Rain does not fall straight down on a roof. It arrives sideways in a gale, creeps upslope under the tails of tiles by capillary action, and sits in the laps between them. How steeply the roof slopes decides whether that water drains away or finds a way in. Each tile on the market has a stated minimum pitch for exactly that reason, and a covering laid below its minimum is a leak waiting for the right storm. The question comes up when you are choosing tiles for a new extension, swapping one covering for another, or wondering why a roof that looks sound still drips in the wind. Here is how it works.
What pitch is, and how it is measured
Pitch is the angle of the roof slope from the horizontal, given in degrees. A roof rising at 30 degrees is steeper than one at 22.5 degrees. Do not confuse it with the ratios builders sometimes use, such as "one in three", and do not confuse it with the height of the ridge. Two roofs with the same pitch but different spans have very different ridge heights.
On a house you can measure pitch from the loft. Place a long spirit level horizontally against the side of a rafter, measure the vertical drop from the level to the rafter at a set horizontal distance, and calculate the angle, or hold a digital angle gauge or a phone clinometer against the rafter edge. From the outside, you can do it from a ladder with the same tools, but the loft is safer and gives a more accurate reading. Our short answer on what roof pitch is and how it is measured covers the method step by step.
Measure at several points. Older roofs sag, and a rafter that has sunk in the middle will give a different reading to one near the ridge. Use the pitch of the rafters as built, not the shape of the tiles, which may have settled.
Why a minimum exists
Tiles shed water because each course overlaps the one below, and the overlap is called the headlap. On a steep roof, water runs down the face and falls off the tail well before it can travel up under the next course. On a shallow roof, water moves slowly, and a gust can push it upslope across the headlap and onto the underlay and batten beneath. Raising the headlap helps, and a lower pitch means a longer headlap is needed, but there is a limit past which the tile cannot do the job on its own.
Manufacturers test each profile and publish a minimum pitch. Their figure depends on the tile's profile, its side lock (the interlock along its edges), its size and the headlap used, and it can change with the exposure of the site. In more exposed conditions, a higher minimum can apply. Never read a general table as a substitute for the data sheet of the actual tile.
Typical minimums by tile type
The figures below are the commonly quoted ranges, given so you can see the pattern. They are guides only. The real minimum for any tile is the one the manufacturer states for that product, in your location, with the laps and underlay they specify.
| Covering | Typical minimum pitch | What changes it |
|---|---|---|
| Plain clay or concrete tiles | About 35 degrees, a little less for some machine-made tiles | Hand-made tiles often ask for more, and a longer headlap needs more pitch |
| Interlocking concrete tiles | Commonly about 22.5 degrees | Some profiles go lower with extra underlay detail and a longer headlap |
| Pantiles and curved clay | Commonly about 22 to 30 degrees | Profile, fixings and exposure |
| Natural slate | Commonly about 20 to 25 degrees | Slate size, headlap and how exposed the site is |
The pattern holds up: small, individually hung tiles need steeper roofs than large interlocking ones, because they rely on overlap alone. Interlocking tiles add a side lock that resists wind-driven water crossing between neighbours, so they can go lower. If you are weighing those two formats, our guide to plain tiles and interlocking tiles sets out the difference.
Headlap, underlay and what they can and cannot fix
Where the pitch is near the minimum, the roof leans harder on its other layers. A longer headlap pulls the tile tails further apart, but it costs extra tiles and lengthens the gauge (the batten spacing). The underlay becomes more important, because on a shallow roof some water will always get under the tiles and the membrane has to carry it to the eaves. Laps must be taped or sealed as the manufacturer directs, the underlay must run into the gutter or an eaves tray, and any sag must be removed. Our note on sagging roof felt shows why a droop in the membrane matters.
