What Is Head Lap on a Slate Roof?

What head lap measures

A slate roof is not one layer of stone. At any point on the slope, water has to get past three thicknesses of slate before it reaches the underlay. Each slate is held by a nail or hook near its top, and the course above sits over it by a set amount. That overlap is the head lap. It is not the same as the gauge, which is the distance between the bottom edges of one course and the next. The two are linked by the slate length: lengthen the lap and you shorten the gauge, so more slates are needed to cover the same area.

Beginners often confuse head lap with side lap. Side lap is the way neighbouring slates overlap each other across the course, and on a standard roof it is built in by the staggered bond, where every slate is centred over the joint below. Head lap is the vertical one, and it is the figure a slater sets out first, because it controls the battens (the timber strips the slates hang from) and the number of slates in the order.

How much lap a roof needs

The lap is chosen to suit the pitch and how exposed the building is. As a rule of thumb, a lower pitch needs a deeper lap, because water runs slowly and wind can push it uphill under the slate. A steep roof sheds water quickly and can get by with less. Trade practice sets the lap anywhere from roughly 75 mm on a steep sheltered slope to well over 100 mm on a shallow one, and BS 5534 and the slate maker's own tables give the figures for a particular pitch and slate. Our page on the minimum pitch for a slate roof covers how low a slope can go.

Exposure moves the number too. A house on an open ridge above Basingstoke takes rain that arrives sideways, and the lap should be increased there, not trimmed to save slates. A terrace tucked behind other buildings can sit at the lower end. The slate size matters because longer slates give more room to build a generous lap without making the gauge too tight. If you are choosing a size, see what size slates you need.

What goes wrong when the lap is short

A short lap rarely shows up as a hole. It shows up as a stain on the ceiling after a storm that comes from one direction, then dries and vanishes. Wind-driven rain and capillary action (water creeping upward through a narrow gap between two surfaces) carry moisture past the top of the slate and onto the nails, battens and underlay. Over a few winters the battens go soft, nail heads rust and the underlay perishes. The slates themselves look fine from the ground, which is why the fault often goes unseen until the timbers have already suffered.

Short lap is most common on roofs that have been re-slated with the gauge stretched. A slater who wants to make a batch of slates cover more area can pull the gauge open, and every millimetre gained is a millimetre lost from the lap. It is also seen where old slates of mixed length were relaid on one set of batten spacings. Slipped or cracked slates are a separate fault, though a roof with both problems leaks far sooner.

Checking the lap on your own roof

You cannot measure head lap from the ground, and we would not ask you to go up and try. What you can do is look for signs from a window or the garden: stains on the loft underlay, damp batten ends, daylight showing between courses, or a ceiling mark that follows the wind direction. On a survey we measure the gauge on the slope and work back to the lap from the slate length, photograph it, and compare it with what that pitch and exposure should have. If the lap is adequate we say so and repair what is actually broken.

Where the lap is short across the whole slope, the repair is a re-slate, relaid at the correct gauge on new battens. A few courses cannot be fixed in isolation. Where only an area is affected, perhaps a patched section, we can often correct that area alone. The details of the job are on our slate roofing in Basingstoke page, and every quote is fixed and in writing after the free survey.