Pitched roof make up

[Tony L]

I’d very much appreciate some opinions on this roof design please:

 

I’ve designed a house myself (with no prior knowledge).  I have planning permission.  An architectural technician has recently prepared a full set of building regulations & construction drawings for me - ie one set of drawings will go to the SE for his input, then to BC for approval, then to builders.

 

Now I’m working through the draft drawings, to make sure I’m happy with all the decisions the arch tec has made.

 

My brief for the sloping roof sections was: standing seam on special SS membrane, then OSB, then 50mm air gap, more OSB, I-joists at 600 centres & Warmcel blown cellulose in between. I suggested 300mm deep I-joists or next size down (240mm), if we’re struggling to make everything fit (there’s a PP ridge height restriction).  Also, I’d seen some of the arch tec’s drawings for another project (with phenolic everywhere) so I told him, to keep costs down, we should only use phenolic in one very small area where we need to avoid bulky insulation.  He’s specified two layers of phenolic, over the entire sloping roof.

 

My own (240mm I-joist based) roof design would be about 354mm thick – so, compared to the arch tec’s roof design, my design significantly reduces the space in two of my in-roof bedrooms, which don’t have dormers.  The extra depth of the I-joist solution will be why he’s done drawn something that doesn’t fit with my brief.

 

Here’s the arch tec’s design:

 

 

[Tony L]

I’ve had a go at plugging my I-joist based design into Ubakus.  I couldn’t find most of the materials I wanted to use & Ubakus won’t let me have my battens for the interior plaster board running the other way from the joists, unless I pay a subscription, but this gives me a rough idea of how my 240mm I-joist roof option might perform:

 

 

[Tony L]

If I increase to a 300mm I-joist, the U-value comes down to around 0.129 W/m2K.

 

The arch tec’s roof is just 290mm thick, which is useful.  The notes say it has a U-value of 0.11W/m2K.  Actually, it says “0.11/m2K”, because he missed the “W” (Watts).  Another thing he’s done, just to confuse us all, is he’s used label numbers that don’t correspond to the positions of the materials within the roof make up – so number 1 is the standing seam layer, on the outside, but number 5 (50mm air cavity) sits on top of the rafters (number 4) & the insulation between the rafters (number 6).  Confused?  I am.  I think he’s drawn the Tyvek membrane on top of the 50mm cavity, rather than beneath it, & he hasn’t spec’d the battens for the 50mm cavity; should he have? Perhaps, 50 x 50mm C24 battens should be specified?  I don’t know.

 

I can’t find K107 & K118 in Ubakus, so I haven’t checked the arch tec’s 0.11W/m2K U-value for his roof design.

 

I don’t like the look of the arch tec’s design.  I don’t like the idea of cutting boards in between the rafters (ie relying on somebody else to do it properly) & I suspect the K118 might be off-gassing more poison than my design too.

 

I think the arch tec’s roof may be more expensive (just on materials) than the design I proposed, & it will be significantly more expensive once the labour is factored in.  Have I got that right?  So far, I haven’t found time to get all the prices & work it all out.

 

What do you think I should be saying to the arch tec about the roof make up?