Cold Flat Roof - walls on 4 sides

[Chris Miles]

Hi, 

I'm looking for any advice on how to get over the issue i have with Building Control (who are also inspecting for the warranty provider) on the above detail. We have a built up cold flat roof which forms a balcony from the main bedroom. It has two full height walls at either end and the bedroom door/window on the back wall. There is a glass balustrade on the 'open' face. In effect the flat roof has 4 walls surrounding it, the lowest of which is 1100mm high (the balustrade) the others are full height to the roof.

Building control will not accept that ventilation running the full width of the end walls (approx 1500mm) inside the balcony will not vent the voids between the joists. See detail supplied by the vent company

 

I'm now very desparate because this is holding up completion so any help this forum can provide would be gratefully received 

[nod]

You can argue till your blue in the face 

Chuck it at your SE That’s what he’s there for

[Mr Punter]

Did you do a full plans application for Building Control?  If so, what detail was submitted?  What area is the balcony?

 

The risk they are concerned with is warm moist air from the room below rising into the joist zone, condensing on the timbers and not being able to evaporate into open air quickly enough.  Would a step up onto the balcony be possible?  That way, you may be able to have a warm roof construction and allay their fears.

[Chris Miles]

1 hour ago, nod said:

You can argue till your blue in the face 

Chuck it at your SE That’s what he’s there for

The SE has conducted a condensation assessment which says it will be fine with the vented void but the BC won't accept it because he says its not vented (ie teh ventilation won't be effective)

[Chris Miles]

40 minutes ago, Mr Punter said:

Did you do a full plans application for Building Control?  If so, what detail was submitted?  What area is the balcony?

 

The risk they are concerned with is warm moist air from the room below rising into the joist zone, condensing on the timbers and not being able to evaporate into open air quickly enough.  Would a step up onto the balcony be possible?  That way, you may be able to have a warm roof construction and allay their fears.

Thanks. I understand the concerns regarding the condensation. We already have a step up onto the balcony to accommodate the thermal PIR in the roof void so have very little to play with above teh joists. We could use tapered insulation on top of the joists but I'm trying to avoid that because it will be less insulation and there will be no distance between teh cill and the deck.

Yes, we did a review and he picked it up but the Principle Designer argued it was acceptable and BC didn't come back to us. We should have confirmed it was accepted at that point but we didn't (its a self build so we are a little naïve) 

[nod]

45 minutes ago, Chris Miles said:

The SE has conducted a condensation assessment which says it will be fine with the vented void but the BC won't accept it because he says its not vented (ie teh ventilation won't be effective)

Then push on 

That’s all you need He will back down once it’s done 

[Nickfromwales]

2 hours ago, nod said:

Then push on 

That’s all you need He will back down once it’s done 

Yup. Once a professional backed with PI insurance says it's kosher, esp your SE, then your BCO should back down. They know a lot about some stuff, and some about a lot of stuff......

 

Ask the SE to speak to the BCO maybe, as they'll be less dissmissive of another paid professional who should overrule them anyways!

[kandgmitchell]

On 16/06/2025 at 21:08, Nickfromwales said:

Once a professional backed with PI insurance says it's kosher, esp your SE, then your BCO should back down.

Mmmm - if only that were true - I spent many years in Building Control and came to the conclusion that many architects hadn't a clue whether their design complied with the requirements or occasionally even if they would be buildable. As for SE's I had to point out to him that the steel bars in the raft he designed for my house wouldn't all fit into the depth of concrete he'd specified given the spacing, so even those with PI can make mistakes.

[Nickfromwales]

47 minutes ago, kandgmitchell said:

Mmmm - if only that were true - I spent many years in Building Control and came to the conclusion that many architects hadn't a clue whether their design complied with the requirements or occasionally even if they would be buildable. As for SE's I had to point out to him that the steel bars in the raft he designed for my house wouldn't all fit into the depth of concrete he'd specified given the spacing, so even those with PI can make mistakes.

My point was more about recourse as well as 'opinions' born of these various 'professionals'. Ultimately, who governs the governors.....? And how does Joe Public know who to ask / trust / rely upon.....? MI's and SE trump BCO.

 

If the architect is tasked with designing to current b regs or above, then they are responsible for defects in their works via the PI insurance.

[saveasteading]

4 hours ago, kandgmitchell said:

even those with PI can make mistakes

Of course. I assume though that the SE agreed the error and you sorted out a solution. 

 

I've had lots of run-ins with bco's  usually LA ones who don't like being answered back to.

 

The thing is here, expertise. An SE doesn't do much on ventilation.

Where it is Engineering I would always expect the bco to accept my proposals, or have a very good reason to disagree.

I've had hecking Engineers say they didn't know the European codes or like Limig state principles.... ie they weren't properly qualified. 

 

But for ventilation a bco won't accept my word and wants to see a proven construction detail.

Even though we are looking at Building Science and effectively fluid mechanics.

Can you find something in a catalogue standard detail that is like your proposal? Even then he will want a formal proposal to put in the file as insurance.

 

 

[Mr Punter]

Could you introduce any ventilation along the open part, along the front?  A drawing or photo may help.

[ADLIan]

What is the span of the roof? More than 5m creates a problem. Do the proposed vents allow cross ventilation to each roof joist void? They appear to comply with the British Standard and Building Regs for open area. Can the vent manufacturer help?

 

On a general note cold flat roofs should not be used with current levels of insulation. Relying on unknown and unquantifiable ventilation levels leaves a big question mark over the long term performance. Perhaps what the BCO is alluding to.

[ETC]

Is the roof built? - if not change to a warm flat roof.

If built can you drill holes through the cross members?

[Temp]

 

On 16/06/2025 at 17:34, Chris Miles said:

the BC won't accept it because he says its not vented (ie teh ventilation won't be effective)

 

If I've understood the problem correctly your proposed ventilation/airflow runs left to right but the joists run front to back so the joists block the ventilation? 

 

Can you lift the top layer and fit say 25mm or 50mm deep counter battens at 90 degrees to the joists to allow cross ventilation left to right? Perhaps that much increased height is a problem?

 

Failing that are the joists deep enough that they could be notched or drilled to allow cross ventilation? 

 

If none of this is possible you might need to take it down and convert it to a warm roof construction that doesnt need ventilation. This has the insulation above the joists rather than between. In practice some insulation can be between the joists if there is enough above them.

Edited Thursday at 21:09 by Temp