A slate roof does not rely on the slates being waterproof joints. It relies on overlap. Every slate sits partly under the course above it, so water running down the slope is passed from one slate to the next and never finds a straight path to the timber below. Three things control how well that works: the size of the slate, the amount each course overlaps the one beneath, and how each slate is held in place. On a gentle pitch, a small overlap that would be fine on a steep roof lets wind-driven rain creep up and under. This guide explains the vocabulary a slater uses, the typical figures published by a slate supplier, and how nails, copper rivets and hooks differ. It also shows how we check the lap on an older roof around Bath before deciding whether the fault is a few slipped slates or the way the roof was set out.
Slate sizes and what the numbers mean
Natural slate is sold by its length and width in millimetres, with the length quoted first. A 600 x 300 slate is 600 mm long and 300 mm wide. The sizes you will see most often on a modern specification are 600 x 300, 500 x 250 and 400 x 250. Larger slates cover more roof per piece and need fewer fixings. Smaller ones suit smaller roofs and tighter detail, and they sit more comfortably on a short slope where a big slate would look out of scale.
On older houses the sizes are less tidy. A Victorian roof may be laid in slates of one size throughout, or in random widths that were cut to suit the quarry's stock. Some historic roofs were laid in diminishing courses, where the slates get smaller towards the ridge. When we repair a roof like that, the replacement slate has to be matched to the course it goes into, not to a catalogue size, which is one reason we keep reclaimed stock for repairs.
Size matters for the lap as well as the look. The figures a supplier publishes for the minimum overlap are given for particular slate sizes, so you cannot read a lap from a pitch alone. A 500 x 250 slate and a 400 x 250 slate behave differently on the same roof. The supplier we draw figures from gives no value for 400 x 250 slate below a 30 degree pitch, which tells you how little margin a short slate has on a shallow roof.
Thickness, texture and colour vary between quarries, and they affect how the slates lie. A rough, uneven slate leaves small gaps between courses that a smooth, even one does not. That is part of why a laid roof of one slate can look and perform differently from a roof of another, even at the same nominal size and lap.
The size you have is easy to find out. On a survey we measure a few slates against the batten gauge, meaning the spacing of the timber strips up the slope, and note the width pattern. Photographs of the underside of a slipped slate usually show the nail holes and the lap in one image.
Head lap: the overlap that keeps water out
Head lap is the overlap between a slate and the slate two courses below it, measured in millimetres. Picture three courses. The slate at the top covers the joints of the course beneath it, and it also reaches down past the top edge of the course below that. That top edge is the head of the lower slate, and the amount of cover is the head lap. It is the safeguard for the case where wind pushes water up the slope: the water has to climb past the lap before it gets to a gap.
Do not confuse head lap with side lap or with the gauge. Side lap is the overlap sideways between the slates in neighbouring courses, which comes from laying them in a staggered bond so that each joint sits over the middle of the slate below. The gauge is the spacing of the battens, the timber strips the slates are fixed to. Gauge and head lap are linked: for a given slate length, a bigger head lap means a smaller gauge, so more battens and more slates per square metre. That is why a roof laid with a generous lap costs more in material.
Two things push the required head lap up. A lower pitch gives water more time on each slate and less gravity to shed it. And exposure matters, because wind-driven rain is forced up the slope in places that are open, high or near the edge of a ridge line. A roof on a sheltered terrace behind a parapet is a different case from a roof on an exposed hillside above Bath.
The supplier guidance also separates moderate and severe exposure. For severe exposure it asks for a larger lap on the steeper pitches. We treat the published figures as typical values that depend on exposure and workmanship, and we check the position against BS 5534 and the slate maker's own data for the roof in question.
The practical point for an owner is simple. If the lap on your roof is too small for its pitch, replacing slipped slates will not cure a persistent leak, because the fault is in the set-out. If the lap is right, a leak almost always has a more local cause such as a failed nail, a split slate or a worn flashing.
Lap against pitch: typical published figures
Pitch means how steeply the roof rises. A slate roof needs more lap as the pitch falls, and the table below shows the pattern. It gives typical head laps for 600 x 300 and 500 x 250 natural slate in moderate exposure, as published by Cupa Pizarras for the conditions set out in BS 5534. Moderate exposure there means wind-driven rain below 56.5 L/m². The figures are guidance from a manufacturer and not a design for your roof. Check the exposure and the slate with the maker and BS 5534 before relying on any of them.
| Roof pitch | Typical minimum head lap |
|---|---|
| 25 degrees | 95 mm |
| 30 degrees | 80 mm |
| 35 degrees | 70 mm |
| 40 degrees | 60 mm |
| 45 to 75 degrees | 55 mm |
| 85 degrees | 50 mm |
Look at how the figure drops as the roof steepens, and how it levels off between 45 and 75 degrees, which is where a great many hipped and gabled houses sit. The same source gives a general minimum pitch for natural slate of 20 degrees, quoting the NHBC standard, subject to the head lap. With a 500 mm slate and a 115 mm lap in moderate exposure, BS 5534 allows 20 degrees. Severe exposure asks for 70 mm on pitches from 45 to 75 degrees, and 65 mm at 85 degrees.
