What Is a Slate Eaves Course?

A slate roof is laid in rows called courses, and each slate is covered by the rows above it. At the eaves there is no row below to do any covering. So the roofer starts with a short slate, an undereaves slate, set with its tail on the edge line. A full-length slate goes on top with the same tail line, and the pair lie together as one thick row at the edge.

Why is the eaves course doubled?

Without it, the joints between the first slates would sit over nothing, and water could run through the gaps onto the underlay or battens. The undereaves slate covers those joints. It also tilts the first full course to the same angle as the rest of the slope, which keeps the rows above flat and neat. Our page on slate headlap and fixing goes into the overlap that the rest of the roof depends on.

Because the doubled row is the first thing fixed, mistakes at this stage are carried upward. A row set too high or too low changes where every course above falls, and the error shows as a crooked line of tails along the whole slope, or as a gap near the ridge where the courses fail to meet.

The slate tails should project beyond the roof edge so that water drops into the gutter and not behind it. A tail that stops short lets rain run back along the underside and onto the fascia, the board that carries the gutter, which then rots. A tail that sticks out too far catches the wind and can be knocked off, or throw water over the back of the gutter.

How do I know if the overhang is wrong?

Stand outside during a heavy shower and watch. Water should fall from the slate edges into the gutter. If it runs down the wall instead, or you can see a damp stain on the fascia or the brickwork below the eaves, the edge is not doing its job. The underlay, which is the waterproof sheet below the battens, should also be dressed over into the gutter, a detail explained in our piece on the membrane under slates.

Gutters themselves sag and clog, so a leak at the edge is not always the slates. We check both, and our gutters, soffits and fascias work covers the roofline when it needs attention.

The eaves take all the water that falls on the slope above, and that gathers at the bottom. Any fault there is tested by every shower. The eaves are also where gutters leak, birds look for a way in and moss collects, so a leak at the edge can have more than one cause.

What goes wrong at the eaves?

Common faults are a missing undereaves slate, one that has slipped, a course cut short, or a rotten batten behind the first slate. Each lets water in where the roof is lowest, and the first timber to get wet is the wall plate, a long timber sitting on top of the outer wall and carrying the rafters. We look for damp marks on the wall plate during a loft check. Our answer on why a slate roof leaks lists the other usual places.

Rot caused by a slow eaves leak is quiet, and it often travels along the wall plate to the corner of the house before it shows. You may see nothing indoors for a long time, until a brown stain blooms on a ceiling close to the outer wall.

The eaves course is set out before the rest of the slope, because every row above is measured up from it. A badly laid first course carries its error to the ridge, and it is much harder to correct once the slates are on. So we check the batten spacing and the height of the first batten before a slate is fixed, and on an old roof we compare them with the setting-out of the rows above so the replacement follows the original lines.

Can the eaves be fixed without re-slating?

A slipped or missing undereaves slate can be replaced if the courses above can be lifted. A badly laid course along a whole slope is a bigger job. We lift a few slates at the survey, share the pictures of what is beneath and say whether it is a repair or a rebuild. Read about the service on our slate roofing page, and when you are ready, send a picture of the eaves and gutter line so we can plan the visit.