Flat roof detail - warm or cold?

[LnP]

I'm interested in comments on this flat roof and parapet detail.

 

1. Am I right in thinking it's a cold roof?

2. And is the 18mm OSB deck in the wrong place and should it be moved to be on top of the firring (red arrow)? This would give the possibility for ventilation between it and the insulation?

3. What do people think about cold roofs and the risk of condensation if the VCL gets perforated? Is the risk manageable or are warm roofs always preferred, assuming you've space for the additional thickness?

4. What about the cold bridge along the bottom of the joist (blue arrow)? If so, what could be done about that?

 

Thanks

Still learning!!

Edited September 4 by LnP

[Redbeard]

27 minutes ago, LnP said:

1. Am I right in thinking it's a cold roof?

 

Yes

27 minutes ago, LnP said:

2. And is the 18mm OSB deck in the wrong place and should it be moved to be on top of the firring (red arrow)? This would give the possibility for ventilation between it and the insulation?

I don't know, because I do not know what the designer had in mind. One assumes either that it is an incomplete dwg - there is nothing shown above the firrings - or, as you surmise, the OSB is in the wrong place. I always like a howling gale on the top side of a cold roof, although I have seen designs for apparently unventilated, fully-Warmcel-filled roofs, which seem to work OK. As that is drawn I cannot tell whether the 'air gap' has ins and outs (cross-ventilation) anyway. Has the detail been drawn by a company specialising in Warmcel-ins'd roofs?

 

27 minutes ago, LnP said:

3. What do people think about cold roofs and the risk of condensation if the VCL gets perforated? Is the risk manageable or are warm roofs always preferred, assuming you've space for the additional thickness?

Warm roofs are theoretically much better, I feel, but the scope for poor execution due to misunderstanding the concept can be significant. If you get someone who is really good at insulated cold flat roofs you *may* get a better job than asking them to do a warm roof that they have never done before.

27 minutes ago, LnP said:

4. What about the cold bridge along the bottom of the joist (blue arrow)? If so, what could be done about that?

 

Is it really a full-depth thermal bridge, or is the tail of the arrow masking a 'mitigating' piece of insulation? Certainly it wants to be 'cloaked' as best it can.

 

[LnP]

@Redbeard thanks for that. There's nothing hidden by the blue arrow. The stud is 140 mm and as drawn there's already only about 100 mm of the joist sitting on the stud. If the joist was shortened to make room for say 50 mm of insulation, there'll only be about 50 mm of the joist sitting on the stud. Is that enough or is there another solution?

 

Thanks

[bob the builder 2]

if this is a plan from a well known timber frame suppler...

 

and

 

the upper surface is external

 

then

 

you need to allow ventilation across that top surface between it and the final external membrane

 

IN which case you need to in  build another layer of osb on battens to place the final membrane and allow air to flow across this cavity 

 

[SBMS]

We’ve had this exact issue with our flat roof design. We are going with 304mm posi rafters with warmcell for our pitched roof.  Originally we were going to do the same with the flat roof portion but then another BH member pointed out the need to ventilate this. Then came the issue that cross ventilation with a parapet wall around the top is incredibly difficult to detail successfully. I decided to switch to a much simpler warm roof design with PIR on top.

 

Any reason you’re not going warm roof for the flat roof? Much easier to detail. 

[DevilDamo]

A warm roof is so much better and easier when dealing with flat roof upstands. Assuming that stud on top is forming some kind of parapet?

[LnP]

1 hour ago, bob the builder 2 said:

if this is a plan from a well known timber frame suppler...

 

Indeed it is .... and one could concluded it's not a good detail.

 

36 minutes ago, SBMS said:

Any reason you’re not going warm roof for the flat roof? Much easier to detail. 

Just that that was what this timber frame supplier proposes. But I'm asking myself if there's a better way, so yes it looks like that detail would need to be changed.

 

The build up on a warm roof will present a bit of a challenge though. We were planning to make this flat roof a balcony/roof terrace and if we put all the insulation on the top , there will be some head scratching to work out how to avoid a big step up from the FF door onto the balcony. Using shallower joists on closer centres would help but maybe not the full solution.

 

Anybody any further thought on the cold bridging question?

 

Thanks

[SBMS]

12 minutes ago, LnP said:

Indeed it is .... and one could concluded it's not a good detail.

 

Just that that was what this timber frame supplier proposes. But I'm asking myself if there's a better way, so yes it looks like that detail would need to be changed.

 

The build up on a warm roof will present a bit of a challenge though. We were planning to make this flat roof a balcony/roof terrace and if we put all the insulation on the top , there will be some head scratching to work out how to avoid a big step up from the FF door onto the balcony. Using shallower joists on closer centres would help but maybe not the full solution.

 

Anybody any further thought on the cold bridging question?

 

Thanks

What’s your current joist depth your cold roof is working to - or to put it another way, what’s your maximum height you’ve got for the warm roof make up and the u value you’re trying to achieve?

