How to spot poor slate roofing workmanship

Why slate rewards a careful eye

A well-laid slate roof is one of the quiet triumphs of British building. Get it right and it will shrug off driving rain and frost for the best part of a century, fading gently to that soft grey-blue we all recognise on a Welsh or Lakeland skyline. Get it wrong, and the first leak can appear within two or three winters, usually somewhere awkward above a ceiling you have only just decorated.

The reassuring news is that poor slate work is rarely invisible. Slating is a trade of gauges, laps and fixings, and when those disciplines are abandoned the evidence shows up in the same handful of places on almost every roof. Take a walk to the far side of the garden with a pair of binoculars, pick a low sun, and start looking.

Uneven courses and a wandering eaves line

Slates should be laid to a consistent gauge, with a uniform headlap where each slate overlaps the one two courses below. A minimum headlap of around 65mm is a sensible benchmark, and you want more on shallow pitches or exposed coastal sites. When the gauge drifts, the courses visibly snake.

• Sight along the eaves and the ridge. Both lines should be straight and parallel.
• Look for slates of wildly mixed length, thickness or colour. Genuine reclaimed slate varies, but a roof made of three different quarries will soon look patchy and perform worse.
• Check the eaves course, which is traditionally doubled, and the verge, where slates should be cut neatly rather than snapped or ground into shape on site.
• Run your eye up a rake or a hip. Ragged, inconsistent cutting is a sign nobody was checking the work.

Waviness is not just cosmetic. Uneven courses usually mean uneven battens, and uneven battens mean the underside of the roof is not holding slates flat or true.

Protruding nails and proud fixings

Nails should be driven home neatly over the batten, flush with the slate and never sitting proud of it. A nail head standing above the surface holds the slate above away from the one beneath, breaking the lap and giving water a route straight through. On a sunny morning you can often see these little bright points glinting across a roof.

• Look for slates cracked or starred at the nail holes, usually a sign of over-tightening or nailing too close to the edge.
• Nails should be copper or stainless steel. Galvanised or bright steel fixings will rust and fail long before the slate does.
• Larger slates, generally those over about 450mm, should be double-nailed at both top corners.
• Where you can see the underside from a loft or an open eaves gap, check for split battens and nails punched through the wrong place.

Excessive sealant, mastic and mortar

This is the single clearest warning sign. Proper slating does not rely on tubes and cartridges. When you see mastic smeared along laps, silicone squeezed under flashings, or cement mortar used where lead belongs, you are usually looking at a leak being disguised rather than cured. Sealant has a working life of a few years; slate has a working life of decades.

Torching — a soft mortar bed spread on the underside of slates — is legitimate when it is thin, even and applied to stiffen the roof against wind. It becomes a problem when it is used to hold badly nailed slates in place or to plug gaps that should not exist. Similarly, cement fillets at abutments and ridges look tidy for a season and then crack, shrink and let water in.

Flashings, ridges, valleys and ventilation

The details at edges decide whether the whole roof works. Lead should be chased into the wall by at least 25mm, in stepped pieces with proper laps, and dressed neatly over the slate. Self-adhesive tape and cement smears are not flashings.

• Valleys should be lead or a properly installed GRP or GRP-style system, clear of debris and correctly lapped.
• Ridges should be bedded and pointed with a lime-based mortar or a dry-fix system, not a brittle cement skim.
• The underlay should be lapped correctly, sagging gently between rafters and with the right face upwards.
• Eaves and ridge ventilation should be continuous and unobstructed, especially where insulation has been upgraded.

Keeping the rest of the house in step

A roof does not fail in isolation. Water that gets past poor slating tracks down rafters, wall plates and chimney breasts, and it often surfaces far from where it entered. In older properties it can show up as damp patches beneath flagstone or York stone floors, salt bloom on the stone, lifting quarry tiles or soft joist ends under a suspended timber floor.

This is why traditional maintenance is best done as a cycle rather than a crisis. Clear gutters and check downpipes twice a year. Repoint in lime mortar rather than cement, which traps moisture in solid walls. Avoid sealing stone flooring with a plastic sealer that stops it breathing; let it dry and it will stay sound. If a leak has already reached the floor, deal with the roof first, then let the stone dry slowly before doing anything else.

Above all, ask questions before you commission work. A roofer worth employing will happily tell you the gauge, the headlap, the fixings and the source of the slate. Vague answers, a quote built entirely around mastic, or a hard sell on coatings and cleaning are all worth walking away from.