Why lead is still the right answer at awkward roof junctions

Lead has been dressed around dormers, chimneys and valleys for centuries, and it remains the best material for the job. It is soft enough to be bossed and shaped by hand over difficult geometry, it moves with the roof through the seasons, and once a grey patina has formed it will last for decades — often outliving the slate or tile around it.

What matters most is thickness. Rolled lead sheet to BS EN 12588 is graded by code: roughly code 3 and 4 for small soakers, code 4 and 5 for flashings and valley linings, and heavier codes for the back gutters behind dormers, where water sits and debris gathers. Thin, floppy or dented lead is the first clue that somebody cut corners.

Around a dormer: the four places to look

• The front apron — the flashing that covers the junction between the dormer face and the slope below. It should be dressed tight, ideally in one piece, with a decent upstand into the wall.
• The cheeks — stepped flashings with individual soakers under each course of slates or tiles. Short soakers, exposed laps or smears of mortar used as filler are all bad signs.
• The back gutter — the trough behind the dormer where the cheek meets the slope. This is the classic weak spot: leaves and grit collect, water lingers, and the lead works hardest for longest.
• The junction into the valley — where the dormer roof discharges into the main valley, the lead must be continuous and lap properly into the lining. Awkward, and often rushed.

Look at the back gutter first. If moss, grit and dead leaves have been allowed to build up, the lead underneath may already be stained, pitted or cracked — and you will usually spot it from a bedroom window long before you see it in the loft.

Valleys: where water has to be moved on quickly

A valley lining takes the combined flow from two roof slopes, so it has to be laid with care. Laps between lead sheets should be generous — around 150mm in a valley — and the lining wide enough to cope with the run-off. Long lengths need to be broken into bays, typically no more than about 1.5 metres, with room to expand, because lead moves a surprising amount between a cold snap and a hot summer day.

Valleys usually fail for dull reasons: blocked outlets, moss washed down from above, laps laid the wrong way, or old lead quietly corroded from underneath by organic acids in trapped debris. A valley that holds water after rain, even shallowly, is telling you something.

Signs that lead is failing or was never fitted properly

• Fine cracks at a bend or fold, or along the line where the lead flexes.
• White or chalky patches, pitting, or a powdery bloom where lead should be grey.
• Mortar smeared over junctions instead of properly dressed lead.
• Self-adhesive flashing tape or mastic used as a long-term repair.
• Loose clips, lifted edges, and lead that rattles or flaps in the wind.
• Green or black staining on render below a flashing, or damp patches on a ceiling beneath a valley.
• Water still standing in a back gutter hours after the rain has stopped.

What good lead work looks like

Correctly installed lead is dressed, not cut about. It laps in the direction the water runs, sits on smooth underlay or boarding, and is held by clips that allow movement without letting the sheet lift. Joints are usually burned rather than sealed with mastic. Where lead touches unseasoned oak or fresh mortar it should be isolated, and a coat of patination oil helps keep the surface even while the natural patina develops.

Any repair should be a proper one: replacing the failed section with new lead of the right code, dressed to fit, and lapping correctly into what remains. Patches of mortar or tape buy you a season or two at best, and often hide a bigger problem underneath.

A simple routine for period properties

Check your dormers and valleys twice a year, ideally in spring and autumn, and again after a storm. From a window, or with binoculars from the garden, look for lifted edges, staining, cracks and blockages. Clear debris from back gutters and valley outlets gently, with a soft brush or by hand — never with a scraper, and never by walking the roof in wet or frosty conditions.

If you spot standing water, cracked lead or a damp patch inside, call a roofing contractor with genuine leadwork experience rather than someone who mainly installs modern sheet coverings. Ask how they will dress the flashing, which code of lead they intend to use, and how the laps will run. A specialist who can answer those questions plainly will usually give you a repair that quietly does its job for another thirty years.