A slate roof should outlast the people who fitted it. Get the basics right and forty, sixty or even a hundred years of service is realistic. Get them wrong and you can be chasing leaks within a decade, replacing slipped slates every winter and wondering why a roof that looks perfectly good from the pavement keeps letting water in.
Most failures we see on British roofs are not caused by bad slates. They come from small decisions made during installation that seemed harmless at the time. Here are the ones that cause the most trouble.
Battens that are too small, too soft or badly spaced
Battens do two jobs: they carry the weight of the slates and they hold the fixings. Get either wrong and the roof starts moving.
The most common error is using battens that are too thin. A roof covering is heavy, and with slate it is heavier than most. Battens should be graded for the job, and the spacing must match both the slate size and the required headlap. Fitters in a hurry sometimes set the gauge by eye, or space battens to suit whatever slates are on the pallet rather than working from the specification. The result is a roof where the lap is a few millimetres short, which is exactly the sort of margin that lets wind-driven rain creep through during a south-westerly storm.
Also worth checking:
- Battens should be from a reputable supplier and marked for roofing use. Cheap, low-grade timber sags and rots.
- Timber should be reasonably dry and stored off the ground before fitting. Wet battens shrink as they dry, loosening fixings.
- Battens must run true and level. A wavy batten line shows through the finished roof and makes slate alignment a fight.
- Fixings into battens should be long enough to bite properly, not just skim the surface.
Underlay treated as a formality
Modern underlay is a secondary line of defence, so it needs to be fitted with care rather than thrown down to keep the rain off the carpenters.
The classic mistake is fitting it too tightly. Underlay should drape slightly between rafters so that any water reaching it runs down the middle rather than sitting against the timber. Pulling it drum-tight creates a flat sheet that holds water and can tear when the roof moves.
Drainage is the other problem. Underlay should run down the slope and discharge into the gutter, ideally over a support tray or into a dedicated lap, not stop short of the eaves. A common shortcut is to leave the underlay draped over the fascia with no support, where it flaps in the wind and eventually splits.
Finally, laps matter. Horizontal laps should be generous and positioned so that water cannot be driven upwards into them. Vertical laps must be sealed as the manufacturer specifies. Tape that is applied to a dusty or wet surface will fail, and by the time anyone notices, the battens beneath are already damp.
Fixings chosen for speed rather than performance
Slate fixings are small, inexpensive and absolutely decisive. Copper or silicon bronze nails and stainless steel screws are the tried and tested options for long life. Electro-plated or galvanised nails are cheaper and will eventually corrode, staining the roof and losing their grip. On a roof expected to last decades, saving pennies here makes no sense.
How the fixings are driven matters just as much. Nails hammered in at an angle, or driven so hard that they cut into the slate, create a fracture point that opens on the first hard frost. Nails that are barely through the batten will pull out in a gale. Each slate should sit flat, with the fixing holding it without pinching.
Nail holes and positioning
Holes should be drilled or punched at the correct distance from the slate's edges. Too close to the edge and the slate cracks. Too high and the slate sits proud. Where slates are fixed with two nails, both should be set at the same height so the slate hangs square rather than twisting.
Ignoring ventilation
A cold roof needs to breathe. Without ventilation, warm moist air from the rooms below finds its way into the roof space, condenses on cold surfaces and quietly rots battens, rafters and sarking. It also encourages moss and algae, which hold moisture against the slates.
Ventilation should be designed, not improvised. In a traditional cold roof, air needs to enter at the eaves and leave at or near the ridge, with a clear path through the roof space. That path is blocked if insulation is pushed hard into the eaves, which is one of the most frequent problems found on otherwise well-built roofs. Baffles and trays keep the airflow open without compromising insulation.
Warm and hybrid roof designs have their own ventilation requirements, and getting these wrong is expensive to correct later. If the design is unusual, it is worth having the ventilation and insulation strategy checked by someone who works with that build-up regularly.
Slates that are graded badly or laid in the wrong order
Slate is a natural material. Thickness and size vary slightly, even within a graded batch. Sorting them on the ground into thickness groups and using the thicker, larger slates on the lower courses gives a flatter, more even roof. Skipping this step leaves a rippled surface and slates that sit unevenly under the next course.
It is also worth checking slates as they come off the pallet. Chipped corners, hairline cracks and delamination are not always obvious, and once a batch is up on the roof it is much harder to reject individual pieces. A quick tap and a look at both faces catches most problems.
Small details that decide how long the roof lasts
Ridges, hips, valleys, verges and abutments are where water concentrates and where shortcuts show up fastest. Mortar bedding that is too dry, under-mixed or applied in cold weather will crumble within a few years. Valleys need the correct width and a proper lining material, with slates cut and laid so water is guided, not trapped. Verges need adequate overhang and support, or the edge slates will work loose.
Flashings are another favourite hiding place for trouble. Lead should be dressed properly around the abutment, with correct laps and enough length to cope with movement. Sealant alone is not a flashing.
Getting it right from the start
Before the first slate goes on, work through the job on paper. Confirm the batten gauge against the slate size and required headlap. Check the underlay specification, lap directions and drainage route. Decide on fixings and make sure they are on site in the right material. Plan the ventilation path and make sure it cannot be blocked by insulation.
On site, keep the slates sorted, take time over the first course and check alignment regularly rather than at the end. If something looks rushed, it usually is.
Finally, if the roof design is complex, if the property is listed, or if you are dealing with a warm roof build-up or unusual pitch, bring in a qualified roofing surveyor or an experienced slater before work begins. It costs far less to change a detail on a drawing than to strip and rebuild a finished roof.
Photo: paulbr75 / Pixabay
