What Is a Slate-and-a-Half?

The bond that makes slates watertight

Slates are laid in horizontal courses, each overlapping the course below, and the vertical joints between slates are staggered so that no joint lies directly above another. This half-bond arrangement is what stops water running straight down a gap. Each joint in one course is covered by the middle of a slate in the course above and by another in the one below. It works well across the middle of the roof where slates sit side by side without interruption.

The problem comes at the edge. If every course starts with a whole slate at the verge, the joints in all the courses line up in a vertical stripe, and water has a straight route down. To avoid this, the starting slate is made a different width on alternate courses. One course begins with a standard slate and the next begins with a wider one, so the joints are set half a slate apart.

Why one and a half, and where it goes

If a standard slate is one unit wide, starting the next course with half a slate would also stagger the joint, but a half slate is narrow and weak. It cracks easily, holds less nailing and is a poor edge for the verge to rely on. Using a slate one and a half units wide gives the same stagger with a solid, generously sized slate instead. On a standard roof, the courses alternate between a full slate and a slate-and-a-half at the verge, and the same trick works on the other side.

The same idea applies at valleys, where slates are cut diagonally to follow the line, and at abutments against a wall or chimney, where the courses end against masonry. Roofers carry slate-and-a-halves as a standard stock item, in the same thickness and finish as the main slates. They can also cut one from a wider slate if needed. Our page on what size slates you need explains how slate sizes are set.

How it is fixed

A slate-and-a-half is nailed or hooked like any other, with fixings in the correct positions for its size. Because it is wide and sits at an edge, it is often fixed with extra nails. At the verge, it is bedded or held with mechanical fixings, since BS 5534 requires mechanical fixing of verge components. The slates must lap correctly, with the head lap the same as the rest of the roof. See head lap on a slate roof for the overlap. At the verge, the slates project slightly over the gable wall and are sealed with a mortar or dry-fix verge system.

Where the edge of the roof is a hip, the same thinking applies in a different form; the cut slates meet in a mitre. Our page on mitred hips shows this.

What goes wrong without one

If a roofer ignores the half-bond at the edge, you get either a stripe of aligned joints or a narrow cut slate. The aligned joints let water through the gap under wind-driven rain, so a damp stain develops under the verge. Narrow slivers crack where the nails go in, and then slip, leaving a gap that the verge has to cover. Both show up as a ragged, uneven edge from the ground.

A repair near the verge needs the right slate. If one is cracked, a standard slate won't replace it, and the repair will need a slate-and-a-half or a cut equivalent. A missing one is a clue that earlier repairs were done cheaply. For hands-on repair methods, how a single slate is replaced explains how the new piece is held.

Cutting and finishing the slate-and-a-half

Slates of this width are sold as a stock size by some suppliers, and others are cut from wider stock on site. Cutting is done with a slater's axe, called a sax, or a cutting tool on a bench. The edge facing the verge or valley may be left as split or dressed straight, depending on the look the job needs. Holes are made in the correct position for the slate's length and the gauge, and the surface is checked for flaws before fixing, because a crack in a wide slate is as bad as a crack in a standard one. A well-cut slate-and-a-half sits flat against its neighbours, and its colour and thickness match the rest of the course.

We look closely at the edges of slate roofs during a survey, because verges and valleys are where shortcuts hide. Our pictures show the bond along the edge, and we give a fixed price for any repair. The full range of this work sits under slate roofing in Basingstoke.