According to BS 5534, 'the nails used to fix battens to rafters should usually have a diameter of at least 3.35mm. The nails should provide a minimum of 40mm penetration into the rafter, so a nail length of 65mm is generally recommended'. Obviously this requirement is achieved, for the counter battens, using the method you suggest Chris. However my question is if we try to comply with BS 5534 the batten nails fixing the slate batten to the counter batten will penetrate both battens through the membrane and into the wood fibre sarking board by 15mm. Apart from the hundreds of puncture holes in the membrane the end 15mm of the batten nail will only be in wood fibre sarking board and therefore offers no pull out resistance. Is this a problem? if so whats the solution?   

I would welcome anyone's thoughts

Tony

Hi Tony

Thanks for your message. We would usually suggest using screws to hold the counter battens on and they need to penetrate the rafters by 40mm as a minimum. You'd therefore need to accommodate the thickness of the batten and the insulation to achieve this.

In terms of punctures, we do have nail sealing tape that can be used to properly seal under the battens but generally the compression of the batten against the surface of the wood fibre boards is enough to seal it from water tracking sideways under the counter-battens and through the breathable membrane.

Hope that helps.

Hi Tony

Yes, the slate batten nails may puncture the membrane but as any punctures will be under the counter-battens this is not a problem. The nails that hold the slates are much shorter and barely protrude from the underside of the battens.

I would still recommend using the breathable membrane over the wood fibre board as an additional layer of protection and also to appease Building Control, who tend not to understand a construction without it.