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The Permavent Easy Slate system rips up the rule book when it comes to roof pitch and slate laps. A unique patented system in the form of a strip that fits under each slate joint, prevents water from creeping sideways. Easy Slate vastly improves the performance of the slates, enabling roof pitches as low as 12 degrees. The slates are still installed in a double lapped pattern, so there is no discernible difference in appearance on the roof.
Traditional double lapped slates have been keeping buildings weathertight for 100’s of years, so the relationships between slate length and width, headlap, side lap and roof pitch are well understood. BS 5534: the British Standard Code of practice for slating and tiling, provides the rules by which we can work out the minimum headlap, side lap and roof pitch for any roof anywhere in the UK.
As rainwater runs down a slate roof, it runs into the gaps between each slate, with a slate below each gap to catch the water and drain it to the next course of slates below. Water is also drawn between the slates through capillary action and can run downward, sideways and even upwards. Gravity acts on the water, so it is not drawn as far by capillary on a steep pitch as it is on a lesser pitch, consequently steeper pitched roofs are more efficient in keeping the water out.
To minimise leakage through the slating, there must be limitations, dependent on the size of slates, on roof pitch and rafter length. But even so, during a roof’s service life, which can be 40 to 60+ years, there will be weather events in which water will penetrate the slating. If this happens with any frequency, there is a risk of long-term damage to the roof sub-structure, battens and fixings. .
Easy Slate increases the gap slightly between each slate, not only making water creep through capillary action far less likely, but also encouraging better air flow between the outside atmosphere and the batten cavity. This improves the efficiency of the vapour or air-permeable underlay in removing unwanted condensation from the roof structure, something particularly important for slates that have smooth surfaces, for example, fibre cement slates. Also, the higher airflow through the slating increases the security of the slates against wind uplift as a larger proportion of the wind load is then borne by the underlay.