The short answer
The name says it all: a slate-and-a-half is wider than the roof's ordinary slates by half again. It goes at the verge, the sloping side edge of the roof beside a gable wall, in every second course, so that the staggered joints carry on to the very edge. Leave it out and two joints can stack above one another, giving rain a straight path through.
The bond that makes slate waterproof
Slates are laid in rows called courses, working up from the eaves. Each slate sits across the joint between two slates in the course below, in the same way that bricks are laid in a wall. That arrangement is called a bond, and it matters because it stops an unbroken gap running from ridge to eaves. Water that gets into one joint finds a slate in the way a course lower down.
The slates across the main field of the roof share one width, so the pattern works itself out neatly mid-slope. At the edge, a problem appears. Every second course would start with a half slate if the joints are to stay staggered, and a half slate is narrow, weak and hard to hold.
The slate-and-a-half solves that. It is wider than the main slate, so the courses can alternate at the edge between a whole slate and a slate-and-a-half, and each joint is still covered by a slate below. Nothing lines up, and no thin, fragile piece is needed at the end of the row.
The result is a verge that is neat, strong and weathertight.
Where it sits: the verge
The verge is where the slope finishes against a gable wall. It takes more weather than most of the roof because wind gets under the edge and tries to lift it. For that reason it needs both a strong bond and a secure fixing.
On a slate roof the slate-and-a-half sits in the last position of alternate courses at the verge. The slates beneath it are held by nails in the usual way, and the wider slate carries extra fixing at the edge, since it is the piece that faces the wind. The verge is then finished either with a mortar bed and a slate undercloak, or with a dry-fix system that clips the edge down.
Which finish is used affects how the slate-and-a-half is cut and set. Our answer on verge on a slate roof sets out the finishes, and the detail is decided when the roof is surveyed.
If you have a gable end and are not sure how the verge was built, a photograph from the garden usually tells a roofer a good deal. Please do not go up to look.
Cutting and fixing the wider slate
A slate-and-a-half is either ordered to size with the main slates or cut on site from a slate of the right thickness and colour. A slater marks the width, scores the line and splits or cuts the slate cleanly, then punches the nail holes in the correct position near the head.
The cut edge is the point to watch. It should be square and clean, with no chips or splits running into the slate, because a ragged edge shows from the ground and weakens the piece. A slate that is cut with a poor edge can crack in the first hard frost.
The slate must also match the others in thickness. A piece that is thicker or thinner than its neighbours will not lie flat against them, leaving a gap that wind can enter. That is a reason to cut from slates of the same batch.
On a reslate or a repair at a gable, we cut each piece to fit the course it belongs to and show you the finished edge. The slate head-lap answer explains the overlap behind the slates that all of this protects.
Checking the edge of your roof
A slate roof gives away a good deal through its verges. A good one has a straight, even line along the edge, with the wider slates alternating regularly down the slope. The courses sit tight to the gable wall, and the finish, whether mortar or dry-fix, is complete and unbroken.
Warning signs are easy to spot from the ground. Mortar that has cracked and dropped away along the verge, slates at the edge that tilt outward and a visible gap between the slate edge and the wall all suggest the verge needs attention. A narrow strip of slate at the end of a course, where a slate-and-a-half should be, is a sign the bond was cut short.
These faults let wind and rain into the edge of the roof and the wall beneath. They often start as small problems and become costly if they are left.
When we survey a slate roof we look at each verge and photograph any defects. The photographs go to you along with a clear recommendation. If a verge needs rebuilding, the work is priced as one written total. Our slate roofing page lists the other repairs we carry out.
