A slate keeps rain out by overlapping. The part of each slate that is covered by the course two rows above is the headlap, and the steeper the roof, the less of it you need, because water runs off faster. On a shallow pitch or a windy site the overlap has to grow, or rain can be driven up between the slates and onto the battens. The slate is then held by a nail through its top, by a tingle, or by a hook that grips the bottom edge, and those fixings carry the load of every gust.
On a coast, both the overlap and the fixing are worth getting right, since this is where the weather tests them hardest. A map will not give the answer. No exposure category is published for a named town, and the Worthing coast faces south, which matters when you read rules written for other coasts. What follows shows how the published tables work and what we check on a particular roof.
Below: how headlap is chosen, what wind does to a slope, where hooks stop being acceptable and how driving rain is rated. Our answer on the minimum pitch for slate sits alongside this guide.
Why headlap matters more on a coast
Picture a slate roof as rows of overlapping stone plates, shedding rain down the face with the overlap sealing each joint. The headlap is the safety margin in that system. If wind pushes rain sideways and upward, a generous overlap stops the water reaching the gap, and a thin one lets it through.
Two things make the margin matter. The first is pitch: a shallow roof sheds water slowly, so the overlap has to do more of the work. The second is exposure, meaning how hard weather hits the roof. A roof on an open seafront, high above its neighbours, takes driven rain from a different angle and with more force than one in a sheltered street. Both push the sensible overlap upward.
The slate size matters as well. Cupa Pizarras, the Spanish slate maker, publishes headlap tables by slate size and pitch. For a 500 by 300 mm slate in sheltered or moderate conditions, the table runs from a 115 mm headlap at 20 degrees down to 50 mm at 85 degrees. For 600 by 300 mm and 500 by 250 mm slates it gives no value below 25 degrees.
A smaller overlap saves slate, so there is always a pull towards the minimum. On a coastal roof we would rather give some of that saving back. Extra headlap is cheap compared with the cost of a leak that only shows in the worst storm, which is exactly when nobody can get a ladder up to find it. The headlap is a number for the specification, set before ordering slate, and not something to adjust on the roof as the work goes.
The table values are for sheltered and moderate conditions. An exposed site needs the manufacturer's advice on top, not a smaller number.
Reading the Cupa headlap table
The figures below come from the Cupa Pizarras guidance on minimum headlap and fixing for slates. They apply to a 500 by 300 mm slate in sheltered or moderate exposure. Pitch here means the slope angle, measured up from horizontal.
| Roof pitch | Minimum headlap, 500 x 300 mm slate |
|---|---|
| 20 degrees | 115 mm |
| 22.5 degrees | 105 mm |
| 25 degrees | 95 mm |
| 27.5 degrees | 85 mm |
| 30 degrees | 80 mm |
| 35 degrees | 70 mm |
| 40 degrees | 60 mm |
| 45 to 75 degrees | 55 mm |
| 85 degrees | 50 mm |
The first line is also the lower edge of the recommendations. Cupa says BS 5534:2014+A2:2018 indicates that the minimum roof pitch for natural slate is 20 degrees, for moderate exposure, a 500 mm slate and a 115 mm overlap. Anything shallower is outside the table, and it needs a different covering or a manufacturer's written specification.
Reading the table is simple. Find the pitch, find the slate size, take the headlap and make sure the roof is set out to it. The difficulty comes when the real pitch is not what the plans say. Old roofs sag and settle, and the pitch of a lower slope or a rear extension may be a few degrees shallower than the main roof. We measure the pitch on site, and where two slopes of one roof differ, each gets its own headlap. That measured figure goes on the written specification we give you along with the photographs.
Always check the product's own data sheet as well. Different slates have different published values, and the data sheet for the slate being laid is the one that counts.
Hooks, nails and the 25 degree line
There are two main ways to hold a slate. A nail goes through a hole near the top of the slate and into the batten, so the slate hangs from it. A hook grips the bottom edge of the slate from below and holds it flat against the course beneath. Hook fixings are quicker on some jobs and they avoid drilled holes, which suits slates that are brittle or thin.
The rule that matters on a coast is the limit on pitch. Cupa’s advice rules hooks out entirely on slopes under 25 degrees. On a shallow roof the wind can lift the lower edge of the slate against the hook, and with no nail or holding at the top there is little to stop it. Our answer on hooks versus nails goes through the trade-offs in more detail.
