Installing Eaves, Ridges, Hips and Valleys: The Four Spots Where Slate Roofs Leak First

When it rains hard, everyone runs inside. I go and stand at the bottom of the building and watch. It's a habit thirty years of roofing left me with — as soon as the guttering starts talking, I want to see how the water is moving. And since I moved back to Jiujiang to run a slate factory, the habit has only got half a new set of eyes: I still watch the roofs, but now I also watch the tiles that go on them.

Real slate barely drinks water at all. Our Lushan material tests at around 0.2% water absorption, so rain landing on a sloped roof has no reason to linger. The water that gets in finds the slates that can't cover the gap — and that means the four junctions, not the field. In all the leaky roofs I've worked on over thirty years, more than eight out of ten problems lived at the eaves, the valley, the hip, and the ridge. The slates in the middle of the roof were rarely the ones making trouble.

The short answer, before we start: the eaves see the water off, the valley gathers two slopes into one channel, the hip splits the water two ways, and the ridge finishes and seals the top. Each junction has a set of numbers worth carrying in your head — the eaves slates should project 20–25 mm beyond the eave line; the valley slates should stop at least 100–150 mm short of the valley centre line; the ridge slates should lap by no less than 75 mm. Get those numbers right and the details mostly look after themselves.

First, Read the Roof: Where a Storm Walks

A roof is not one flat plane. It's a few slopes stitched together. Rain lands on a slope, runs down the laps of the slates, and only when it reaches the bottom of a slope — or the seam where two slopes meet — does it arrive at a junction. For the water, the four junctions are four different roads:

Junction 1: The Eaves — Let the Water Leave Cleanly

The eave is the water's last step on the roof. If the jump is clean, nothing happens. If it's hesitant, the water runs back along the wall — black streaks down the brick, damp at the corners, and in winter ice that pushes the slate tails up.

Three details decide the jump

• The eaves slates overhang by 20–25 mm. A small number with a big job: the water drops off the slate edge clear of the wall below, straight into the gutter. Less overhang and it creeps back onto the brickwork; more and a strong wind gets a finger under the tail. Our factory-trimmed tiles come out square with eased edges, so an overhanging tail takes the load evenly instead of chipping.
• The underlayment must turn down. This is the step most crews skip. The underlayment reaches the eave and should not just be cut off flat — it folds over the eave edge, drops into the gutter, and is angled down by a tilting fillet (a wedge, roughly 40–50 mm high, nailed along the eave). Even if water gets under the slates, the membrane now steers it out and down, instead of lying flat and holding it. What membrane to buy and how it goes under the battens is the previous article's job — this is just the final fold.
• Stagger the first course. The bottom row of slates must break joint with the row above, offset by half a tile. A vertical seam running straight from the eave up into the second course is a motorway for water.

Winter is where the eave earns its keep. Frost-thaw cycles hit the eave line hardest, and ice dams usually start where the membrane was left flat instead of turned down — water pooled overnight, then froze. Our Lushan slate absorbs next to nothing, so ice on the surface doesn't get a hold; but if the tile survives and the detail under it doesn't, the water will still find the way in. A roof that holds water at the eave is a roof with an eave problem.

Junction 2: The Valley — Where Two Roofs Combine

Where two slopes meet on the inside, you have a valley — the one place on the roof with the most water, moving the fastest. The standard way to finish a slate valley is an open valley: a metal tray laid the full length of the valley, from ridge to eave, with the slates stopping short on both sides so a clear channel stays visible.

How to choose and lay the tray

• About 100–150 mm wide (measured at the junction of the two slopes). The flatter the pitch, the wider the tray. Lead (code 3 or 4), copper or zinc-coated steel, roughly 0.6–0.7 mm and up — lead bends around everything, copper and zinc are harder and last longer. One honest warning: in a coastal climate, do not use galvanised steel. Salt spray turns it white in a year.
• Cut the slate tips. The quietest detail in valley work: every slate that reaches into the tray gets its bottom corner trimmed off at an angle, so the water runs past the tip instead of being stopped by it. Our export slates arrive already cut to size; this one diagonal cut is done on site, and the tidier it is, the cleaner the channel.
• Keep slates at least 100–150 mm from the valley centre line. In plain language: thumb's width of clear metal each side. No slate crosses this line. Leave the room and the water has a road; stand at the eave and look up the valley, and you should see a continuous metal line from bottom to top.
• Finish the bottom of the tray. At the eave end, fold the metal back with a drop so the collected water pours straight into the gutter instead of sliding down the brickwork. It's the last millimetre of the whole detail, and the one surveyors look at first.

The saddest valley failure I've seen wasn't the slates — they were good, right out of the crate. The tray was narrow and shallow, and the slate tips had been pushed hard into the centre line. One heavy storm, the channel backed up, and water poured out over the slate edges into the loft. The crew was furious. And the fix was just: widen the tray to 120 mm, cut the tips, done. One point on a whole roof.

