Cellulose-filled, cold, flat roof. Design & specification challenges help

[Dunc]

Our plans have a flat roof section of 5.8m x 2.5m. This is sandwiched between a 45 degree pitched roof on one side and a vertical wall on the other, along the long axis, such that ventilation and drainage are available only at the eaves on the 2.5m wide ends.

The proposed build up is internal VCL, 421mm posi-joists with blown cellulose, OSB deck, firrigns (providing a minimum 50mm air gap), OSB deck, EDPM.

 

I'm a bit stuck beween my architect who reckons this can't be ventilated appropriately, and the TF manufacturer who has provided an interstitial condensation analysis which shows the build up of doesn't pose a condensation risk. However they can't tell me what the length of the ventilation path is in the analysis (presumably there must be a limit) and just pointed me at NHBC guidance. All can see in there is section 7.1.10 which basically says "cold roofs are a bad idea; if you must do one the ventilation path should be less than 5m long".

 

Anyone got a similar roof could you share details of how it's ventilated and why it works?

Anyone point me at regs or guidance which would convince that the ventilation over the 5.8m length would be OK?

I'm in desperate need of help here as this issue has only come up last minute and it's feeling rather catastrophic. thanks.

Edited Saturday at 18:07 by Dunc
edited for some shocking spelling :(

[Mike]

4 hours ago, Dunc said:

I'm a bit stuck beween my architect who reckons this can't be ventilated appropriately

Why did he design it then?!

 

The best solution would, of course, be switch to a warm roof design.

 

Failing that, you could add breather vents in the flat roof - Klober do a range, for example, but so do others. I'm not sure where the Building Regs have hidden the provisions on ventilating roofs, but it used to require vents equivalent to a % of the roof area in the case of large / complex roofs. If nobody else chips in, then the manufacturer should know, if your architect doesn't.

[Nickfromwales]

I've just moved clients into an MBC TF (PH) that had this exact detail, and the GF flat roof ran between the 2 separated upper floors (house had two staircases) and extended out into a 5m x 5m balcony.

 

No issues there and I have been on site there on/off for a few years, including being up in the cellulose looking for a lost MVHR connection (that was just a delight....). 

 

1 hour ago, Mike said:

Why did he design it then?!

I am finding out, at a saddening rate of knots, that the types of people in the industry I used to look up to are well educated f*ckwits. I've only met 2 or 3 architects over the last decade who have impressed me, others I have recommended the clients dismiss (and go with the one I rated).

 

It's shocking how the public are left to rely on such people to help them and trouble-shoot, even at the design stage (pre-construction) but are just useless on a good day, expensive to the point of destruction on the bad days....

[SBMS]

We had the same issue and we did find it was too difficult to ventilate a cellulose filled cold roof. But we did have a parapet wall which really made things more difficult. In the end warm roof construction was much more simple in my view. 

[Dunc]

Thanks for the thoughts.

 

The same guidance that limits the vent path to 5m also suggests that Mushroom vents don't work.

 

To be fair to the architect, the planning drawings were done before TF manufacturer was selected. I imagine he expected a warm roof and a standard 140mm frame at that point....although having selected the cellulose fill TF it would have been nice for someone (architect or TF supplier) to point out the problem months ago.

 

And if I'd done my research I'd have known cold flat roofs are generally challenging. I wasn't even aware of the cold vs warm thing until this came up...but then we pay professionals so we don't have to know this stuff, don't we? 😞

 

@Nickfromwales any details on the flat roof you know about? Size? Vent path length?

 

It does seem that we'll have to change to a warm roof and have that installed/insulated by someone other than the TF manufacturer. Disappointing.

 

[Nickfromwales]

1 hour ago, Dunc said:

@Nickfromwales any details on the flat roof you know about? Size? Vent path length?

All I can say is it was the 5m x 5m ish balcony, and then a strip running off that about 1.5m wide x 7.5m long, basically a flag shape section of flat roof. No ventilation, MBC PH TF, and currently been weathertight, insulated (fully pumped with cellulose), and dry for prob 3-4 years, been lived in for 6 months and zero issues.

[Nickfromwales]

1 hour ago, Dunc said:

I wasn't even aware of the cold vs warm thing until this came up...but then we pay professionals so we don't have to know this stuff, don't we? 😞

You can go for a hybrid roof there also, but the execution needs to be flawless. Needs a VERY contentious installer. 

