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. 

[YodhrinForge]

1 hour ago, Gus Potter said:

-space snip-

 

Okay, useful feedback thanks, but I'm not quite ready to give up on it yet it seems refinable. To address your points:

 

Firstly sorry, I wasn't clear - the roof buildup currently has a mortar fillet, I was trying to say that was different to the example image. My intention when replacing would be to ask for something much more like the example with leading and a formed gutter edge under the edge of the slates.

 

Regarding the internal battens - wood fibre batts are flexible, you need to set them between something, so I can't just fix the plasterboard directly atop them. I could use rigid boards instead, but those are a big enough faff using lime adhesive and hammer fixings on a wall, getting them in place and fixing them with thermally broken screws against a pitched roof would be a proper pain. The bridging issue also isn't a problem if the external insulating sarking is used since it will place all of the roof timbers within the insulation envelope.

 

On the membrane: the Intello is functionally impermeable unless it's beneficial for it not to be. It's air-tight if properly taped and under most conditions it's as impermeable to vapour as regular plastic barrier sheets, but if temperature and relative humidity levels between interior and exterior are right and there is sufficient vapour pressure from the building assembly on its exterior face(ie, if there were ever any water penetration which soaked into the structure and did not dry to the outside), its pores open and allow the assembly to dry to the interior as well. 

 

The air gap isn't necessary in this proposal since the assembly is not air-open(I specced an air-open membrane on the exterior only so the surface of the woodfibre sarking would dry even faster if there were ever any minor rain penetration), instead it relies on the three layered waterproofing(slate, membrane, woodfibre sarking which is rated as waterproof at my roof's pitch) and vapour diffusion to manage moisture levels. Historic Environment Scotland have done multi-year test installations where they compared vapour open materials when installed with and without an air gap in traditional roofs and the data from the probes they buried inside showed both approaches resulted in humidity levels within the roof materials that were almost identical, well within the margin of error for the test. And IIRC that was done with no VCL at all, fully vapour-open to both interior and exterior, so with an Intello type membrane it should be improved significantly since no humid interior air(not that it'll be *that* humid, I'll be using a whole house MVHR) will be absorbed. My understanding is air gaps are recommended practice on the assumption that any insulation being used will be completely vapour-impermeable like PIR, or will have less ability to diffuse vapour through its entire mass like rockwool, in either case meaning the timbers would struggle to stay dry without constant airflow, while woodfibre is both permeable and very effective at evenly distributing vapour through its mass until conditions allow it to leave the structure.

 

Regarding the edges, in terms of the gable the plan would have a continuous layer of wood fibre insulation rising up the outside of the exterior wall(a lime harl protective coat would be applied), over the top by replacing the existing skew(which wouldn't be wide enough for the insulation anyway) with faux-stone coping(basically blocks of insulation wrapped in fibreglass then covered with resin-based material that superficially resembles cut stone, they can colour match it to your existing stonework) and then a thin layer of something like a PIR down the inside cheek to meet the woodfibre sarking(all of which would be covered over by the membrane and then the lead). At the rear of the property there won't be any weak spots as that will also be insulated externally and the rear eaves will be ammended accordingly, resulting in continuous insulation. The only remaining weak point will be at the front wall which must be insulated internally because of the conservation area(and because I like it), however only the very ends of the bottom of the rafters where they meet the wall plate won't be completely enveloped in insulation so thermal bridging will be extremely minimal and that part of the roof will be just as capable of dispersing and then releasing moisture vapour as the rest.

 

On the marine ply you're right I didn't account for its impermeability, it would impair the vapour openness of the assembly. For the purposes of the nails biting properly regular ply or lengths of regular wood board should be fine though. I'm not sure why I would have issue with fixing span? The rafters are 600 on centres and 50mm wide so a 25mm surface is all you'd get on a normal butt joint between two pieces, and since they're inset the fixings would be more like 530mm apart with a 550m span, plus each would be getting nailed both right down into the runner and diagonal into the rafter - it seems like it would be solid enough? If not, I wonder if you have any suggestions on how I might refine it, would pushing up to 40x40mm for the runners do the job?

 

On the slates, you'd know best how it's done, I recalled reading about the two nail offset method but now I think back it might have been on an American's blog and I just muddled that with the youtube videos I watched from Scottish roofers. 

 

I know it must seem mad to be so intent on making this nonsense work, but for me this is probably my "forever home" and my one opportunity cost-wise to have it seriously renovated, so I'm really keen on a "buy once cry once" approach and it really is amazing the difference in comfort and ongoing cost between insulating inside vs outside(as much as possible) - for the walls you can't put anywhere near as much insulation on or you can cause problems with moisture condensing inside and degrading your stone walls, my rough calcs would put it almost a full point down with a u-value of barely 0.3 vs 0.2 or even 0.18 if done outside. And the difference in a mass-y roof of wood fibre compared to PIR is huge; it's a bit worse for thermal performance(though not as much as you'd think given you can't fully fill the rafter voids with the PIR approach), but the wood fibre roof has *amazing* decrement delay so should never overheat in summer, and it's like stepping into a soundproof box.  I'm really keen to figure something out if I can.

 

Edited 7 hours ago by YodhrinForge