Technical FAQ – interlocking slates

Tom Woodhouse, site services manager at Marley, answers contractor questions about interlocking slates

Offering a middle ground between the top and bottom of the roofing price range, thin leading edge, interlocking concrete slates are a quick and affordable way to create a stunning slate effect roof. They can be used for both new and refurbishment projects and, unlike traditional slates, they don’t require specialist skills and can be installed by any roofer.

Here we answer frequently asked questions about interlocking slates:

1. How much do they cost?
As a cost-effective alternative to both natural and fibre cement slates, interlocking concrete slates are the most economical way of achieving a slate aesthetic. If you take material costs alone, natural slate can cost between £70 and £100 per m2, whereas a concrete slate like Edgemere can be as little as a third to half the price. This is partly because of the high coverage rates of Edgemere, with just 9.7 tiles per m2, compared to around 13.4 per m2 for fibre cement slates and as high as 18 per m2 for natural slate.

Concrete slates are also an affordable upgrade from standard slate interlocking concrete tiles. For example, for a premium look interlocking concrete slate, like our Edgemere product, the difference can be as little as 3% extra on the total cost of a standard domestic roof.

2. What minimum pitch can they be used at?
The minimum pitch will vary according to which type of interlocking slate you buy but usually they can be used at a lower pitch than natural slate. For example, our Edgemere slates can be used down to a low minimum pitch of 17.5 degrees. This means they have less limitations than natural slate and have the versatility to be used on a wide range of projects in any geographical area (when fixed according to BS 5534). They can also be installed using standardised and uniform supports, which means that weight loading is more predictable.

3. How can I make a concrete slate roof look more like natural slate?
Some thin leading-edge concrete slates are able to replicate the look of slate very effectively, but in order to choose the right one, you need to consider colour, thickness, format, texture and accessories. For example, our Edgemere range is just 18mm thick and the Smooth Grey and Anthracite colours have been designed to more closely resemble natural slate. We also offer a textured slate, Riven Edgemere, which has a variegated surface for a closer match to natural slate, and is often accepted by planning departments.

Also think about the slate aesthetic you want to create – if you want to replicate the look of a small format slate, then our Duo Edgemere has a mock bond down the centre of the tile. You could also consider adding accessories to create a traditional slate aesthetic, for example, contrasting red coloured concrete ridges to give the likeness of clay ridges installed with natural slate.

4. Why do the Edgemere slates need to be installed with staggered interlocks (broken bonded)?
Not only is this aesthetically more appealing (in line with a slate like appearance which the tiles are designed to simulate), but it aids the performance of the tiles. The broken bond allows any water draining into the interlock on one tile to disperse safely onto a flat surface of the tile below. Duo Edgemere are laid to a ¾ bond to achieve this broken bond pattern.

Edgemere slates should never be installed with aligning interlocks (straight bond) as this would run the risk of rainwater draining directly from one interlocking channel into another and possible flooding of the interlocks, leading to water ingress.

5. How can I install them to BS 5534 more quickly?
Since BS 5534 was revised in 2014, all interlocking tiles are subject to more stringent fixing specifications, which means there is an element of clipping on all roofs laid in single lap tiles.

Traditionally, these clips and nails come in separate bags and need threading together prior to installation, which is fiddly and adds extra labour time to the job. To save time, you can use the SoloFix one-piece clip and nail. It’s comparable in price to a standard aluminium clip, but can save as much as 30% on roof clipping time.

All of the Edgemere slates are also available to purchase as part of a complete Marley roof system, including battens, underlay, fixings and accessories, all backed up by a 15-year guarantee.