Flat roof make up

[Tony L]

I’m checking through my architectural technician’s drawings (these are for BC & construction) & I’d like some advice, please.  Due to planning constraints, my  new build is going to have 4 large flat roof areas – see roof plan, below. 

 

The flat roofs make up well over a third of my roof covering, so I’m keen to get these right.  Here’s the arch tec’s first draft roof plan.

 

[Tony L]

& here’s part of the detail drawing for the bathroom roof.

 

[Tony L]

I don’t like this because:

 

1) There’s a big wedge of cold, damp air in the middle

 

2) Even if we fix 1) above, I’m concerned the 120mm Rockwool above the ceiling may insulate well enough to make the OSB supporting the PIR cold enough for condensation to form (& the VCL is above this OSB, so it will have a significant supply of moisture to it).

 

3) It’s a bathroom – even if we swap out the KNAUF Soundshield plasterboard for a water resistant plaster board, we may still need something else to stop moisture rising into the structure .  Would we?  I don’t know – I’m a novice.

 

The same roof make up is specified over the bedrooms too.  I need to ensure these rooms will be quiet.  I’d be happy to hear the rain as I drift off to sleep, but rest of the household demands absolute silence.

 

I’m thinking I should change the design so we have, from top down:

 

1. roof covering – which may or may not be not be GRP (let’s not get into that now)

2. 18mm OSB (can I get away with 11mm to save £330 over approx. 55m2)?

3. 200mm PIR  with top layer  Kingspan TR47, which has built in falls, meaning firings will not be required lower down (I haven’t priced TR47 yet).  Taped at joins & in two layers with offset joins . 

3b & is it worth the expense of Gapotape (or perhaps cheaper alternative) all around the perimeter, which will mostly be OBB3?

4. VCL – If somebody could tell me which one to go for or what the spec ‘should be (in terms of SD value? – I don’t know) that would be helpful. 

5. 18mm OSB

6. 170mm C24 joists, or perhaps fatter 150mm joists, if SE allows

7. unvented air between the joists

7b. For outside walls, do I stuff 100mm (or more) rockwool against the sides, (along the joists on the long side & between the joists on the cheeks), so the roof insulation is “tucked in”?

8. 100mm Rockwool fitting tight between the rafters – this is for sound insulation, really, but it’s making the void cold.  This sits directly on top of 9)

9. 12.5mm plaster board (water resistant type if it’s a bathroom)

10. resilient bars

11. another 12.5mm plaster board (water resistant type if it’s a bathroom)

 

Once I’ve settled on a make-up, I can start worrying about the edges/interfaces.

Edited Tuesday at 20:37 by Tony L

[ETC]

Big cold bridge.

[Roger440]

1 hour ago, ETC said:

Big cold bridge.

At the topof the wall i presume you mran?

 

[Iceverge]

 

I've very much gone off the idea of GRP and EDPM roofs stuck straight down on top of OSB and the PIR like all warm roofs are specced. 

 

The GRP is your only barrier against the elements and it's subject to the full rigors of heating and cooling and the associated expansions and contractions. The roof surface could quite easily see a 60deg temp swing over 24hrs in early summer. I very much fear this will lead to leaks down the road for too many roofs. 

 

I would far prefer to see a metal roof over a ventilated cavity over insulation over rafters. 

[Iceverge]

What kind of metal have you covering the remainder of the roof and I might have a go at a detail later.

 

Also a section view would help. 

[ADLIan]

A few comments.

• It’s not good practice to add further insulation between the roof joists in a warm flat roof. This creates a hybrid roof that will be prone to condensation issues. 
• The choice of VCL will depend upon how the roof is being fixed - either fully bonded or mechanically fixed.

The insulation manufacturer’s advice should be sought on both of the above.

 

• The thermal bridge can be minimised by packing the roof edge, between top of wall and underside of the deck, with mineral wool insulation- a section perhaps 300mm wide.
• Tapered roof boards on such a small and simple roof will be expensive. Simpler & cheaper to create the falls in the roof structure

[Tony L]

Thank you @Iceverge & @ADLlan.  I'm having a horribly busy day at work.  I'll try to get back to you within a couple of hours.

 

Anyone: why aren't my @nametags working as they usually do?

