Code Lead Flashing and Hydraulic Lime Mortar Waterproofing Junctions for Corbiestep Rakes

Scotland’s maritime climate is among the most hydraulically punishing in Europe. In regions such as the Highlands, Aberdeenshire, and the western isles, rain does not merely fall downward; propelled by gale-force winds exceeding sixty miles per hour, it is driven horizontally and forced vertically upward across building surfaces. When this driving deluge strikes the stepped rake of a crow-stepped gable, the junction between rising sandstone masonry and descending roof slates becomes the most vulnerable point in the entire building envelope.

Historically, early medieval builders attempted to seal this interface with mortar fillets (*scuncheons*)—sloped ramps of lime and horsehair plaster troweled between slate and stone. However, because roof timbers flex under wind buffeting and seasonal snow loads while masonry remains rigid, mortar fillets inevitably fracture. Once cracked, water is sucked into the interior roof structure through capillary action, saturating rafter tails and inducing catastrophic dry rot (*Serpula lacrymans*).

To create a permanently watertight, flexible bridge between moving timber roofs and rigid sandstone corbiesteps, master roofers turned to milled sheet lead. Governed today by British Standard BS EN 12588, sheet lead is categorized by code numbers corresponding to standard thickness and weight per square meter. For exposed Scottish Baronial gables, masons and slaters mandate Code 5 lead (2.24 millimeters thick, weighing 25.4 kg/m²) or Code 6 lead (2.65 millimeters thick).

Code 5 lead offers the ideal metallurgical balance: it is sufficiently malleable to be dressed tightly into complex ninety-degree masonry corners using wooden bossing sticks and dressers without thinning or splitting, yet dense and heavy enough to resist being lifted by negative wind suction during coastal storms. Its natural self-healing oxide film renders it immune to ultraviolet solar degradation, giving a properly installed lead junction a functional service lifespan exceeding a century.

The genius of modern and classical corbiestep weatherproofing is the dual-layer flashing principle: separating the primary water barrier from the secondary cover apron. The primary barrier consists of lead soakers. These are individual L-shaped pieces of Code 4 or Code 5 lead cut to match the length of the roof slates.

As the slater lays each horizontal course of dark Scottish Ballachulish or West Highland slate up the roof slope, a lead soaker is inserted beneath the slate edge abutting the corbiestep. The horizontal wing of the soaker is clamped between the overlapping slates, while its vertical wing turns up seventy-five millimeters against the sandstone wall. Because every individual slate has its own interlaced soaker overlapping the one below, water running down the valley between slate and stone is gathered on an unbroken, stepped metallic channel and discharged safely onto the lower roof eaves.

To protect the vertical upstand of the soakers from rain running down the face of the corbiesteps, masons install stepped lead cover flashings. The top twenty-five millimeters of the lead sheet is folded into a horizontal flange and chased into the masonry bed joints (the *raglet*) behind the steps. The lead is mechanically secured inside the stone joint using tightly folded scrap lead wedges spaced every three hundred millimeters.

Finally, the raked chase is pointed solid with Natural Hydraulic Lime mortar (NHL 3.5) blended with sharp, washed quartz sand. Hydraulic lime is critical: unlike modern brittle Portland cement, which chemically attacks lead and cracks under thermal expansion, hydraulic lime remains micro-flexible and breathable. It accommodates the daily thermal expansion and contraction of the sheet lead without losing adhesion. Once treated with patination oil to prevent white lead carbonate runoff, the completed flashing junction provides an impenetrable, maintenance-free seal that preserves Scotland’s architectural heritage across centuries of storm and sea spray.