03 April 2018

What lap treatment is required for fibre cement sheeting?

When installing profiled fibre cement roof sheeting, the type of lap and sealing treatment to be used, either sealed or unsealed, depends both on the expected wind loading conditions, which vary in different regions of the UK, as well as the roof pitch. Minimum end laps for fibre cement roof sheeting range  are either 150mm  or 300mm. Sheets should always be installed with 2 fasteners per sheet width per purlin or rail.

Determining weather exposure of a site

When specifying profiled sheeting, determining the wind loading and exposure of the site is critical to ensuring the optimal sealing and fixing of the sheets. 

Firstly, it must be established whether the site is classified as being subject to sheltered to moderate weather exposure or moderate to severe exposure. Sites situated in the northern Highlands of Scotland or outer Hebridean Islands are subject to special requirements due to the extremely high wind speeds in these areas. 

Please refer to the UK map in BS 8219 for more information about these three categories: 
● sheltered to moderate: less than 56.5 litres/m2 wind driven rain per spell
● moderate to severe: equal to or greater than 56.5 litres/m2 wind driven rain per spell
● Highlands and Islands: consult technical advice

When buildings stand above their surroundings or are situated in open country with no windbreaks, including sites on or near the coast, or on hill tops, they must be considered subject to severe exposure.

Please refer to BRE Digest 127 ‘An Index to Exposure to Driving Rain’, or the BSI Draft for Development DD93, taking account of the recommendations regarding localised effecton high buildings, on buildings of any height, on hill slopes or hill tops, in coastal districts, or in other areas where higher exposure gradings are likely.

Calculating centres of support for fibre cement roof sheeting

When using Marley Eternit’s Profile 6 fibre cement roof sheeting the purlin supports should be spaced at maximum centres of 1375mm for a superimposed loading up to 1.89kN/m2.

For Profile 6 wall sheeting, support centres (i.e. rail spacing) should not exceed 1825mm for a superimposed load of up to 1.07kN/m.

For Profile 3 roof sheeting, the purlins should be spaced at 925mm maximum centres for loadings up to 1.8kN/m2. 

Where wind loadings exceed this level, please contact our technical support team, as reduced purlin spacing will be required.

Wind loading should be calculated in accordance with BS EN 1991 Eurocode 1: Actions on structures – Part 1-4: General actions – Wind Actions.

Lap and sealing requirements for sheltered to moderate sites

For sheltered to moderate sites, exposed to less than 56.5 l/m2 of wind-driven rain per spell, the minimum lap requirements for profiled sheeting are as follows:

Roof pitch End lap (mm) End lap treatment Side lap treatment
22.5° and over 150mm Unsealed Unsealed
15° and over 300mm Unsealed Unsealed
15° and over 150mm Sealed Unsealed
10° and over 150mm Sealed Sealed
5° to 10° * 300mm Double sealed Sealed

The minimum pitch for Profile 6 roof sheeting is 5°. Where slopes are between 5° and 10° the maximum slope length should be 15m with double-sealed end laps and single-sealed side laps. The minimum recommended pitch for Profile 3 is 5º.

Lap and sealing requirements for moderate to severe sites

For sites exposed to moderate to severe levels of wind-driven rain (over 56.5 l/m2 per spell), the minimum lap requirements for profiled roof sheeting are:

Roof pitch                      End lap (mm)      End lap treatment         Side lap treatment
25° and over                   150mm                 Unsealed                         Unsealed
17.5° and over                150mm                 Sealed                             Unsealed
15° and over                   150mm                 Sealed                             Sealed
10° and over                   300mm                 Sealed                             Sealed
5° to 10° *                       300mm                 Double sealed                 Sealed

On roofs over 10° pitch, where parapets might allow the build-up of snow, 300mm double-sealed end laps and single seal side laps are recommended. 

For more information, please download our Profiled Sheeting Design Guide, paying particular attention to pages 28-29.

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