Why Slate Abutments Need Careful Flashing

Slate is a superb roofing material — dense, hard-wearing, and capable of lasting a century or more when it is laid well. What slate is not, however, is forgiving. Each slate is thin, rigid and unforgiving of movement, which means every junction where the roof meets a wall, chimney or dormer has to be detailed properly. Water does not simply run downhill on a slate roof; wind drives it sideways, and capillary action will pull it into any gap it finds, however small.

That is why flashings matter so much on slate. A slate covering has no redundancy — there is no thick tile or heavy overlap to hide a poor detail. If a soaker is too short, a chase too shallow, or a cover piece fitted in the wrong order, the result is usually a damp patch on a ceiling some weeks later, well away from the actual defect. Getting the sequence right at the outset is far easier than tracing a leak in January.

Soakers: The Hidden Layer That Does the Work

Soakers are the real waterproofing at an abutment. Each one is an individual piece of lead, dressed against the wall and laid between courses of slate so that it laps over the slate below and under the slate above. Because they are stepped in with the slating, they shed water course by course rather than trying to resist it in one go.

• Size: a minimum of 150mm wide, with an upstand of at least 65mm against the wall and enough length to give a 100mm lap over the slate beneath.
• Material: Code 4 lead is the usual specification, though Code 5 is worth considering on exposed sites or where the upstand is taller than average.
• Position: the upstand should sit tight to the wall, and the bottom edge should be dressed so it does not hold water or sit proud of the slate line.
• Fixing: soakers are held by the slate above, not by nails, and should never be fixed through the upstand.

The golden rule is that soakers go in as the slating proceeds. Trying to slide them in afterwards never produces the same result, because the lap over the slate below cannot be formed properly.

Step Flashing, Cover Pieces and the Order of Work

Above the soakers sits the cover flashing — sometimes called step flashing where it follows a stepped abutment up the roof. This is the visible leadwork chased into the wall, and it is what keeps the bulk of the weather out. The order is always the same: soakers first, laid with the slates, then the cover flashing dressed down over them.

The chase into the wall should be at least 25mm deep and cut cleanly, with the lead wedged and then pointed. Where the abutment is stepped, each length of cover flashing should lap the one below by a good 75mm and be dressed neatly into the corner so no water can sit behind it. Horizontal runs should be kept to around 1.5m in Code 4 lead, with 100mm laps between lengths to allow for thermal movement — lead expands and contracts, and a continuous run will eventually tear or buckle.

On a raking abutment, such as a gable wall rising alongside a pitched roof, the flashing follows the slope rather than stepping. The principle is identical, but the pieces are longer and the dressing needs more care, as a single poorly formed length can funnel water straight into the roof space.

Valleys and Other Junctions

Valleys take the greatest volume of water of any part of a roof, so they deserve a wider lining than most people assume. Code 5 lead is the traditional choice, laid on a smooth, continuous underlay such as plywood or boards rather than directly over battens, so the lead has full support. Laps should be generous — 150mm is sensible — and the valley should be wide enough that the slate edges do not overhang into the flow of water.

Chimney back gutters are the other classic trouble spot. The gutter must be wide enough to cope with the run-off from the slope above, with a decent upstand and a proper apron at the front, and the sides need soakers and cover flashing just like any other abutment. Where a chimney sits in a valley, take extra care — this is a genuinely difficult detail and shortcuts rarely hold.

Materials, Mortar and Compatibility

Lead remains the best material for flashings on slate for most situations, but it must not be in direct contact with copper, uncoated steel or zinc, as dissimilar metals will corrode one another in the presence of moisture. Use suitable separation where different metals meet.

Mortar matters too. On older properties, a lime-based mortar is far more appropriate than a hard cement mix, because it moves with the building and allows the lead to expand without cracking the pointing. Pointing should be slightly recessed rather than flush, so water runs off rather than sitting on the joint.

Maintenance Checks Worth Doing Each Autumn

Flashing rarely fails suddenly; it usually gives warning. A short inspection before winter will catch most problems early.

• Look for lifted or slipped soakers, especially after gales.
• Check the pointing in the chase — cracked or missing mortar lets water straight behind the lead.
• Clear debris and moss from valleys and back gutters, where blockages cause water to back up under the slates.
• Watch for white staining on lead, which suggests corrosion, and for green or black streaks that point to water tracking where it should not.
• Inside, check ceilings and loft timbers below abutments and valleys after heavy rain — early staining is much easier to deal with than rotten battens.

If you are having slates replaced or a roof recovered, insist that the flashings are renewed at the same time. Reusing old lead over new slates rarely works, and the labour to revisit the detail later will cost far more than doing it properly the first time.