Nail direct to sarking vs batten/counter batten; Hooks vs nails; and slate size.

[Dunc]

I have a couple of questions on slating. They seem well-worn questions but I’m not sure I’ve found any specific answers.

 

Nail direct to sarking vs batten/counter batten; Hooks vs nails; and slate size.

 

Context for this specific roof: This is in Scotland. 45 degree pitched. Hybrid/Warm roof – cellulose between rafters (but no insulation above) the rafter. This needs to “breath”. In-roof solar PV in part of the roof (needs ventilation behind). Roof will have some Velux windows. New, regular sized slates (Cupa 3 heavy or similar).

 

I gather that slating in Scotland is traditionally done by nailing directly to sarking boards (with a membrane between boards and slates). For this warm roof I would need to vent behind the sarking, so will have to stand the sarking off the rafters on battens, with eaves and ridge vents (rafter, counter batten, sarking board, membrane, slate). At the ridge, the membrane would have to be cut to permit ridge ventilation (i.e. the membrane can’t span the apex of the roof). This worries me because if the dry ridge vent fails then water could penetrate behind the membrane at the apex. Is this an unreasonable concern? Any way to mitigate the risk?

 

In a battened roof (rafter, sarking board, low resistance membrane, counter batten, batten, slate) the membrane can cover the apex of the roof so any failure in the dry ridge just runs down the membrane and out at the eaves. Penetration of the membrane is limited to nails holding down the battens. So, what weather conditions does Scotland provide that would make this battened construction less desirable? Wind uplift behind the slates? Is the nail-direct construction just a hangover from using uneven sized slates?

 

Then for fixing the slates, hooks look to be a nice idea for long-term maintenance. Any down sides to hooks? And if I need to go the “nail-direct” route can hooks be used in this context?

 

And finally, various places on t’interweb seem to suggest that smaller slates are more common in Scotland (400x250 vs 500x250 down South). Other than tradition, any reason to use smaller slates (more fixings, more time)?

 

Thanks.

[JohnMo]

Our is insulation between rafter, sarking boards, these dry to leave small ventilation gaps. Breather membrane, then natural slate nailed to sarking boards.

 

No ventilation spaces, other than between sarking boards.

 

We have GSE integrated solar, again this is breather membrane (same as rest of roof) and GSE panel mount trays directly attached to sarking.

29 minutes ago, Dunc said:

For this warm roof I would need to vent behind the sarking, so will have to stand the sarking off the rafters on battens,

Why? Use as membrane that needs no ventilation gaps.

 

Due to the natural slate not being flat you have thousands of small gaps to breathe through. 

 

Fixing - do what is local to you.

 

If man made slates/tiles none of the above applies, they have to be treated the same way a tiles with battens etc.

[Dunc]

thanks, @JohnMo some interesting points.

I was looking at the NHBC diagrams (https://nhbc-standards.co.uk/7-roofs/7-2-pitched-roofs/7-2-15-ventilation-vapour-control-and-insulation/)

Whether or not the roof covering is air permable, the suggestion is for a conitnuous ventillation gap which allows air flow. This seems quite different to gaps in the sarking which allow vapour diffusion.

 

 

[JohnMo]

You need to careful looking at English references as the roof build is completely different. England doesn't use sarking boards, instead the membrane is drooped to form valleys for water run off, so they need to provide a free space between insulation top of rafter.

 

Your sarking boards provide a natural airspace, as long as the membrane is breathable.