotted, and rebedding with proper fixings.

Loose ridge tiles are worth dealing with before a winter of gales, because a tile that leaves the top of a roof can break slates on its way down. Our ridge and verge repairs page goes further into the choice between rebedding and dry-fix, and what each costs to put right. If you can see a gap along the top of the roof from the street, or find pieces of mortar on the path, that is the time to call.

A roof keeps water out easily across the open slope. The difficulty comes wherever the slope meets something else: another slope, a wall, a chimney, a dormer or a window. At every one of those junctions the slates stop and a second material has to take over. On Bath's period roofs that material has traditionally been lead.

Valleys

A valley is the inward angle where two roof slopes meet, the opposite of a hip. Water from both slopes gathers there, so a valley carries far more water than any other part of the covering. A lead-lined valley is formed of sheet lead laid on boards running down the angle, with the slates cut back to leave an open channel. Long valleys are laid in lengths that lap over one another, because a single sheet that is too long cannot move freely and will crack. Where two parallel roofs meet in the middle of a terrace, the valley becomes a level box gutter between them, and that gutter has to drain somewhere, often through the house to the back wall.

The Bath Preservation Trust's comments on 33 St James's Square, a Grade I house, give a sense of how this work is viewed. Its roofs are predominantly Welsh natural slate, and the Trust supported re-leading the central and side valley gutters as part of the scheme.

Flashings and soakers

A flashing is a strip of lead that seals the joint between the roof and a wall or chimney. Its top edge is turned into a mortar joint in the masonry and wedged, and its lower edge is dressed down over the slates. Along the side of a wall or stack, soakers do the hidden work. They are small pieces of lead, each slipped between two slates and turned up against the wall, so that every course has its own lead. A stepped cover flashing over the soakers stops water running down behind them.

Mansards add more junctions. A mansard is a roof with two pitches on each side: a steep lower slope, often holding dormer windows, and a shallower one above. Commenting on Lambridge Place, the Trust noted that a mansard with integrated dormers is more visible than a pitched roof set back behind the parapet line, which is why the materials on such roofs attract attention. Every dormer cheek and window head on a mansard is another lead detail to get right.

Lead code

Sheet lead is sold by code, a number that stands for its thickness and weight. Different parts of a roof call for different codes, with heavier lead where a detail takes more wear or more water. We state the code for each detail on the quote so you can see what you are paying for.

When lead fails it is usually down to movement rather than age: cracks along over-long lengths, splits where it was dressed over a hard corner, or a flashing pulled out of its joint. We look for each of these, and for old repairs in mastic or bitumen that cover up the defect underneath.

Valleys and flashings exist on tiled roofs as well as slate ones, and the same rules about laps and movement apply. Our pitched roofing page covers clay, concrete and slate pitched roofs across the city, including the lead details that keep each one dry where it meets a wall or a chimney. If you are not sure what covering your roof has, the survey photographs will show you, slope by slope.