Signs your slate roof is failing

Begin with the outline. A healthy slate slope has straight horizontal courses, even edges and a flat plane. Anything that breaks that pattern deserves a second look.

Gaps are the clearest sign. A slate that has dropped leaves a dark rectangle where its neighbours should overlap it, or a slate sitting lower than its row. Slates found on the lawn, in the guttering or on a lower roof are the other giveaway, and finding more than one after a single gale suggests the fixings are going rather than the weather being unusually fierce.

Lines that no longer run true

Look along the roof from the side. A dip or hollow in the slope can mean a batten has rotted or a rafter has sagged. A ridge that is no longer level, or a slope that looks wavy, suggests the structure underneath has moved. That is a structural matter, not a repair, and calls for a proper inspection first.

Uneven courses where slates have crept down in a rough diagonal suggest a line of failed nails. So does a patch of slates that stand slightly proud of the surface, because they are lifting at the tail.

Check the details as well:

• cracked, chipped or missing ridge cappings
• lead flashings that have lifted away from a wall or chimney
• a chimney stack with open mortar joints or leaning pots
• gutters that overflow, droop or have sprouted plants

Moss and lichen are common on slate, and a little is harmless. A thick mat that holds water against the slates is a different matter, because it keeps the stone wet and can push the slate edges up. The cleaning side of the work is on our slate roofing page.

The loft is where a slate roof tells the truth. Go up on a bright day, switch the lights off and wait for your eyes to adjust. Pinpricks or slivers of daylight between slates mean the covering has gaps. A few, near the eaves, may be old and harmless. A scatter across a whole slope is not.

Look next at the timbers. Dark staining on rafters and battens, particularly below a gap, shows that water has been getting in, even if the ceiling below is dry. Rafters that feel soft to a screwdriver tip, or that have a powdery surface, are a more serious sign. So are rusty nail tips poking through the battens, which are what a nail looks like when the slate it was holding has stopped depending on it.

Underlay and insulation

Many older slate roofs have felt that has dried, split or fallen away between rafters. If you can see slate undersides directly, with nothing below them, the roof has no second line of defence, and wind-blown rain can reach the insulation and the ceiling in a storm.

Damp insulation, a musty smell or condensation on the underside of the slates point to another problem, which is ventilation. Moisture that cannot escape from the roof space ends up in the timber. Every survey includes a look at how air can move from the eaves to the ridge.

Photograph anything unusual, set a pen beside it for scale, and note where it sits relative to the chimney or a window below. A good survey starts from where you saw the sign, and photographs help a roofer find it quickly. Stay on the joists or boards when you move about, never on the plasterboard between.

The most common reason for a slate roof to fail is the one you cannot see. Nail sickness is the term roofers use when the fixings that hold slates have rusted, loosened or lost their grip, and the slates begin to slide. We explain it in full in our guide to nail sickness on slate and tile roofs.

The pattern is distinctive. Slates slip one at a time, in no obvious order, across a slope that is otherwise sound. You repair one or two, and a few months on, two or three more have gone. Each repair is genuine, and each one is useless against the underlying cause. The nails across the slope are all the same age and all in similar condition.

A way to tell a bad winter from a failing roof

A single severe gale can lift a few slates from any roof, and one repair is then enough. The warning sign is repetition. If slates have slipped in three different places in two or three years, or the same slope keeps shedding slates in moderate weather, the fixings are the problem.

Another sign is a tingle on a slate you can see from the street. Tingles are the lead or copper strips used to hold a replacement slate in position, and one or two are normal on an old roof. A scatter of them across one slope shows a history of piecemeal repair. Our answer on why slates slip describes how they work.

Nobody can confirm nail sickness from the pavement, so we photograph the loft side and sample the fixings on a slope before we give a verdict. If the nails are going, the right answer is to relay the slope, and our re-slating guide shows what that involves. If only a few are affected, a repair will hold.

Slate is a layered rock, and the quality of that layering varies from quarry to quarry. Good stone weathers slowly and stays hard. Poorer stone splits along its bedding planes, sheds thin flakes and eventually goes soft and crumbly at the edges.

The giveaways are easy to see up close. Flakes or chips on the roof below, on a path or in the gutter. A slate with a layered, rough surface and a dull colour where its neighbours are smooth. Edges that look frayed. A slate that gives a dull thud when tapped from below in the loft, where a sound one rings.

Cracks and breaks

A crack across a slate lets water through, but may be invisible from the ground until the slate is seen from the loft side. Common causes are the weight of someone walking on the roof, a dropped branch or ice that has formed in a natural weakness. Occasional cracked slates on an old roof are normal. Cracks scattered over a whole slope suggest the stone itself has little useful life left.

Not every slate is the same. A roof covered in imported slate of varying quality can show a mix, with some courses crisp and some tired, and that alone is a reason to examine the whole slope instead of repairing one slate at a time. The cause and range of this problem is set out in our answer on what makes slates flake or delaminate.

Checkatrade's slate cost guide describes slate as capable of lasting more than a hundred years. That is a typical figure, not a promise, and it assumes decent stone, sound fixings and ordinary maintenance. Where slates are flaking across a slope, the stone has stopped doing its part of the bargain, and relaying with the sound slates and new ones for the rest is the likely answer.

Not every leak on a slate roof is the slates. Some of the commonest causes are the metalwork and drainage around them, and it matters to find the real source, because repairing the wrong thing will not stop the water.

Lead flashing is the first place to look. It seals the roof against a chimney, a wall or a dormer, and it is exposed to temperature swings that make it creep and split. Signs include lead that has pulled clear of the brickwork, a crease or crack in the sheet, or a stain on the ceiling beside a stack. Lead can serve for a very long time, and MyJobQuote puts flashing at over 80 years, but it is not permanent, and a split shows up as a leak.

Ridge cappings and valley gutters

Along the top of the roof, ridge cappings that have loosened or cracked allow wind and rain to enter. Look for gaps in the bedding below them. Valleys, the inward-turning joins between slopes, carry a great deal of water, and a valley lined with lead can fail where debris has sat or the metal has split. Leaves and moss in a valley are a warning in themselves.

Gutters can mislead too. Overflowing at the eaves wets the bottom courses and the fascia, and the stain on the wall below may look like a roof leak. A blocked downpipe is much cheaper to put right than a roof.

Our guide to finding a roof leak explains how water travels along timber and appears far from where it entered. A mark on a ceiling is where the water arrived, and the entry point may be some distance up the slope. So the whole roof gets looked at, not just the corner above the stain.

One sign on its own is rarely a reason to panic. A slipped slate after a gale is normal. A mark on the loft timber that has long dried may be history. What counts is the pattern, and the way to read it is to ask three questions: how many, how widespread, and how fast.

One or two signs in one place point to a repair. Several signs on one slope, or the same signs returning after a repair, point to the fixings and suggest a relay. Signs on every slope, with soft timber or a sagging line, call for a full survey of the structure before anyone prices a covering. The verdict we give has three possible outcomes, repair, patch or replace, and photographs back it up so you can see what we are describing. We would sooner recommend a small fix that holds than a large job that is not needed.

What to do next

Photograph what you have seen, with the date. Note where the water appears indoors. If water is coming in, catch it in a bucket and move belongings clear; a bulging ceiling is a reason to keep people out of that room. Then arrange a survey. After a storm, speak to your insurer before booking repairs, since a roof left to deteriorate counts as wear and tear in the example Which? gives, and not as insured damage.

The slate roof repair guide shows what a repair involves and what it typically costs, so you can weigh a fix against a relay with figures in front of you.