Hook fixings are not recommended below 25 degrees on that guidance. Where a roof is close to the lower limits, the answer is not to stretch the figures. It is to use a larger lap, or to reconsider the covering. We do not give a minimum pitch for historic stone slates, because there is no authoritative figure to quote, and they are treated roof by roof.
Finding your pitch does not need a ladder. A phone with a level app held against a gable-end rafter, or the side of the roof in a photograph, gives a close figure. We measure it on the survey and write it down, because every other decision, from the lap to the fixing, depends on it.
Nailing: how a slate is held
Most natural slate roofs are nailed. Each slate has two holes, and a nail goes through each into the batten. There are two usual positions. Centre nailing puts the holes about the middle of the slate's length, so the slate is held at its balance point and the tail can lift a little in the wind. Head nailing puts them near the top edge, which holds the slate firmly, and it is used where wind uplift is a greater concern. Which one is chosen depends on the slate, the exposure and the specification.
Nail material matters more than almost anything else in this method. A nail that corrodes lets go, and then the slate slips. Copper nails, and other nails made to resist corrosion, are the right choice on a new job. Plain steel is not. On an old roof the original nails may have rusted through, and a slate that looks sound from the ground may be held by nothing. We deal with that failure on our page about nail sickness, which is the usual reason an otherwise good roof starts shedding slates.
Two nails per slate is standard. The nails are driven so the head sits flush, and never so hard that it cracks the slate or tight enough to stop it moving a little. A nail left proud lifts the slate above, and one punched through too far splits it. Quality of work shows in these small things, and they are hard to inspect once the roof is finished, which is why a good slater lays carefully and why we photograph the roof before and after.
Nails also fix the lap in place. A slate nailed too low or too high changes the lap on the course beneath it, which is a quiet way for a roof to end up below its specification even when the batten gauge was right.
If you can see slates sitting a little proud at the tails across a slope, that is often the nails working loose. A single slipped slate may be a one-off, but a spread of them points to the fixings.
Hooks, clips and rivets: when nails are not the answer
A slate hook, sometimes called a hook fixing, holds the slate by its bottom edge. It is a stainless or other corrosion-resistant strip that is nailed to the batten and curls over the tail of the slate, so the slate hangs from it without a hole through the slate. It is used on roofs where the slates cannot be nailed in the usual way, such as slates that are too hard or too thin to be holed, and some roofs where nail holes would weaken a slate. Slate hooks are also what a roofer reaches for to re-fix a run of old slates in situ when the old nails have failed, because hooks let a slate be refixed without disturbing the course above. Our short page on slate hooks looks at that use.
A rule you will find in the published guidance is that hook fixings are not used below 25 degrees. Below that pitch the slate is not heavy enough to lie against the hook reliably, and wind can lift it. For the same reason, hooks must be spaced and sized for the slate and the exposure, and a roof with hooks should be inspected after a storm in the same way as a nailed one.
Copper rivets are a third method. A rivet fixes a slate with a copper tab that is nailed or riveted at the head of the slate, and the same word is used for the tabs on fibre cement slate. The Marley Eternit fixing guide for its fibre cement slate describes rivet and nail fixing as generally suitable for roof pitches of 20 degrees and above. That is manufactured slate, not natural, and its own fitting guide rules.
Each method has its place. Nails give a neat, traditional roof and are right for most natural slate. Hooks suit slates that cannot be nailed and repairs where the nails have failed. Rivets and clips belong to particular products. What is wrong is mixing methods without a reason, or leaving unsupported slates on a roof where the wind lifts them. Our page that sets slate against tiles covers the broader choice of covering.
Why the right lap matters on your roof
The lap is the one dimension you cannot see once the roof is finished, and it is the one that decides how the roof copes in a storm. Two roofs of the same slate can behave very differently if one has a lap suited to its pitch and the other does not. The first sheds the rain, and the second lets it in at the exposed corners and at the lower, shallower parts of a hip or a dormer cheek.
On an older house around Bath, the pitch is not always the same across the whole roof. A main slope may be steep, while a rear outshut, a porch or a bay window roof sits at a shallower angle. Those lower slopes are where leaks begin, because the lap that suits the main roof is too small for them. We check each slope by itself rather than assume the whole roof matches.
When we are on a roof, we look for these signs of a lap problem rather than a fixing problem:
- Staining or damp on the underside of the slates near the tails, with the nails sound.
- Leaks that show when rain blows hard from one quarter, not in every wet spell.
- Moss and debris collecting at the lap lines of a shallow slope.
- Slates that have been replaced more than once in the same place.
Where the lap is too small for the pitch, the options are to re-lay the slope with a smaller gauge, to add an underlay beneath the battens as backup, or to change the covering to suit a shallow pitch. Underlay is a sheet laid over the rafters that carries away any water the slates let through. It does not replace the lap, but it helps. If a full re-slate takes the work over the thresholds, the Building Regulations apply, and the Planning Portal page on re-roofing sets out when. Our slate roofing service covers the repair and re-slating side.
After the survey you receive a written report with pictures of your own slopes, the pitch, the lap we measured and the work we recommend, then a fixed price. Where repair works and the lap is right, repair is what we propose, and the work carries a 10-year written guarantee.