Fibre cement slates are becoming more popular due to their affordability combined with aesthetics that replicate the look of natural slates. They are being used in new homes, in public sector applications such as schools and healthcare projects, and in roof replacements or refurbishments.
As their popularity increases, it is important that good quality guidance is available for roofing contractors, covering technical questions such as design and specification, and fixing. Here are some of the things we are frequently asked.
In conservation areas, local authorities have traditionally stipulated that traditional materials, such as natural slate or clay must be used for roofs, both for new build projects and refurbishments where the roof covering is to be replaced. However, many councils will now allow fibre cement to be used instead of natural slate because it gives an authentic natural stone look. Marley Eternit fibre cement slates, such as the Rivendale, Thrutone and Birkdale ranges, have been specified for schools and residential buildings in conservation areas. We can provide technical support and further information to assist with the planning process – contact our technical team for help.
In one project, we worked with housebuilder Cambrian Homes to persuade to local council planning officers to allow fibre cement for a project in Hawkshaw, a green belt conservation area. We were able to demonstrate that fibre cement slates would meet the fixing method required for the site’s exposed location as well as blend with the natural landscape.
For other examples, read our compilation of case studies showing how fibre cement slates have been used in conservation areas. This covers heritage refurbishments, scheduled monuments and new build projects in areas such as London’s Clerkenwell Green.
If you are using standard fibre cement slates vertically, you can typically follow normal roofing design principles. Slates are secured to timber battens set at the appropriate gauge over counter-battens and underlay. This type of installation is suited to fairly small areas, such as the sides and faces of small dormer windows. However, it is possible to use vertical slating on larger surface areas.
Vertigo fibre cement slates are designed specifically for vertical applications and follow a rainscreen cladding principle, being rear ventilated. They are more suitable for large areas of vertical cladding, such as complete building elevations. They can be fixed to battens over supporting counter-battens; or, if they are to be used on walls with external insulation, can be installed directly onto vertical timbers using specialist adjustable brackets.
If you are designing with slates in a vertical application, the need for an underlay, and the type of underlay, will depend on various factors. These can include the type of wall construction (masonry or framed) and the location of the insulation.
BS 5250 ‘Code of practice for control of condensation in buildings’ is the best place to look for guidance on the relevant design considerations.
BS 5534 confirms that a suitable underlay can be used to provide secondary weather resistance and vapour permeability where required. Read more about BS 5534.
Fibre cement slates are between 20.4kg/m² to 21.3kg/m², depending on the size of the slates used and the installed headlap.
Natural slate can vary between 28kg/m² and 42kg/m², again depending on the size of the slates and the headlap. Slate is a natural material and will be supplied in a range of size and thickness, which affects the weight.
Fibre cement slate, however, is machine made in more finely controlled conditions, and is produced to a nominal thickness of 4mm. Marley Eternit fibre cement slates weigh 20.4kg/m² when installed with a 100mm lap or 20.9kg/m² with a 110mm lap.
These quoted weights are typically calculated by comparing 600mm x 300mm slates and 500mm x 250mm slates. Smaller format natural slates can weigh significantly more, making the difference between fibre cement and natural slate even more pronounced.
Fibre cement slates should be fixed using there standard method below so that they can withstand wind suction loadings.
All slates should be nailed at least twice using 30mm x 2.65mm jagged copper nails, as well as being restrained at the bottom edge with a copper disc rivet. The correct rivet should have a 19mm x 2mm stem and a 19mm x 0.5mm base. When fixed correctly, the maximum wind suction loadings that fibre cement slates can withstand are as follows:
However, a fixing specification may not be required if the maximum wind suction loading values will not be exceeded for your particular site and building. Consult a qualified structural engineer during the design stage, who will check the wind suction loadings that are expected to be exerted on the building.