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Interlocking Tile Fixing - Quality and Quantity by Chris Thomas

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Depending upon the type of building that is having a new roof, and who is placing the roofing contract, so the information provided for tender/pricing can vary enormously from nothing to paperwork overload.
In most instances one very important piece of information that is not provided by the specifier, and left to the roofing contractor to determine, is the Tile Fixing Specification
Cost estimating
It can be assumed that each contractor will only win about 20-25% of the projects that they quote for, and therefore unlikely to spend valuable time obtaining accurate fixing specification for the 75-80% that are lost, unless the size of the project or its exposed location warrants it. Once a contract has been won it is rare for a fixing specification to be obtained unless there is some doubt about the facts contained within the tender/quotation. In reality this means that a high percentage of tiled roofs have been fixed to an unsafe/incorrect fixing specification.
Whilst estimating the cost of an element of roofing work is not a science, neither is it an art, it is a calculated guess. The guess involves calculating the quantity of materials, plant, labour and administration, placing a cost against each, adding a percentage profit and tax to give an overall cost. Where the project is very competative each contractor will be trying to find the quickest/cheapest solution to gain an advantage over the competition. Every item on site is questioned and reduced to the minimum. Unfortunately in most cases the result of this process is the cheapest possible materials fixed with less tile fixings than is safe to resist hurricane force winds that can occur at least once in any 50-year period. Unless an accurate fixing specification is obtained prior to tendering, it is impossible to place an accurate cost on the fixing element of the tiles for both material and labour.
Once on site the fixing specification used for quotation purposes may need to be justified. To do this can result in fraudulent data manipulation. To obtain a tile fixing specification to comply with BS5534, The code of practice for slates and tiles: Design, approx. twenty pieces of data are required. While some data is more critical than others, and some data can not be altered, it is possible to obtain a wide range of fixing specifications for one location. Where roofing failures have gone to court, compliance with fixing specifications can be a critical factor. The suitability of the fixing specification is directly related to the input data for the calculation. The person or company that provided the 20 pieces of data to allow the manufacturer to provide a fixing specification is therefore critical and will make the provider of that data liable if it is considered to be incorrect.
Fixing options
If we look at the typical fixing specifications for slates and tiles; for double lap centre nailed slates there are only two specifications, all slates must be twice nailed, or twice nailed and tail riveted. Whilst the nail diameter and length can change, this is the same for all tiles and slates.
With double lap plain tiles, there are five recognized fixing specifications. In each case the top and bottom two courses, and the end tiles in every row must be twice nailed. Whilst in the remaining area the tiles can be either twice nailed every course, 2nd, 3rd, 4th or 5th course nailed.
With lightweight resin slates there may be only one fixing specification stipulated by the manufacturer. But with interlocking tiles there are thousands of fixing specifications. Deciding which is correct for the roof in question requires scientific knowledge of the components and familiarity with British Standard 5534. To determine the safest minimum fixing specification for interlocking tiles requires a long winded set of calculations and some data from the manufacturer (tile clip resistance figures).
To try and guess the results of the fixing calculation is almost impossible. The only way of safely guessing a fixing specification is to use a maximum fixing specification or one that is almost as onerous.
Picture 1 The left-hand verge slates should have been clipped with a verge clip or covered with a lead cover flashing, which acts as a continuous verge clip.

The wind suction generated over the parapet, lifted and snapped the loose edge slates, which in turn released the next slate in a progressive failure.


Having obtained an accurate minimum fixing specification from the manufacturers, it is allowable to enhance the fixing specification, but it should never be reduced. A plain tile specification is not applicable to an interlocking tile due to an interlocking tile being lapped along two edges, while plain tiles are only lapped along one edge. The amount of the lap, the size of the tile, its relative weight, and being laid straight or broken bond, all make a difference.
To save cost product substitution is often used. The substitution can be the tiles themselves or the fixing clips. Whilst two tiles may look alike, their weight may be different, and the clip used may be stronger or weaker. Using a tile clip from one manufacturer with another's tile may work, but will automatically invalidate the fixing specification provided.
Picture 4 Marley Modern Verge clips used with a Redland Mini Stonewold. While the roof tiles are similar the clips are very different.

Note how the middle clip does not secure the tile.

The interpretation of the fixing specification on site can be very complicated. In most instances the specification is provided as a verbal description, which needs to be transposed into a pattern on the three dimensional roof. Where there are projections clarification may be needed. In theory any roof slope can be divided into approx. ten zones, each with different load conditions. Most fixing specifications simplify this down to between one and three zones, by taking the worst case for a group. To try and simplify still further requires an understanding of the issues and an enhancement of the fixing specification.
For example; the calculated minimum fixing specification for a roof may require the edge tiles to be clipped, the three tiles next to the edge tile to be clipped and the remaining tiles twice nailed. To simplify this specification it is acceptable to clip every tile, but not nail every tile. In this example an edge tile is the eaves row, top row, the verge tiles, and the end tiles at a side abutment, hip or valley. At 45 degrees and above, all interlocking tiles must be fully nailed and may also need to be clipped. In this case once nailing a tile with two nail holes is not adequate, neither is changing from nails to clips, as at steeper rafter pitches the nail is used to hold the tile on the batten, which a clip may not be capable of doing.
Picture 3 With the rafter pitch over 45 degrees, all the tiles on this roof should have been twice nailed.

The eaves course was once nailed and the second course not nailed at all.

In a perfect world the specifier should provide the fixing specification to ensure a level playing field for the roofing contractors to price against, without jeopardizing the security of the roof. The fixing specification should be obtained from the tile manufacturer using accurate data from the specifier. On site the fixing specification should be followed too the letter, and in that way a wind related roofing failure should be avoided, and if it does happen, it is less likely to be the liability of the roofing contractor.
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