[ETC]

Go with a warm roof.

[DevilDamo]

10 hours ago, LnP said:

how to avoid a big step up from the FF door onto the balcony.

Is there scope to lower the roof for that element so you’ll have a bulkhead below and therefore flush levels at first floor level? The bulkhead could be made out to be a feature too with a coffer for example.

[SBMS]

Is it the MBC flat roof detail (300mm joists achieving 0.14)?

[LnP]

On 04/09/2024 at 20:44, SBMS said:

Any reason you’re not going warm roof for the flat roof? Much easier to detail. 

No reason apart from ignorance 🙂!! I think we'll change it!

 

23 hours ago, SBMS said:

What’s your current joist depth your cold roof is working to - or to put it another way, what’s your maximum height you’ve got for the warm roof make up and the u value you’re trying to achieve?

Good question. I don't have that information at the moment.

 

23 hours ago, ETC said:

Go with a warm roof.

Good advice which I'm going to follow!

 

13 hours ago, DevilDamo said:

Is there scope to lower the roof for that element so you’ll have a bulkhead below and therefore flush levels at first floor level? The bulkhead could be made out to be a feature too with a coffer for example.

Nice idea thanks. A creative solution. We'll give that some thought!

[Dave Jones]

make sure its ventilated. Warm roof much easier to detail but you need the build height which is what most people struggle with.

[G and J]

1 hour ago, Dave Jones said:

make sure its ventilated. Warm roof much easier to detail but you need the build height which is what most people struggle with.

I think that’s exactly what we are facing.  We’ve a 3m x 5m flat roof with a fair bit of that taken up with a skylight.  I think the difference between internal ceiling height and the top of the parapet that surrounds the flat roof is between 450mm and 500mm.  Posi joists will be 254mm, plus we need to be able to walk on the flat roof for maintenance/window cleaning.  
 

Reading this thread it sounds like we’ve no choice but to have a cold roof, but how on earth do you ventilate it?

[DevilDamo]

1 hour ago, G and J said:

Reading this thread it sounds like we’ve no choice but to have a cold roof, but how on earth do you ventilate it?

Ventilated upstands at each junction.

[SBMS]

1 hour ago, G and J said:

I think that’s exactly what we are facing.  We’ve a 3m x 5m flat roof with a fair bit of that taken up with a skylight.  I think the difference between internal ceiling height and the top of the parapet that surrounds the flat roof is between 450mm and 500mm.  Posi joists will be 254mm, plus we need to be able to walk on the flat roof for maintenance/window cleaning.  
 

Reading this thread it sounds like we’ve no choice but to have a cold roof, but how on earth do you ventilate it?

With difficulty.  It's possible but once I started looking into it, I realised that the detailing had to be exact and a warm roof was much easier.

 

The detail you've posted looks like MBC's flat roof with cellulose.  Are you sure the joists are 254mm and not 300?

 

254mm cellulose filled wouldn't be a great U Value.. probably 0.16 or something?

[Dave Jones]

2 hours ago, G and J said:

I think that’s exactly what we are facing.  We’ve a 3m x 5m flat roof with a fair bit of that taken up with a skylight.  I think the difference between internal ceiling height and the top of the parapet that surrounds the flat roof is between 450mm and 500mm.  Posi joists will be 254mm, plus we need to be able to walk on the flat roof for maintenance/window cleaning.  
 

Reading this thread it sounds like we’ve no choice but to have a cold roof, but how on earth do you ventilate it?

 

[SBMS]

If you could reduce your joist depth to 202mm - 2 layers of super quilt plus 40mm PIR making 242mm yields 0.15 u value. All warm roof. We’ve got a 202mm posi joist with green roof loading… 
 

alternatively if it’s the Mbc detail then it’ll be a 300mm joist (0.14 u value with cellulose) which you could do with a 202mm with 1

layer super quilt and 80mm PIR.

[mjc55]

13 minutes ago, SBMS said:

If you could reduce your joist depth to 202mm - 2 layers of super quilt plus 40mm PIR making 242mm yields 0.15 u value. All warm roof. We’ve got a 202mm posi joist with green roof loading… 
 

alternatively if it’s the Mbc detail then it’ll be a 300mm joist (0.14 u value with cellulose) which you could do with a 202mm with 1

layer super quilt and 80mm PIR.

 

How was the posi-joist sizing in relation to green roof loading?  We got a quote for posi-joist some time ago but at that stage we were not specific on roof construction.

[SBMS]

51 minutes ago, mjc55 said:

 

How was the posi-joist sizing in relation to green roof loading?  We got a quote for posi-joist some time ago but at that stage we were not specific on roof construction.

Not that much of an impact. The engineer at Pasquill has over specified it and a 202mm joist will suffice. I think he specified it for a traditional soil + greens roof makeup whereas we are using a lighter tray system. 

Edited September 6 by SBMS