The Short Answer: What the Curve Actually Does
If you lay two completely flat pieces of clay or glass on top of each other in the rain, water doesn't just run off—it gets sucked up between the two flat surfaces. That is called capillary action (or capillary draw).
By firing a deliberate curve (camber) into a traditional 10.5 x 6.5 inch clay plain tile, the tail of the upper tile touches the tile below at a steep angle while leaving a tiny 3mm to 5mm air gap just behind the contact point. That pocket of air breaks the surface tension of the water, stopping rain from creeping uphill under your tiles while stopping wind from lifting the tail.
How Capillary Action Ruins Flat-Laid Clay Roofs
When I get called out to inspect a persistent leak on an older period cottage around Faversham, Canterbury, or a Victorian terrace in Sittingbourne, homeowners often expect me to find a smashed tile or a big hole. Nine times out of ten, the leak is more subtle than that.
Traditional clay plain tiles do not have interlocking side channels like modern concrete tiles. Instead, they rely entirely on double lapping—every tile overlaps the tile two courses below it by at least 65mm (2.5 inches). Because water can pass straight into the side joint between two neighbouring tiles, the tile directly underneath has to carry that water safely down to the next course.
If the tiles were dead flat, water sitting in the overlap would bridge the two surfaces and draw upwards over the nibs and onto the timber battens. The curved camber forces the bottom edge of the tile to sit tightly down on the back of the course below, while opening up a ventilated breathing space underneath the belly of the tile. That air gap also lets the underside of the clay dry out quickly after heavy winter downpours, preventing frost spalling and rotten battens.
Single Camber vs Double Camber: What is the Difference?
When I am sourcing reclaimed or new clay plain tiles to repair a roof, one of the first things I check on the roof ladder is whether the existing tiles are single camber or double camber. Mixing the two together on a patch repair will leave the replaced tiles sticking up like a sore thumb and create a wind trap.
1. Single Camber Clay Tiles
A single camber tile curves in one direction only: from the head (top) to the tail (bottom). Across its 6.5-inch width, the tile is completely flat.
- ✓ Creates the essential air gap between overlapping courses to stop capillary draw.
- ✓ Sits with a cleaner, flatter horizontal line across the roof slope.
- ✓ Very common on handmade Kent peg tiles and traditional machine-made clay roofs.
2. Double Camber Clay Tiles
A double camber tile curves from top to bottom and has a slight arch across its width from left to right (known as the cross camber).
- ✓ Channelling effect pulls rainwater towards the centre of the tile, away from the side joints.
- ✓ Gives a more textured, rippled shadow line across the roof in sunlight.
- ✓ Requires dedicated hip and valley tiles shaped to match the steeper crown.
Why does that cross camber on a double camber tile matter? On a standard single camber tile, heavy rain running down the face can drift sideways into the butt joint between two tiles. Because a double camber tile is slightly crowned across its width, water is naturally Shed away from the edges when it hits the back of the lower tile. However, if you try to slide a double camber tile into a single camber roof during a repair, the raised middle pushes the two tiles above it upwards, leaving a gap that driving coastal rain from the Swale estuary will blow straight into.
Why Clay Plain Tiles Need a Strict Minimum 35° Roof Pitch
One of the most common mistakes we see on bungalow extensions and rear kitchen lean-tos across Sittingbourne, Rainham, and Herne Bay is someone laying traditional clay plain tiles on a shallow roof pitch so it matches the main house.
Here is the rule every decent roofer sticks to: machine-made clay plain tiles require a minimum roof pitch of 35 degrees, and handmade clay or Kent peg tiles should never be laid below 40 degrees. Look up at the older cottages in Faversham or Canterbury and you will see pitches of 45 to 50 degrees as standard.
There are two practical reasons why the camber demands a steep pitch:
- The Effective Pitch of the Tile is Flatter Than the Rafters: Because each tile sits on top of the double-lapped tiles below it—and because the tile itself is curved—the actual angle of the tile sits roughly 8 to 10 degrees shallower than the timber rafters underneath. If your rafters are built at 30 degrees, the top half of a cambered clay tile is actually lying at barely 20 degrees. At that angle, wind-driven rain simply washes sideways over the side joints and straight over the top of the nibs.
- Slow Drainage Leads to Frost Damage: Clay is a natural, porous material. It absorbs a small amount of moisture when it rains and releases it when the wind blows. On a pitch below 35 degrees, rainwater sits on the tail and in the overlap far too long. When a sharp Kent frost hits overnight, that trapped water freezes, expands, and flakes the face right off the tile (known as spalling).
If your extension roof pitch is shallower than 35 degrees, we will advise you straight away not to put plain clay tiles on it. Depending on the pitch, you are much better off with a low-pitch interlocking clay pantile, a natural slate system, or a continuous GRP fibreglass flat roof that will stay bone dry without relying on gravity overlaps.
What to Watch Out For When Repairing Clay Plain Tiles
When you ask me to look at a clay tile roof, I don't just price up from the pavement. Getting a repair to sit flat and stay watertight on a cambered roof comes down to three details on the roof itself:
Matching Imperial vs Metric Tile Gauge
Traditional clay plain tiles measure 10.5 x 6.5 inches (265mm x 165mm), usually laid to a 4-inch (100mm) batten gauge. However, handmade tiles and older regional batches vary in thickness and camber height. On our clay tile repairs and Kent peg tile repairs, I bring reclaimed tiles to match the exact camber profile so the tail beds tightly without kicking up.
Tile-and-a-Halfs at Verges and Valleys
Because plain tiles have to lay in a broken bond (like brickwork), every alternate course at the gable verge, chimney abutment, or lead valley needs a wider tile-and-a-half (roughly 9.75 inches wide). Cutting a standard tile in half leaves a narrow strip that cannot hold a nail and inevitably slips out in high winds.
Eaves and Top Course Under-Eaves Tiles
To maintain the exact same camber angle right down to the gutter and right up to the ridge, shorter eaves tiles (around 7.5 inches long) have to be fitted as a double course at the bottom and top of the slope. Without them, the bottom course drops flat into the gutter and allows birds and wind under the tails.
Building Regulations & Conservation Rules in Kent
If you only need a dozen slipped or frost-damaged clay tiles swapped out, you do not need Building Control approval. However, if the timber battens and old bituminous 1F felt underneath have perished and you need a full strip and retile covering 25% or more of the roof area, Building Regulations Part L applies. That means upgrading your loft insulation to current thermal standards and notifying South Thames Gateway Building Control Partnership (covering Swale, Medway, and Canterbury) or your local council.
Furthermore, if your home is in a conservation area such as central Faversham, Rochester, or Canterbury, planning rules require like-for-like clay materials so the camber, colour, and profile of the streetscape are preserved. You can check official local guidance on our Local Roofing Guidance & Building Control page.