[saveasteading]

8 hours ago, Nickfromwales said:

Needs a VERY contentious installer. 

Is that freudian? Consciencous?

 

EDIT: This typo is just too good to edit out. She's a keeper  

Edited 19 hours ago by Nickfromwales
Legendary typo behaviour afoot!

[ADLIan]

Do not use a cold roof design here. The 5m max span is mentioned in BS 6229 on flat roofing. The BS 5250 calculation method does not accurately reflect the effect of the ventilated cavity that should be present. It’s possible to get the condensation calculation to show a safe construction quite easily. From past experience condensation will be predicted on the underside of the lower osb deck.

 

Redesign as a true warm roof.

[Dunc]

8 hours ago, Nickfromwales said:

All I can say is it was the 5m x 5m ish balcony, and then a strip running off that about 1.5m wide x 7.5m long, basically a flag shape section of flat roof. No ventilation, MBC PH TF, and currently been weathertight, insulated (fully pumped with cellulose), and dry for prob 3-4 years, been lived in for 6 months and zero issues.

I don't understand how there could be no ventilation at all, unless the flat roof is a warm construction!

[Nickfromwales]

7 hours ago, saveasteading said:

Is that freudian? Consciencous?

Oh bollocks......not enough sleep happening atm lol.

 

Consciencous / conscientious. LOL....................................we're both shite at this haha  

[Nickfromwales]

3 hours ago, Dunc said:

I don't understand how there could be no ventilation at all, unless the flat roof is a warm construction!

If cold air cannot get into the void, then delta T and interstitial temp differences diminish massively. Also helps that the blown-in cellulose slumps back under its own weight so is not in true full contact with the underside of the OSB. The residual air gap is stagnant, and the humidity therefore controlled to a point that this is just non-problematic.

 

I'd prefer this option vs a hybrid, as you have to be very conscientious (there, I spelt the bastard right this time lol) going down any other routes.

 

Critics will choose safety in the face of fear, as in they doubt themselves and take the safe way out.

[Mike]

22 hours ago, Mike said:

I'm not sure where the Building Regs have hidden the provisions on ventilating roofs, but it used to require vents equivalent to a % of the roof area in the case of large / complex roofs.

Just been looking for an answer to that question and found a partial answer in this guide to BS5250 : https://assets.ctfassets.net/cd81nrotvav2/ujVUWjeQFoihtJkYoWsxY/878731b5e0c188d85afad93ba1dac78b/BMI-BS5250-Pitchedand_flatroofingguidanceAugust2021.pdf

 

Page 7 shows a section of a cold pitched roof with a cold flat roof at the apex - not quite what you have, but on the right lines - that says "If the span of the flat roof apex is more than 5 m then additional ventilation to the cold roof space to a minimum free area of 5,000 mm²/m should be introduced, as close as practicable across the midpoint of the flat roof apex."  You'd have to ask if anyone will confirm that as suitable for your monopitch version.

[Iceverge]

This will be totally fine if built correctly. As usual everyone is tied up in knots as they don't understand the actual issue.

 

Most flat roofs fail because moist internal air condenses on a cold surface of the underside of the OSB. 

 

The normal method is to get rid of the "cold" part by shifting all the insulation above the OSB. 

 

Personally I'm not a fan of the usual, OSB, Vapour membrane, PIR, OSB EDPM sandwich. I think we'll see more failures as time goes on. 

 

The top layer of the OSB is sandwiched, in the cold, between two layers of impermeable materials. It's a recipe for rot. There is no secondary barrier to rain water, as soon as you get a tiny hole in the EDPM, then water will get into your roof. A primary covering and secondary covering like a normal tiled roof or what you're suggesting is much more robust. 

 

There is another approach here, stop the moist air getting anywhere near the OSB deck in the first place. 

 

Airtighess is the secret here. Proper robust 100% sealed  AIRTIGHTNESS. 

 

No designer ever specs it because they A. don't understand it. B if they do, they don't trust it to be done well on site. (Fairly justified in my view)

 

If you make the internal layer perfectly sealed and you'll have zero issues. Espically with 400mm of dense pack cellulose. 

 

Add some mushroom vents on the flat roof if you want.