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?

 

 

[Russell griffiths]

His design is pretty crap

he's obviously not considered building it or paying for it. 
 

couple of points, your roof is nearly identical to mine. 
are you sure you can use osb under the metal roof, the screws are really small and the pull out strength in osb is poor, I thought they said it needed to be plywood or solid timber. 
 

we went with knauf omni fit between the rafters and 70mm pir board underneath. 
 

ditch the insulated plasterboard and use a service batten then fit normal plasterboard. 
 

 

[saveasteading]

29 minutes ago, Russell griffiths said:

pull out strength in osb is poor, I thought they said it needed to

The pullout force on each screw is huge and osb is weak.

If any dampness was to find a way in the osb would fail anyway.

I assume the cladding fixings come where they come and dont hit rafters, so you need a batten running along each line of them... and so more height... or use noggins/ dwangs.

 

What qualifications does this person have? And PI cover?

 

Remember that standing seam does not work well with any penetrations. It is for looks more than performance. No vents, chimneys , dormers etc.

[Tony L]

Thanks, @Russell griffiths & @saveasteading.  Pagurek offer this make up, below, as one of their suggested options.  I will perhaps choose to upgrade from OSB to wood.  I'm not keen on ply as it warps so much when it dries out after becoming damp.

 

I'm happy enough with the standing seam above the 50mm air gap.  It's the other aspects of the roof build up I need help with, please.

 

 

[Tony L]

26 minutes ago, saveasteading said:

What qualifications does this person have? And PI cover?

 

 

Sorry, I'm too busy to dig that information out for you.  I engaged him quite some time ago, although I have only recently received the full set of draft drawings that I was supposed to receive early in July.  He came highly recommended by a prominent BH forum member.

[Russell griffiths]

From inside to outside mine is. 
plasterboard

service batten

pir board, taped and sealed to make vcl

joists

knauf omni fit between joists

breather membrane

spacer batten

solid timber boarding

special metal roof membrane

standing seam. 

[Nick Laslett]

One of the YouTuber’s I follow, actually posted a pitched warm roof video today. 
 

 

[Tony L]

Thanks, @Russell griffiths & @Nick Laslett.  I've watched quite a few of Steve Roofer's YT videos, but I hadn't seen that one.  He's very good.  This is so difficult - even an expert like Steve Roofer doesn't have all the answers.  He's got tons of good ideas & he explains things very well, but rather than say, "Do it exactly like this.", he's saying (in this video), "Talk to the manufacturers.", to choose a suitable membrane.

 

Very interesting that he recommends sandwiching the outer membrane between the battens & cross battens - so it's in the middle of the 50mm vent cavity under the tiles, standing seam or whatever - that idea wouldn't have occurred to me.

[Russell griffiths]

You need to bear in mind that pir insulation is rubbish at keeping out noise, with a metal roof that is a major consideration 

[Tony L]

There’s another aspect of the roof design I would also like to receive advice on.

 

Due to the LA planning dept. limiting the ridge height, my roof design has two conventional sloping sides meeting at a flat roof running the full width of the house.  The flat roof is 14200 x  1800mm.

 

I am happy that the arch tec hasn’t drawn a parapet flat roof, but I don’t like the flat roof design he’s come up with, especially the way it interfaces with the sloping roof sections & the way the two steel ridge beams run the entire width of the house, with next to zero insulation on the outside edge & only 72mm insulated plasterboards across the whole of the bottom of 102mm wide steel beams.

 

This looks like 28+ metres of unnecessary cold bridge to me.  Am I right to be concerned about this?  Maybe I shouldn’t worry about it & I should just focus on building something I can afford, & making sure it’s super air tight.   What does everybody think?

 

Here’s an early draft roof plan from my SE, showing steel beams, in dark blue.

 

 

Edited 2 hours ago by Tony L

[Tony L]

Here's what my arch tec has come up with...

 

 

 

 

 

 

 

 

 

 

 

[Tony L]

The detail picture shows the vent at the top of the standing seam is open to the rain – not just the odd raindrop - another drawing shows that all the rain from the flat roof will drains off both long edges (14m along the front of the house, 14m along the back of the house) onto the sloped roof.  Surely this detail should show the flat roof edge overhanging the top of the standing seam (perhaps right on top of the seams) – so air just vents out where the standing seam finishes & rainwater from the flat roof drips onto the standing seam 80-100mm beyond the end of the standing seam?

 

 

 

[Tony L]

What improvements could I suggest?  One solution would be to put the steels lower down, so their tops are where their bottoms have been drawn.  This eats into my narrow vaulted ceiling space, but it could be a price worth paying.  Another solution would be to have them lower by half the height of the steel & the sloped roof joists could then have a cut in to accommodate the steel, which the joists would rest on.  

 

I don’t think we could maintain the height of the steels & move them closer to one another (even just 100mm, to allow 100mm of insulation to be added), could we?  I don’t know how we’d deal with the connection of the sloping roof joist ends if we did that – perhaps just by adding blocks of C24 where we already have C24 bolted into the steel I-beam, to make the connection to each joist end.  I wish I had time to draw all these ideas.

 

Perhaps I should be aiming for a warm flat roof with up to 200mm of PIR on top of the rafters.

[Tony L]

30 minutes ago, Russell griffiths said:

You need to bear in mind that pir insulation is rubbish at keeping out noise, with a metal roof that is a major consideration 

 

Thanks for the reminder - same probably goes for GRP, which I may end up with.  I understand GRP can be very noisy as it expands & shrinks, although expansion ridges can be built in to mitigate this.

 

My arch tec has specified phenolic insulation for the sloped roof, which I suspect is almost as bad at providing noise insulation as PIR.  He's specified PIR under the GRP flat roof.

Edited 2 hours ago by Tony L