A roof with too little slope lets water take its time getting down, and water that takes its time ends up getting in. Each tile model has a minimum slope, which rises with the length of the roof plane, with exposure to wind and with the part of the country. This guide shows the values published by one Portuguese manufacturer, explains how to read them, what happens below them and how to measure the slope without climbing onto the roof. We do not name firms or give prices.
Updated on 3 November 2026. General technical information, not a substitute for the manufacturer’s data sheet or a technician’s design.
The essentials
- There is no single value. The minimum slope depends on the tile model, the climate zone (1, 2 or 3), the exposure (sheltered, normal or exposed) and the length of the roof plane.
- The numbers in this guide come from one manufacturer. They are from Cerâmica Torreense. The APICER and CTCV guide has no table, and other manufacturers may give different values.
- Below the minimum, the manufacturer requires extra measures to keep the roof watertight. Without them, rain driven by wind can get in under the tile.
- It is measured as a percentage: rise divided by the horizontal distance, times 100. Do it from the loft or on a photograph, not on top of the roof.
- When replacing a roof, ask before changing the slope. Changing the shape of a roof may need a procedure with the town hall (Câmara).
How slope is read
Slope in percent = rise ÷ horizontal distance × 100. A roof at 30% rises 30 centimetres for every metre horizontally, about 17 degrees. The APICER and CTCV guide calls a «pitched roof» one above 8% (about 4.6 degrees); below that it is a flat roof, with other rules. The steeper the roof, the larger its real area: our area calculator does that sum.
The values of one Portuguese manufacturer
Cerâmica Torreense publishes a table of «recommended minimum slopes» for each model. This is the one for the lusa tile (the Milénio has the same values), in percent:
| Roof plane | Exposure | Zone 1 | Zone 2 | Zone 3 |
|---|---|---|---|---|
| up to 6 m | sheltered | 21 | 25 | 30 |
| up to 6 m | normal | 23 | 28 | 33 |
| up to 6 m | exposed | 27 | 33 | 38 |
| 6 to 10 m | sheltered | 23 | 28 | 33 |
| 6 to 10 m | normal | 26 | 31 | 37 |
| 6 to 10 m | exposed | 30 | 36 | 42 |
| over 10 m | sheltered | 25 | 30 | 36 |
| over 10 m | normal | 28 | 34 | 40 |
| over 10 m | exposed | 33 | 39 | 46 |
For the manufacturer’s other models, the same table gives, in normal exposure, for a plane of 6 to 10 m (zones 1, 2 and 3): marselha 36, 40 and 45; canudo 44, 50 and 55. Across all situations, the marselha runs from 29 to 57 and the canudo from 36 to 70. The lusa runs from 21 to 46.
How to read it:
- Roof plane: the table calls it «pendente». We read it as the length of the roof plane, from eaves to ridge. Confirm with the manufacturer.
- Exposure: the manufacturer defines sheltered (an area surrounded by high ground, protected from winds), normal (flat ground, with slight undulations) and exposed (coast up to 5 km from the sea, cliffs, islands, narrow peninsulas, estuaries, bays, mountain regions and plateaus; also very tall buildings).
- Zone: it comes from a manufacturer’s map that combines wind and rainfall. Your municipality’s zone is on that map, not on this page.
- With a lining, the value may drop. The tables apply to roofs without a lining (open rafters). With a lining, the manufacturer says it can be reduced by 1/7, citing a French standard (DTU 40.21). For example, 33% would become about 28% (our arithmetic).
What happens if the slope is low
- The manufacturer requires reinforcement. With a slope below the recommended one, Torreense says it is mandatory to use extra elements that keep the roof watertight. It also says that if its specifications are not followed, it is not responsible if the tile fails in its basic function.
- Rain with wind. The same manufacturer explains that the worst case for watertightness is rain and wind together, which can make water move up the roof.
- The tile is not enough. The APICER and CTCV guide reminds us that a tile’s waterproofing does not by itself guarantee a watertight roof: slope and correct fitting count. Its list of defects in ceramic tile roofs includes «excessive or insufficient slope» and «insufficient drainage (low slope and gutter size)», both with their origin in design.
- Signs (our conclusion, no source): ceiling stains only with rain and wind, tiles always damp or mossy, standing water in gutters. See the causes of roof leaks and the guide on damp at home.
And if it is very steep?
The same guide says that with a steep slope or strong wind, the canudo tile needs reinforced fixing, because it can slide. Snow calls for steeper slopes, to avoid overloading the structure. And a steeper slope helps against frost. More on climate in the guide to tiles by climate.
How to measure without climbing up
These steps are ours, not from a source:
- In the loft: on a rafter (the sloping beam), mark 100 cm horizontally with a spirit level and measure the rise vertically. The centimetres are the percentage. A phone inclinometer app, resting on the rafter, gives degrees.
- From the ground: if you have a triangular wall (gable), photograph it from far away and straight on, holding the phone level. The height of the triangle divided by half its width gives an approximate value.
- Only if the loft floor is safe. Do not step on ceiling linings.
If the measurement falls below the table, talk to a technician. Do not climb up: see work at height.
What to ask the contractor
- What is the measured slope, and what is the model’s minimum for my zone, exposure and roof plane?
- If it falls below, what extra watertightness measures will be added, and where are they in the quote?
- Does the slope change? If so, has the Câmara been consulted? (see the works permit guide)
- Do the manufacturer’s and the works warranty stay valid? (see the works warranty)
What we could not confirm
- Other manufacturers’ tables. We looked for a second Portuguese manufacturer’s and the page would not open. Values may differ.
- The zone of each municipality. The map is an image, with no list of municipalities.
- Concrete, slate, metal, sandwich and fibre-cement. We did not read dated data sheets.
- A legal minimum slope. We did not find one.
- The exact meaning of «pendente» in the table, and the table’s date: the page does not show it.
Sources (read on 3 November 2026)
- Cerâmica Torreense, lusa tile, technical specifications, marselha tile, canudo tile and Milénio tile (tables, lining note, warranty).
- Cerâmica Torreense, Slope (Inclinação) (pitched roof above 8%, exposure, extra measures) and the glossary of the same site (rain with wind).
- APICER and CTCV, Guia para Projeto e Aplicação de Telhas Cerâmicas, 2018 (defect table, canudo, frost, snow, 8% convention). It has no minimum slope table.