Nails fail in a different way. The iron nail rusts and the slate slips, which is the problem described under nail sickness. The cure is to choose the fixing for the roof you actually have, not the one that is quickest to fit. On a steep roof in heavy weather that may be a nailed slate with extra headlap, and on a shallow one it will be nails and sound slate.
What we check before choosing
We look at the pitch, the slate size and thickness, the batten condition and the roof shape. A long unbroken slope in a gale takes more load than a short one, and a corner or a verge sees higher suction than the middle of the roof. Each of those moves the fixing choice.
We do not name a nail metal as the answer for the whole coast, because it depends on the slate and the manufacturer's advice. The specification we write down for your roof names the fixing, the size and the number per slate.
Wind, suction and the shape of the roof
A gale presses on the windward slope and pulls at the rest. It lifts. Air accelerating over the ridge creates suction on the downwind side and on shallow slopes, and Marley's wind guidance warns that below a 30 degree pitch the windward slope can face strong uplift from suction. That is why shallow pitches get more care than steep ones.
For the actual number, BS EN 1991-1-4 supplies the method: start from the site's basic wind speed, then modify it with allowances for the lie of the land, nearby ground levels and the size of the building, to reach the pressure on the roof. That is a calculation for one building, and wind speeds for a given street come out of it, not out of a general guide.
Roof shape feeds into it too. Edges, corners and the ridge see higher loads than the main field of the slope, so slates in those zones are fixed more securely. Verges are held down with mechanical fixings, not mortar alone.
Near Shoreham airport there is one extra check. Marley points out that large aircraft create air vortices on take-off and landing, meaning a roof near an airport may meet local gusts stronger than the BS 5534 calculation allows, and it directs designers to the Building Research Establishment's Digest 467. For sites inside 6 km of an airport, Marley asks that its Technical Advisory Service be contacted about the fixing specification. Some Adur addresses may fall inside that radius; we name no streets, since the maker judges each site.
All of this feeds one conclusion: the fixing is specified for the site, then checked against the manufacturer's data sheet before the first slate goes on.
Driving rain, long slopes and when to ask the maker
Alongside wind, there is the question of driving rain. Marley explains that the UK is split into four driving-rain exposure categories, based on BS 8104 and the Building Research Establishment's Report 262, and that this applies to buildings up to 12 m at the ridge. It adds that where the roof slope exceeds 6 metres in length, and the same applies to a site in the harshest driving-rain category.
Those rules matter on tall Victorian terraces and villas as well as on newer homes, where long slopes are common. A rear roof that runs from the ridge to the eaves over a deep house can easily exceed the figure. The consequence is not that slate cannot be used. It means the specification needs the maker's confirmation, and a bigger headlap and secure fixings are likely to feature.
Some roofers repeat a rule that anywhere within 30 miles of a west-facing coast is highly exposed. Cupa is the source, and it is a general rule for west-facing coastlines. The Worthing coast faces south, so it does not simply carry over, and we do not state an exposure category for any named town. The category for a particular site is decided from the building and its surroundings.
What this means in practice
For a typical house in a street, we use the published headlap for the measured pitch, add margin if the site is open or the slope is long, and check the slate's data sheet. For a tall building, a very open site, or a slope near the limits above, we ask the manufacturer and write the answer into the specification.
When slates are reused, the same rules apply, and the headlap is set out from the sound slates you have. Our guide to reclaimed slate covers what to look for in second-hand stock.
Conservation settings and what we do on the day
Many slate roofs on this coast are in conservation areas or on listed houses, and there the fixing has two jobs: to hold in a gale and to leave the roof looking as it did. The A2:2018 amendment to BS 5534 accepts that the code may not fit a heritage roof, so the planning authority and conservation specialists come into the conversation. In practice we keep the original slate size and laying pattern, and choose a fixing that does not change how the roof reads from the street. Our guide to slate roofs in conservation areas explains when consent applies.
On the survey, we measure each slope, lift a few slates to check the nails and battens, and photograph what we find. You see those photographs with a written quote that carries a single total. The specification names the slate, the headlap for each pitch and the fixing.
On the day, battens are checked or replaced where they are weak, and underlay is fitted where the roof has none and the work calls for it. Slates are laid to the headlap on the specification, not to what looks about right. Fixings are checked as the work goes on, since any under-fixed corner is where a storm will find it first.
We leave the site clear, remove the waste, and finish with a walk along each slope to check that no slate sits proud of its neighbours. A coastal roof still gains from a look each year, and the slate roofing overview and our guide to slate roof maintenance explain what to check yourself and what to leave to a roofer.
