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Slate is one of the finest roofing materials ever quarried in these islands, and a well-laid slate roof can serve a Georgian or Victorian terrace for a century or more. The trouble is that moss loves it too. Our damp, mild, westerly climate is ideal for it, and any slope that faces north, sits in the shade of a chimney or tree, or lies in a valley between pitches will colonise within a few seasons.
The moss itself does not eat the slate, but it does something subtler and more damaging. It holds water against the surface, keeping the slates permanently cold and wet, and in a hard winter the freeze-thaw cycle nibbles away at the vulnerable lower edges. Rootlets creep into the overlaps and joints, prising slates apart by fractions of a millimetre each year. Then the dead moss and debris wash down into the gutters and valleys, blocking the flow of water so it backs up under the laps and into the loft.
Pressure washing a slate roof is one of those jobs that looks spectacular for a fortnight and costs you dear for a decade. A jet of water at 100 bar or more will do all of the following, usually without you noticing at the time:
Add the sheer weight of saturated moss in the gutters below and the slip hazard of wet slate underfoot, and you have a job best left undone. A pressure washer has its place on a concrete path, not on a roof.
The safest method remains the oldest one: scrape it off by hand, patiently, on a dry day in late spring or early autumn when the moss is brittle and comes away in sheets rather than smears.
Once the bulk is off, a proprietary moss treatment will slow the return. Apply it by brush or a low-pressure spray on a still, dry day, and read the label properly — most need a dry roof and a rain-free window of several hours.
Take care with run-off. A biocide washing into a water butt, a pond or a border of mature shrubs does real harm, so cover what you can and divert what you cannot. Copper and zinc strips fixed along the ridge are an older, slower, gentler approach: rainwater picks up traces of metal and carries them down the slope, where they inhibit new growth for years. They stain light-coloured stone below, so think about where the water lands.
Nothing about slate responds well to brute force, and the same is true of the stone floors in a traditional house. A pH-neutral cleaner matched to the stone, a soft mop, and a reseal every few years will keep flagstones and limestone looking right for generations. Sweep grit before you mop, avoid acidic cleaners on limestone and marble, and resist the urge to scrub a stain harder than the stone can bear.
Walk the perimeter twice a year and look up. In spring, check for slipped slates, cracked ridge mortar and new moss. In late autumn, after leaf fall, clear the gutters again and look for staining on ceilings below. Anything you can see from the ground is worth a closer inspection by someone competent with a slate ladder — and anything that involves actually walking on the roof is worth paying a professional to do. A roofer's invoice is a bargain compared with a fall, or with a loft full of sodden insulation.
Treat a slate roof as the long-lived, slightly delicate thing it is, and it will quietly outlast us all.
Slate & Stone shares practical, down-to-earth guidance on slate roofing, stone flooring and traditional home maintenance for uk properties for readers across the UK.
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