Water Permeability of Roof Tiles

We are often asked questions – and sometimes receive complaints – about the water permeability of roof tiles. In this article we answer the most common questions and help avoid the installation mistakes that lead to damp problems in tiled roofs.

1. Introduction

Our company places top priority on the quality of its products. It is a management commitment and standing policy to supply bricks and roof tiles of excellent quality, made from the best materials and tested in our fully equipped laboratories.

The specifications of roof tiles for the European market are set out in standard EN 1304, which refers to a series of standards for carrying out the tests. We apply factory production control based on ELOT EN 1304 and carry out all the tests it describes in our own laboratory, with fully trained scientific staff. The impermeability test is described in standard EN 539-1, which specifies two methods (Method 1, Method 2).

After testing, tiles are classified into two categories, 1 and 2. The manufacturer must declare which method was used, and results are assessed on that basis.

2. Impermeability – Standard EN 539-1

The standard specifies two test methods for determining the impermeability of clay roofing tiles, which are considered equivalent. Tiles are tested as sampled; if they were already installed on a building, the stresses they have suffered are taken into account when interpreting the results.

Test method 1

It determines the quantity of water passing through the body of the tile within 48 hours, per cm² of surface, under a constant 10 cm water column. Sample size: 10 tiles.

1. PreparationThe tiles are immersed in tap water for 48 hours, dried with hot air until the water has fully evaporated and left for at least 4 hours at room temperature.
2. SpecimensSquare specimens of 45 ± 2 mm × 25 ± 2 mm or round ones of Ø 50 ± 2 mm are cut from the thinnest section. A glass tube of Ø 38 ± 1 mm and 150 ± 2 mm height is fitted on the weather-exposed surface, and the cut surfaces are sealed.
3. 48-hour immersionTube and specimen are placed in water, 1 cm above the exposed surface, on two thin non-porous supports.
4. 10 cm water columnThe tube is filled to 10 cm above the water level of the container and kept constant by a graduated continuous-supply vessel. After 48 hours the volume of water that passed through is recorded, V₁.
5. Evaporation correctionAn identical tube with a non-absorbent glass plate is placed alongside. Its weight difference after 48 hours is the evaporated volume, V₂ (1 g/cm³).
6. CalculationThe impermeability factor IF, in cm³/cm² per day, is calculated with the appropriate formula.

Test method 2 – the one we apply

The Panagiotopoulos S.A. laboratory uses method 2:

Testing the water impermeability of a clay roofing tile or fitting by determining the time elapsed before drops start to fall, after water is applied to the surface of the tile normally exposed to the weather.EN 539-1, Principle of method 2

1. PreparationSample of 10 tiles. Immersion in tap water for 48 hours, drying with hot air and at least 4 hours at room temperature.
2. FrameA frame is fitted on the exposed surface of the tile (excluding parts covered on the roof) and sealed with waterproof material. The tile is supported horizontally, within ±5°.
3. WaterTap water is added continuously inside the frame, to a level of 60 ± 5 mm at the lowest point and 10 ± 5 mm at the highest.
4. MeasurementA device under the specimen, connected to a timer, measures the time until the first drop falls to within ±15 min. Maximum duration 20 hours, at 20 ± 2 °C and 60 ± 5 % humidity, without draughts.
5. ResultThe impermeability coefficient IC is calculated and the tiles are classified according to EN 1304.

In practice, the method 2 categories mean:

All Panagiotopoulos S.A. roof tiles drip only after 4 hours and are classified in the best category of the standard. Most batches complete the maximum test duration (20 hours) without a single drop falling (IC = 0).

The test in our laboratory

After preparation, a metal frame sealed with silicone is fitted to the tiles. Once dry, each specimen is placed on the measuring rig and filled with water to the levels of EN 539-1.

Under the supports there is an electronic board connected to a digital timer. When a drop falls, the water closes the circuit on the board and the timer stops.

3. Tile impermeability vs. roof watertightness

Regarding watertightness, the following distinction should be made:

Parameters that affect the watertightness of a tiled roof:

1. Fit and interlocking* of the tiles
2. Roof pitch
3. Free drainage of water
4. Correct installation of the waterproof underlay and drainage
5. ClimateHeavy rainfall with strong wind.
6. SurroundingsNearby trees and vegetation that can block the drainage paths of rainwater.

These parameters act in combination. The roofer, guided by the supervising engineer, should consider them all together, from the site survey and design through to handover.

*Interlocking, per EN 1304: the design system for joining two adjacent tiles or fittings, usually comprising raised parts (“ribs”) and recessed parts (“grooves”).

Interlocking on the Roman roof tile

A Roman-type tile has folds and geometric features that allow tiles to interlock with each other and be laid on the roof:

1Fixing hole2Transverse interlock (rib)3Transverse interlock (groove)4Longitudinal interlock (rib)5Water drainage rib6Longitudinal water-stop rib7Longitudinal interlock (groove)

Rib-and-groove interlocking lets rainwater run off the flow surfaces. If moisture passes the rib, it is stopped by the longitudinal water-stop rib (6) until it reaches the drainage rib (5) and is led back to the flow surface. Similar interlocking exists on the transverse axis, so that the tiles are laid overlapping.

How Roman roof tiles are laid:

Why a waterproof membrane is needed

With this interlocking and Category 1 tiles, a watertight roof is possible but not guaranteed, because with:

• a low pitch
• heavy rainfall
• wind
• adverse surroundings
• unfavourable design parameters, or a combination of these

the flow surfaces may not be able to carry away all the rain. They become saturated quickly and, by the principle of communicating vessels, water can pass the interlocks and reach the layers beneath the tiles.

Roof tiles are designed to shed rainwater through their overlaps and the pitch. But they have many gaps, so in strong wind water ingress is very likely: the joints loosen under wind pressure while rain is pushed into the gaps.

Pitch and drainage

Correct drainage design is essential: suitable pitches prevent water from pooling and the tile channels from filling quickly in heavy rain.

On a parapet roof, without correct detailing, water soaks the parapets and the collecting gutter, runs back under the tiles and wets the sloping slab or structure. The gutter is built between the roof edge and the parapet, with downpipes at the lowest points.

4. Common installation mistakes

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