Scottish slate roof Part 2: Atypical Assembly Boogaloo

[YodhrinForge]

Quick recap of the type of roof I'm talking about: 

 

With two addendums; it uses a mortar fillet in place of the lead skew flashing, and it has already had a velux conversion done to it by a previous owner so the rafter assembly is different. As you can see there are no battens, slates are nailed directly to the sarking through the felt. Also my specific situation(conservation area, semi-detached) means changing the exterior dimensions of the roof significantly is not an option so no insulating sarking with counter-batten setup on top.

 

So the previous time I asked about my roof you guys helpfully established that unfortunately nailing into modern wood fibre insulating sarking boards is a non starter, but I've been loathe to give up the idea of externally insulating it's just so much more efficient and so much better for the building fabric in the long term, and it's also looking more and more like this roof will be at end of life within a few years anyway so if I need to go for a full reslating anyway that opens up some options. What I'm wondering from people with experience is firstly, how reasonable do you find BCOs to be in terms of a-typical buildups? Can they be persuaded if you demonstrate your solution will be safe and functional, or are they generally Computer Says No types when it comes to roofs? And secondly, whether the a-typical design I had the notion to build would even work in your better informed opinion.

 

My thinking is this: if the problems are that insulating sarking won't reliably hold a nail and that I can't make the roof assembly significantly thicker as seen from the street, can I add a substrate to securely hold the nails within the existing depth? Illustrated badly in the attachment, you'll have to forgive my terrible "plans" but MSPaint is about the limit of my artistic ability on computers. I'll list out what it shows inside(bottom) to outside(top):

• Plasterboard
• 50x50mm horizontal battens with 50mm wood fibre insulation batts between
• Intello VCL membrane
• existing 150x50mm rafters, fully filled with more wood fibre batts
• wood runners fixed to the inside of each rafter along its whole length
• 25mm marine ply fixed to runners and joists sitting flush with outer surface of existing rafters
• 35mm Steico tongue & groove woodfibre sarking
• vapour and air open waterproof membrane
• ~8mm "heavy" slates with 75mm+ stainless or copper annular ring nails, two per slate centre & off-centre to prevent rotation

 

Result is essentially a solid assembly, thermally broken, three layers of water protection, breathable to the exterior at all times and to the interior in conditions that would benefit the roof but otherwise prevents transfer of moisture from interior. The slates are on top of insulated sarking, but the actual grip for the nails is provided by the marine ply sheets fixed between the rafters and supported by the runners. It would be a few millimetres thicker than the existing assembly, but so little that the slight ridge created by the necessary hidden splitting gutter would disguise it relative to my neighbour's slates. In addition to bringing the whole roof structure within the thermal envelope of the building, it would also allow continuous insulation over the whole structure with the exception of the front wall: you can't see on the image above but the stone wall is actually twice the thickness of the skew/coping and the whole area which in the illustration is a lead valley is underneath that sarking sitting on top of the bare stone of the rest of the wall, which would be a huge honking thermal bridge but with this approach needn't be.

 

Anyway enough blathering: am I on to something or should I just resign myself to insulating the whole property internally despite it being much less effective?

[Gus Potter]

1 hour ago, YodhrinForge said:

With two addendums; it uses a mortar fillet in place of the lead skew flashing,

Stick with lead for the skews. 

 

1 hour ago, YodhrinForge said:

What I'm wondering from people with experience is firstly, how reasonable do you find BCOs to be in terms of a-typical buildups? Can they be persuaded if you demonstrate your solution will be safe and functional, or are they generally Computer Says No types when it comes to roofs?

If you put forward an evidence based design then most accept. Like in all walks of life there is a bit of "personality" involved at times, younger BCO's and designers are, by default, less experienced in communicating which can lead to crossed swords. Client's sometimes are the same, maybe they don't allocate enough funds to the design fee element. In summary if your design is sound then BC will accept.. you often need patience. 

 

1 hour ago, YodhrinForge said:

My thinking is this:

 

Ok I can see what you are trying to do, a few comments in line with your text which I've turned into italic.

• Plasterboard
• 50x50mm horizontal battens with 50mm wood fibre insulation batts between

You horizontal timber battens on the inside introduce a repeating bridge. You can test this repeating bridge effect by using say Kingspan online calculator. I often use a continuous layer. But you need to check that the plasterboard can be fixed through the wood fibre on the inside.. it works for PIR. 

• Intello VCL membrane

Not sure about this. I would be inclined to use something impermeable like plastic. 

• existing 150x50mm rafters, fully filled with more wood fibre batts

This sounds ok in the main body of the roof and on paper. But around the roof edges, these are weak spots. leave the full fill insulation 10 -20mm shy of the underside of your ply just to let the water gas / moisture disperse. . I'm not going to to explain in great detail here but the principle of the sarking is to have gaps between the boards. You full fill is not compatible with your marine ply.. which is pretty impermeable. 

• wood runners fixed to the inside of each rafter along its whole length

Ok see that you are doing. But you have structural problems in terms of the edge distances of you fixings. 

• 25mm marine ply fixed to runners and joists sitting flush with outer surface of existing rafters

Now marine ply sounds great.. but trying to drive a long copper nail into this is not that easy.. ask an experience slater. Again it may be ok in the main body of the roof.. but you are going to have massive problems at the edges and any roof lights / service penetration. 

• 35mm Steico tongue & groove woodfibre sarking

Ok.. if you follow the manufacture's fixing detail.

• vapour and air open waterproof membrane

Ok.. for now.

• ~8mm "heavy" slates with 75mm+ stainless or copper annular ring nails, two per slate centre & off-centre to prevent rotation

I would not slate this way on you job, rather I would adopt the traditional route. In Scotland as we do a lot of traditional sarking we do one nail in the head, every third course cheek nailed in the main body of the roof. This lets us maintain the roof easily. Commonly in England the slates are fixed to battens. 

1 hour ago, YodhrinForge said:

Anyway enough blathering: am I on to something or should I just resign myself to insulating the whole property internally despite it being much less effective?

I think I can see your design intent.. but I very much doubt you can get anyone to build it dilligently. TYou design has technical  flaws.  In summary.. my advice is to say "I had a good go" but let pragmatism and budget cost prevail.