The standard that governs the fixing of tiles also sets minimum headlaps by pitch. BS 5534 covers tiled roofs, and our explainer on BS 5534 and tile fixing shows how those rules connect. The point is simple: you can adjust headlap and underlay detail within the manufacturer's limits, but you cannot extend the limits themselves. A tile rated for 35 degrees is not made safe at 25 by adding a few more millimetres of lap.
Roofs that are too shallow for the covering
This is the practical problem we see. A homeowner buys tiles for a roof that looks steep enough, or a builder reuses a covering from another job, and the finished roof sits below the rating. A few situations come up repeatedly.
- Extensions and dormers. A lean-to extension against a two-storey wall often has a shallow pitch, because it must stay below an upstairs window sill. The roof may be too low for plain tiles and can need a lower-pitch interlocking tile, a slate rated for the pitch, or a different covering altogether.
- Porches and canopies. These are small and cheap to cover, and they are often the shallowest roofs on the house.
- Roofs that have sagged. An old timber roof can lose pitch where the rafters have bowed, so the slope is shallower in the middle than the design intended.
- Swapping coverings. Going from a low-pitch-rated concrete tile to a plain clay tile on the same roof can put you below the new tile's minimum.
When the pitch is too low, the options are to choose a covering rated for it, to raise the pitch by altering the structure, or to use a different roof type such as a flat roof system on a low slope. Raising a pitch is a structural job with its own questions about height, planning and neighbours. If the shape or height of the roof will change, ask the local planning authority before any timber is cut. Our answer on converting a flat roof to a pitched one raises the same points.
Where two pitches meet
Few houses have one simple slope. Hips, valleys, dormer cheeks and porch roofs all create junctions where one pitch runs into another, and the minimum has to be met on every plane. A valley is the place where this bites hardest. The two slopes feed water into a channel, and the water arrives faster and in greater volume than it does on an open slope. A hip, by contrast, can have a slightly shallower effective pitch along its rafter than the main slopes have. If a house has a rear extension with a low roof tied into a steeper main roof, the extension is the part that decides which tile is safe, and the junction between the two is where leaks start.
That is why we look at every plane before we advise. A roof can be above the minimum on the front slope and below it on a small dormer or a rear lean-to, and a single covering for the whole house then fails on the weakest plane. Our guide to roof valleys explains how the channel itself should be formed and lined.
Pitch and the weight of the covering
Choosing a tile for a low roof also means checking what the structure can bear. Heavy clay or concrete tiles load the rafters, and a covering that is much heavier than the one it replaces may need the structure strengthened. A new covering 15% or more above the weight of the old one is a matter for building control. Our guide to concrete and clay tiles covers the weight side, and the answer on how heavy roof tiles are lists the points to check.
Exposure, wind and where you live
A minimum pitch is not a fixed number across the country. A house on an open ridge, or one close to the top of a hill, takes more wind-driven rain than one tucked among other buildings. The data sheets recognise this and often shift the limits or ask for a longer headlap in exposed locations. North Hampshire has both kinds of site: sheltered new estates on the edge of Basingstoke, and open downland where the wind comes straight across the fields. When we survey, we look at where the house sits as well as how the roof is built.
Tree cover matters too. Branches shade a roof and keep it damp, so moss builds up in the laps. Moss holds water against the tile and, on a roof near its minimum pitch, it can push water uphill. Keeping laps clear is part of keeping a low roof dry. Our page on cleaning slate, clay and concrete roofs explains how that is done without damaging the tiles.
How we check pitch before we quote
If you are asking us about new tiles, we measure the pitch in the loft or on the roof first, photograph the rafters and the existing covering, and compare the result with the data for the tile you have in mind. If it does not comply, we tell you plainly and show the alternatives that do. If an existing roof is leaking in wind and the pitch looks marginal, we trace whether the cause is pitch, headlap, underlay or fixings, because the fix differs. Our written quote follows the survey and sets out the covering, the headlap and the underlay, and the finished work carries our ten-year written guarantee on workmanship. For more on the wider service, see pitched roofing in Basingstoke.
