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Home Contractor's CornerAn Inspector Calls A good fit: securing your interlocking roof project

A good fit: securing your interlocking roof project

by Matt Downs

In our regular monthly column – ‘An Inspector calls’ – Total Contractor has teamed up with the experts in pitched and flat roofing solutions at BMI UK & Ireland to help you avoid the common pitfalls that can often cost you both time and money, and ultimately help you achieve roofing success.

This month the Inspector unlocks the secret of getting interlocking tiles to look good on your roofing project.

When it comes to interlocking concrete roof tiles, you have to wonder how anyone can get it wrong. The nibs hang on the battens, the nail goes through the hole and each tile interlocks with the one next to it.

Failing to plan is planning to fail – that is why it is so important that before any timber or a single tile is laid, you need to strike out the roof to guide the laying of the tiles

It’s that simple – isn’t it? Unfortunately no, it’s not. Like many skilled trades, what appears to be simple is often far more complex than it seems and installing interlocking tiles falls into that camp.

While the tile itself may be simple to understand, the roof is not. For starters, roofs are very rarely straight; the battens certainly won’t be and there is a very good chance that the designer did not come up with a roof that was purpose-made to accommodate the linear cover of the chosen tile.

Striking out

In all things roofing, as we’ve said many times in this column, failing to plan is planning to fail. And that is why it is so important that before any timber or a single tile is laid, you need to strike out the roof to guide the laying of the tiles.

The chalk line is the roofer’s oldest friend here, as it places markings on the roof that allow us to ensure both battens and tiles are laid out to fit with the roof area and each other.

Firstly, marking a chalk line for the battens highlights bends in the batten timber. After years and years of installing thousands and thousands of metres of graded timber, of every colour, one can only be convinced of one thing: there is no such thing as a straight batten. If one mistakenly trusts the timber and fails to strike those lines, the tile courses will inevitably resemble a banana rather than a straight line.

Secondly, and with battens in place, one measures and strikes out the vertical lines down the roof. This is generally in 900mm centres to allow three tiles per column. Most large format concrete tiles have a linear cover of 300mm: that is to say, the tile width minus the interlock. By striking these lines, it is easier to keep an eye on where the tiles should be sitting in relation to the roof space and ensure they do not wander off along the batten.

Each tile has around 3mm shunt. That means they can be moved together or apart 3mm without affecting the bond of the interlock. This shunt is necessary to make the tiles fit the roof space. Yet it can also be the enemy of best because if not watched, and each tile is out by 3mm, by the time we get to the other end of the roof, it will be off by half-a-tile either way.

Striking vertical lines allows tiles that are too spread – or bunched, like those in the image  – to be spotted immediately.

Top tip

Pay attention to the verge detail, especially if using a dry verge system. The batten overhang varies and can have a significant effect on the tile positioning. Generally, the tile should sit around 5mm in from the end of the batten for a dry verge, but read the manufacturer’s instructions for their particular system: and do not deviate!

So yes: interlocking tiles are simple, but the roof is not. When installing them, the work is in the preparation and cutting corners on the measurements and markings will result in nothing less than poorly fitted tiles.

www.bmigroup.com/uk

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