Junction 3: The Hip — Split the Water in Two

The hip is the outward corner where two slopes meet — the exact opposite of a valley. The valley collects, the hip divides. Water runs up to the hip line, gets split on the ridge edge, and each half goes back down its own slope. So the hip's problem is not the water pressure; it's the cut line and the last slate.

Two ways to dress a hip

The usual way is a cut hip: the slates on the hip course are cut diagonally, each with a straight slope edge, and the line of cut edges forms a clean ridge, capped with a row of narrow hip slates or a metal hip covering. The look is tidy and every slate has its own fixing. The price is waste — the cut corners become scrap.

The other way is a mitred hip: the slates from both sides butt straight against each other, the point of the ridge up, and a dedicated hip slate or flashing covers the seam. It saves material, but the seam has to be protected by something reliable. On old European roofs you still see mitred hips done well and dry for decades; done badly, they hold moss and wind-blown rain.

The one part on a hip you can't skip: the last slate

The last hip slate hangs in mid-air. Under it there is nothing — and if it's not supported, wind and frost will gradually work it out over the years. The rule is to set a hip blade (also called hip iron) under that last tile: a metal bracket nailed into the batten system below, with a small upward lip that catches the slate's tail. It's a small piece of metal. It is the entire difference between a hip that stays for fifty years and a hip that drops a slate during the first serious storm.

Junction 4: The Ridge — Where the Water Ends

The ridge is the highest line on the roof and the windiest. Water up there is the least of your problems — it runs off the ridge in a second. What tests the ridge is wind, and what leaks is wind-driven rain finding a gap in the mortar or a failed seal. Almost every ridge leak I've traced came through a gap, not through slate.

Wet ridge vs dry ridge: two ways of thinking

• Wet ridge (mortar) — the traditional way: ridge slates are lapped at least 75 mm and bedded in mortar. A rough mix that works: 1 part cement, 1 part lime, 3 parts sand, wet enough to work but not runny. It's cheap and forgiving — a slightly crooked ridge line can be levelled up in the mortar. The catch: the mortar is the shortest-lived part of the whole roof. It cracks after a few years, and wind finds the crack. So a wet ridge gets inspected every three years — that's normal, not a failure.
• Dry ridge system — ridge slates held with mechanical clips and rubber seals, no mortar. The payoffs: the ridge can take a ventilation vent (if the roof design calls for ridge ventilation), and any single ridge tile can be lifted later without hacking off mortar. The requirement: the ridge line has to be straight. On a squiggly ridge, a dry ridge system won't seal. If the roof below is built with a breather membrane and a counter-batten system, the ridge ventilation is part of the design — and dry ridge becomes the natural partner.

The ridge has two ends, and each ends its own way: one end runs into the gable and gets a ridge-end cap; the other meets a chimney, and that's flashing's business — a separate article, and a better one than I can write in a paragraph.

Build Order: Valley First, Ridge Last

There is a right order to these four junctions, and the phrase that carries it is: do the leak-first parts first, the visible last parts last.

1. Set the valley tray first — the most water, so it gets the first choice of the whole roof.
2. Then the first eave course — the "water leaves the roof" route is decided here: the overhang, the underlayment fold, the stagger, all at once.
3. Then lay the field, and finish the hip last — the cut, the hip slates, the hip blade.
4. The ridge goes last — it's the highest, and the crew will be walking past it for days; the later it's installed, the less it gets walked on.

There's a quiet advantage to this order. The valley and the eave are where the water arrives first, so they get made first and can be inspected first; the hip and ridge are where it arrives later. If something is wrong, the earlier you find it, the fewer tiles you pull.

The Rainy-Day Checklist: Water Is a Free Inspector

Don't sign off the roof the day the last tile goes on. Wait for a rain. Or when it's raining, take an umbrella and stand for twenty minutes, and walk the four corners of the roof in your head:

• Under the eave: is the water dripping straight into the gutter, or running back along the wall? The second one means the overhang or the fold is wrong.
• At the valley: is there one clean line of water running down the tray? Is any slate tip backing the flow up and pushing it over the edges? The 100–150 mm clearance exists for that line.
• At the hip: is water splashing back over the cut line, or hanging off the last slate?
• At the ridge: in the windiest moment, is any water beading out of the mortar or dry ridge seals?

Twenty minutes under the eaves will tell you more than any instrument. Water is a free inspector — anywhere it won't travel cleanly is somewhere the roof wasn't built well. And when you find something, fix it, and write it in the site record. That little note is the seed of the next customer's trust.

Related Reading

• Natural Slate Roof Installation Overview — where these four junctions sit in the full sequence from deck preparation to final ridge fixing.
• Roof Deck, Battens and Underlayment — the membrane this page folds into the eave starts its life in the layers below.
• Understanding Slate Headlap, Gauge and Exposure — the gauge and offset that decide whether your eave and valley joints line up.
• Minimum Roof Pitch for Roofing Slate — how shallow slopes widen the valley tray and change every closing detail.
• Jiujiang Slate Test Results — the EN 12326 / ASTM C406 numbers behind the slates that carry this detail work.