Ventilation of cold flat roof with parapet

[vivienz]

My house design has 2 deep, long balconies off the bedrooms on the upper floor.  The decks have been designed with parapets; the build is by MBC Timber Frame and it will be a cold roof construction.

 

The architect is doing all the necessary for building regs sign off and it's nearly there, but they are flapping about ventilation of the decks.  Their concern is that the cold roof on the decks must be vented, but they don't want to put any vents through the vertical face of the cladding that will be on the outside of the parapet.  As yet, I haven't seen the type of vent that they are trying to avoid, but does anyone have any other solutions for venting the roof other than going through the cladding?  I've seen a few ideas where the air flow goes to the edge of the deck then up through the inside of the parapet, but I'm not sure if this would give sufficient air flow and achieve the necessary ventilation level.

 

Any suggestions welcome.

[lizzie]

I have a cold roof on top of the MBC warm roof.  MBc provided us with a flat deck and then we built another shaped roof on top to meet our design/planning requirements.

 

Cold roof definately needs ventilating.  We have large overhanging soffits and have a continious ventilation grill running all the way around (thats how the buzzing stripey things got in to nest). I believe that TRADA have some guidelines for the amount of ventilation needed. We have a lot and it is crossflow too but we are venting +250sqm of roof.

 

Our vent strips are painted the same colour as the soffits so they are less visible. They are invisible unless you look up under soffit you would not know they were there looking at the house as fascias are not affected.  Not sure if photo of ours is clear enough to show vents.

 

 Can you vent top and bottom on the internal face and lose it in your finish rather than through the cladding which I agree with architects would not be a good thing to look at.

[Mr Punter]

Is it too late to change to a warm roof?   Seems more popular with building control / warranty providers too.

[Temp]

There are mushroom vents but you don't really want those cluttering up the deck. I would switch to a "warm roof" (insulation above the structural elements) if possible but the increased thickness might be an issue?

 

[vivienz]

Hmm.  I have a meeting with the architect tomorrow morning.  I will see what they and MBC have to say about switching from cold to warm.  I could see if we really need the parapet, as I think this is a design rather than a structural issue.

 

Thanks, chaps.

Edited May 23, 2018 by vivienz

[Weebles]

We have had long discussions with MBC about cold vs warm roofs.

We also have a parapet on a flat roof and the ventilation solution is not through the cladding but the design flows air over the parapet.

The solution seems reasonable but we are definitely going to end up with a "cold" roof though not in the traditional sense.  

 

Nothing built yet but in theory the solution should work well.  Not got building regs sign off though yet as haven't got final drawings from MBC to submit yet.

 

[vivienz]

Bumping this thread for information and comment.  MBC won't budge on the cold roof, the architect would really prefer the warm roof, but I would have to find a different supplier to fit a warm roof onto the MBC structure, which I really don't want to do. The difficulty is the parapet at the edge of the balconies. The architect has been in touch with a firm that specialises in flat roofs and they have come up with the following solution; I don't know how much this will cost me yet, but it seems better than the alternative of sticking big ugly vents on the outside of the cladding.

 

Any comments?

3.3 Skirting Details - using the Flashing Profile..pdf

[Ian]

21 minutes ago, vivienz said:

Any comments?

3.3 Skirting Details - using the Flashing Profile..pdf

That looks like a nice detail which should work. The potential weak spot will be at the joints in the protruding metal flashing - any leak at those joint positions and the water is straight into the roof. Also I'd want to know exactly how the insect/rodent mesh will fit to ensure complete coverage of the air gap.

[Mr Punter]

Be useful to see the drawings from the architect as they need to factor in supporting structure, decking / floor finish, insulation, waterproofing, drainage, door threshold details, wall abutment detail and parapet detail.  They cold roof means you also have ventilation / condensation risk.

 

For a warm roof, why can't MBC just build the structure to appropriate falls for you roof contractor to add vapour barrier, insulation and waterproofing?

 

With the cold roof proposed, is this completed by MBC with all required ventilation / waterproofing?

[vivienz]

The design shown is to give ventilation to the cold roof finish; all the other details have been completed satisfactorily as the insulation, drainage and door threshold details are taken into account with the MBC build, as with the waterproofing.  It's the method of ventilation that this is looking to solve, as the traditional methods are very unattractive and would spoil the whole look of the build, hence the alternative solution.

 

It's worth checking whether the roofing contractor can add the insulation and vapour barrier, but I will need to check out the price.  Something tells me that they will be less competitive than MBC!