[Tony L]

The sloping roofs will be standing seam.  & I plan to get the roof make up below the standing seam changed to this, although with deeper Rockwool & insulated PB.   There are small rooms under the sloping ceilings at the back, so I don’t want to make the roof much thicker than the 296mm drawn; this might include 200mm Rockwool + 72.5 Kingspan K118 (only 60mm of the 72.5 is phenolic insulation).  More insulation would be better, I know.

 

[Tony L]

Here's a section drawing.  I’ll also upload 3x pdfs so you can see this more clearly.

 

 

TonyL 300-04 - Section AA.pdf TonyL 300-08 - NB 1st RT03 is RT02 Proposed Build-ups.pdf TonyL 300-03 - Proposed First Floor Plan.pdf

[Tony L]

Here's an early Sketchup image, to help you understand what's going on with the galleried landing & porch.

 

The front of the house faces north - the north pointers on the arch tec's drawings are all incorrect.

 

 

[Tony L]

I must explain:  the ridge height absolutely cannot go any higher. I could get away with adding 100mm to the height of the dormers if I have to, although I think reducing ceiling heights by 100mm, or more, is probably a better strategy (due to PP restrictions).  There’s plenty of spare space inside my vaulted ceiling, in the section under the long flat roof, so this could be used to make this roof section deeper.  The eaves across the front of the house is higher than PP allows, & I will ask my arch tec' for the wall plate height across the front to be reduced by 140mm to bring the eaves height down to where it should be, at around GL+2700mm. This will have the effect of pushing the 42 degrees front roof towards the middle of the house, making the flat roof across the ridge narrower by about 150mm.

 

Rather than have the flat dormer roofs sloping backwards to drain onto the standing seam, as drawn, my plan is to ask for these to be changed to drain into a front corner, into a downpipe concealed behind the cladding then straight into the hidden gutter (from the 5450 wide dormer in bottom left) & onto the standing seam for the other two dormer flat roofs.

 

The drip edges on the long flat roof that spans the house have not been properly drawn.  I’ll get these changed so the drip edges overhang the standing seam ends by 100mm+, then the 50mm void under the standing seam will vent between the seams, under the long drip edge, which will protrude from the flat roof & lay on the seams.

 

I realise there are many bad things going on in this section drawing.  My plan was to ask about the spec’ for the make up under the flat dormer roofs first, then ask for help with some of the other details once the flat roof make ups have been resolved.

 

I’m very grateful for your interest in my project.  Thank you, @Iceverge & anyone else that's able to help.

Edited 18 hours ago by Tony L

[Tony L]

7 hours ago, ADLIan said:

It’s not good practice to add further insulation between the roof joists in a warm flat roof. This creates a hybrid roof that will be prone to condensation issues. 

 

All good points from your post, above, thank you, @ADLIan.  Yes, the arch tec's design with extra insulation beneath the warm roof was an alarm bell for me, which is one of the reasons I've asked about these roofs. But, I've seen at least one builder here, who seems to know what he's doing, saying he's done almost exactly what my arch tec' has proposed.

[SBMS]

2 hours ago, Tony L said:

 

All good points from your post, above, thank you, @ADLIan.  Yes, the arch tec's design with extra insulation beneath the warm roof was an alarm bell for me, which is one of the reasons I've asked about these roofs. But, I've seen at least one builder here, who seems to know what he's doing, saying he's done almost exactly what my arch tec' has proposed.

Not necessarily true. Unilin approved a 150mm PIR warm Roof with 100mm of Rockwool. Showed an annual moisture accumulation of 0Kg/m2. I checked this myself on ubakus.de which showed the same. 
 

why is there ‘cold damp air’ in middle? Isn’t that warm side of insulation?

Edited 15 hours ago by SBMS

[Tony L]

31 minutes ago, SBMS said:

why is there ‘cold damp air’ in middle? Isn’t that warm side of insulation?

 

It should be, but I don't think it will be very warm in the middle of this make-up.  The section I'm talking about is above the Rockwool, & in winter, cold air will be flowing through the void behind the cladding (a wide void due to cross battening for vertical cladding) then there's nothing but a sheet of OSB between this flow of air & the void made up from the firings & the empty space between the joists.  I will ask the arch tec' to swap the fall around anyway, so rain drains towards the front rather than towards the back.

