Performance and Appearance

When we quote minimum pitch, what does it mean?

The pitches we quote in our literature and on our website always refer to the pitch of the rafter, and not the pitch of the tile. When laid, tiles sit at a slightly reduced angle as they have to overlap with the ones below. This can be as much as 5-8°, and is higher for double lapped products such as plain tiles. We recommend that when using pitch to describe the angle of the roof, you always refer to the rafter pitch.


What is efflorescence?

Efflorescence or “lime bloom” is a natural phenomenon resulting from the reaction between cement and water which produces calcium hydroxide 'lime'. In certain conditions the lime will move through the concrete to the surface of the tile where it reacts with carbon dioxide in the atmosphere to produce a deposit of calcium carbonate crystals causing white patches. Although not attractive it is not harmful and the effect is temporary. Wind and rain will gradually remove the deposits leaving the true colour of the tile. This process can take between 12 to 36 months depending on weather conditions but once the efflorescence disappears it will not come back. The removal process can be hastened by treating the tiles with a dilute (5%) solution of hydrochloric acid. However great care must be taken with access to the roof and the safe application of the acid treatment.


How do the European and older British Standards differ for concrete tiles?

The European Standards for concrete tiles and fittings came into effect in August 1995 replacing the old British Standards. The Standards BS EN 490:1994 and BS EN 491:1994 for concrete roofing tiles and fittings were quite different to the old British Standards they replaced. There is a popular misconception that European Standards are not as stringent as existing British Standards. In the case of concrete roof tiles this is certainly not true. Where the old British Standard was prescriptive laying down the correct dimensions for a tile, the BS EN standard focuses more on the product's performance. The notion that there is no point having a tile that looks good if it leaks or cracks is fundamental to the new European Standard. British concrete tiles now have to meet strict performance criteria in terms of strength, impermeability to rainwater, and freeze-thaw resistance if they are to comply with the European standard. The European product and test standards for concrete tiles and fittings are revised approximately every five years with the current versions being BS EN 490: 2011 and BS EN 491: 2011 respectively.


Does rafter length affect minimum pitch for interlocking slates?

Flat interlocking slates such as Cambrian Slate, Richmond 10, Saxon 10, Landmark 10 Slate, MockBond Richmond 10, Stonewold II, Mini Stonewold and MockBond Mini Stonewold require their minimum roof pitches to be increased when the rafter length exceeds 10m. Wind tunnel testing has shown that water flows can be so great on very large roofs that the side interlocks of flat tiles can become overloaded. It is recommended to contact Redland Technical Solutions for further advice if this applies on your project.


Do the markings on the underside of tiles indicate name and colour?

Concrete interlocking tiles complying with the British Standard have since the mid 1950s been required to have a distinguishing mark to identify their origin. Compliance with the code is referred to on the underside of the tile. A Registered Design Number usually starting “RegDes” followed by a six or seven figure number is also normally included. This can be checked with us, the Design Registry Department of the Patents Office or the Science Reference Library in Holborn, London.

Redland tiles include either our name or the product name on the underside. There will however be no reference to its colour or surface finish as the moulds used are for all colours of the same profile. The original colour of the tile can be obtained by looking at the headlap area of the tile. If the tile is through coloured the colour on the underside of the tile should be checked as this will not have been affected by weathering. However new tiles will take years to weather down to the colour of the existing roof surface. If only a few tiles are required it may be advisable to use salvage tiles that have already weathered on a roof to match more closely. For a large roof area it may be better to strip an elevation and re-tile in new tiles to ensure a consistent colour match and use the salvaged tiles on another part of the building. Pre-1970s roofs may use Imperial tile sizes that are not compatible with their metric equivalents but this should be made apparent by a change in the Registered Design Number of the tile.


How do plain, profiled and interlocking tiles differ?

Plain and profiled refers to the appearance of the tile. Plain tiles are small flat tiles while profiled tiles as their name suggests have a shape or contour. A pantile is an example of a profiled tile. However these are only two of the three possible categories of roof tile, the third being natural slate or slate appearance tiles. Interlocking is a description of the way some tiles fit together. Plain tiles are made to the traditional design dating back to the Middle Ages with no added extras apart from nail holes, a slight camber and nib on the back where the tile sits on the batten.

Interlocking tiles are generally much larger - hence they can be laid much more quickly - and have a grooved 'interlock' down either side. The interlock allows the tile to be simply connected to the adjoining tiles forming a weathertight joint. There is no difference between the quality of the tiles. However the smaller size of the plain tiles, along with their lack of an interlock, makes them a better choice for roofs with intricate detailing and for curved roofs or those with turrets. Interlocking tiles are larger than plain tiles and so better for simple roof shapes and lower pitches. Plain tiles in general should never be used on pitches below 35°, albeit certain specific proprietary plain tiles may be used to 30° rafter pitch when other precautions are taken such as specific underlays used below the tiles. Consult Redland Technical Solutions for more information.


What is the effect of laying a tile below its minimum pitch?

Under severe wind driven rain conditions, a tile laid below the minimum recommended rafter pitch will allow rain to be blown over the headlap or through the side laps of the tiles. Once this occurs the underlay is the only thing preventing water from reaching the rest of the building. If water is allowed to regularly flow down the underlay it will begin to decay where the flow is greatest, near the eaves, and allow water into the building. How long this construction will last depends on the type of underlay, the number of layers and the quantity of water flowing down the underlay, but could be anything between five and thirty years.

For a boarded roof, once the underlay has failed, it will cause the boarding below to rot. The only solution is to strip the tiles, battens, underlay and boarding and replace it with the same labour cost as a total reroof. When the tiles are laid below the minimum recommended rafter pitch, the normal manufacturer’s tile guarantee will also be invalidated, so unless the underlay or a functional weatherproof sub-roof system below the tiles has a guarantee the roof is unlikely to be covered by any guarantee.


What is the difference between weathertightness and durability?

Durability is defined as the ability to last, maintain its primary function and resist wear and tear. With roof tiles this means the individual roofing product will withstand deterioration caused by frost, acid rain, ultra-violet light and heat for a specified period of time. However durability relates only to the composition of the tile or other roof covering material.

In order to keep the rain out of a building under all weather conditions the roof system as a whole must be weathertight. This means that the arrangement of tiles and roofing underlay as a system must be able to resist the ingress of wind driven and deluge rainfall. This system must also resist the suction effect of high winds that would lift the tile and allow rain in through the headlap.

However weathertightness should not be confused with watertightness which means that if the roof were turned into a receptacle it would hold water which clearly a tiled roof would not. A roof is not hermetically sealed for good reason since allowing some air movement through roofs is a good thing as it helps to allow the building to breathe. Consequently, small quantities of water can penetrate through the junctions of tiles under certain driving rain conditions and this normally is not a problem so long as the roofing underlay is installed properly. Other situations also give rise to water presence under the tiles e.g. when the dewpoint corresponds with the atmospheric conditions of the batten cavity, temporary condensation may form. Also, wind driven snow can be blown in through the smallest of joints and will eventually thaw leaving water in the batten cavity, which will run down the underlay and into the gutter.