[ETC]

The wall and roof insulation must meet.

[ETC]

Why are you having a standing seam flat roof if you can’t see it?

[SBMS]

45 minutes ago, Tony L said:

 

It should be, but I don't think it will be very warm in the middle of this make-up.  The section I'm talking about is above the Rockwool, & in winter, cold air will be flowing through the void behind the cladding (a wide void due to cross battening for vertical cladding) then there's nothing but a sheet of OSB between this flow of air & the void made up from the firings & the empty space between the joists.  I will ask the arch tec' to swap the fall around anyway, so rain drains towards the front rather than towards the back.

I can’t see the other section orientation but the vented cavity shouldn’t be venting into the firrings void. The whole point of a warm

roof is it is unventilated. But also, as etc says this wouldn’t be an issue if your cavity insulation carried on all the way up to meet the PIR… 

 

 

[Roger440]

10 hours ago, ADLIan said:

A few comments.

• It’s not good practice to add further insulation between the roof joists in a warm flat roof. This creates a hybrid roof that will be prone to condensation issues. 
•  

At the risk of hijack.................

 

I know this is percived wisdom, but ive only got 300m total height to worrk with so a conventional warm roof wont work. I put my proposed design into ubakus.de and it says all is well. What am i missing?

 

 

[SBMS]

10 minutes ago, Roger440 said:

At the risk of hijack.................

 

I know this is percived wisdom, but ive only got 300m total height to worrk with so a conventional warm roof wont work. I put my proposed design into ubakus.de and it says all is well. What am i missing?

 

 

Sorry just seen the picture. You’ve designed a cold roof that needs ventilation?

Edited 14 hours ago by SBMS

[SBMS]

29 minutes ago, Roger440 said:

At the risk of hijack.................

 

I know this is percived wisdom, but ive only got 300m total height to worrk with so a conventional warm roof wont work. I put my proposed design into ubakus.de and it says all is well. What am i missing?

 

 

You should put a very vapour resistant layer above the osb. Ubakus is calculating with no waterproof layer so it’s basic condensation analysis might assume any moisture can dry freely and quickly?

 

it might also be because it’s assuming the 25mm pir under/over prevents any vapour transfer. On paper yes, in practice with imperfect airtightness, moving timber, gaps between pir this is totally unlikely. Even a 1mm gap over time will produce condensation and rot timbers… I suspect that ubakus doesn’t account for that. I bet if you switch that 25mm to 1mm it’s exactly

The same. But remove the 25mm and you’ll see condensation. That’s ubakus simulating ‘gaps’ or vapour paths through the insulation (the timbers) which is your actually likely as built scenario….

Edited 13 hours ago by SBMS

[Tony L]

35 minutes ago, Roger440 said:

What am i missing?

You need to fill in No. 6 with a roof covering - you can't leave OSB as your top layer.

[Roger440]

16 minutes ago, SBMS said:

You should put a very vapour resistant layer above the osb. Ubakus is calculating with no waterproof layer so it’s basic condensation analysis might assume any moisture can dry freely and quickly?

 

it might also be because it’s assuming the 25mm pir under/over prevents any vapour transfer. On paper yes, in practice with imperfect airtightness, moving timber, gaps between pir this is totally unlikely. Even a 1mm gap over time will produce condensation and rot timbers… I suspect that ubakus doesn’t account for that. I bet if you switch that 25mm to 1mm it’s exactly

The same. But remove the 25mm and you’ll see condensation. That’s ubakus simulating ‘gaps’ or vapour paths through the insulation (the timbers) which is your actually likely as built scenario….

 

Yes, i know i need a vapour barrier. But surely, between the plaster board and first layer of PIR.

 

Yes, clearly if there are gaps etc, that would be an issue. As id be building it myself, i can make sure thats not the case.

 

Problem is, ive got circa 300mm and need to use 200mm (ish) joists.

[Roger440]

Just now, Tony L said:

You need to fill in No. 6 with a roof covering - you can't leave OSB as your top layer.

Yes, of course. That would be felt, grp or epdm etc. Just playing with ideas at this stage,