Roof ventilation

[Breesey]

Scenario: Flat roof dormer in pitched roof loft conversion, build finished within the last month.

Could someone please confirm that if an equivalent 25mm continuous vent gap is required in the soffit of a flat roof by the building regs then this equivalent gap must also exist in the noggings which cross the 50mm gap between the insulation and the roof deck, to allow a clear air passage upto, and out of, the ridge vent (10mm equivalent continuous gap).

Does any one know if it is possible to provide 25mm equivalent continuous gap using 70mm round soffit vents? (I don't think you can)

If it is shown that the min. vent gap required by the building regs is not installed, either in the vent or the noggings, am I in a position to go back to the builder and ask him to rectify the problem.

What I don't understand is why the building inspector did not pick this up.

There is now a condensation problem!

Many thanks for any advice.

 

[Mantamad]

Depends on how the flat roof was constructed, if its a 'warm roof' where the insulation (rigid type) is laid above the deck then felted on top then the flat roof requires no ventilation.

If its a cold roof, where the insulation is placed between the flat roof joists, then the gap above the insulation must be ventilated.

 

[Breesey]

Depends on how the flat roof was constructed, if its a 'warm roof' where the insulation (rigid type) is laid above the deck then felted on top then the flat roof requires no ventilation.

If its a cold roof, where the insulation is placed between the flat roof joists, then the gap above the insulation must be ventilated.

Thanks for the reply. As I said in my post there is a 50mm gap between insulation and roof deck ie it is a cold roof. The building is finished and the building inspector came as required. My query is whether or not the 25mm continuous gap must also be put into the noggings, and in any case why the inspector wasn't bothered that the soffit vents do not equate to 25mm cont gap.
Ventilation regs are not something I looked into until after the build was finished
Thanks again

 

[masona]

Scroll down to roof ventilation, plenty of pictures for you to look at, if you're not sure get back to us.
See here

Does any one know if it is possible to provide 25mm equivalent continuous gap using 70mm round soffit vents? (I don't think you can)

Yes you can providing you meet the calculation. Don't quote me on this, I think it use to be 70mm sofit vent every 200mm compare to 10mm continuous soffit vent but have a read here if you can't use 70mm soffit vents.

 

[Breesey]

Scroll down to roof ventilation, plenty of pictures for you to look at, if you're not sure get back to us.
See here

Does any one know if it is possible to provide 25mm equivalent continuous gap using 70mm round soffit vents? (I don't think you can)

Yes you can providing you meet the calculation. Don't quote me on this, I think it use to be 70mm sofit vent every 200mm compare to 10mm continuous soffit vent but have a read here if you can't use 70mm soffit vents.

Hi Masona, thanks for the input

When I say 'I don't think you can' I meant from a practicle point of view, not that it wasn't possible.

I have a 5 metre soffit. The regs require a 25mm wide gap or equivilent, all the way along. This is 125,000mm2. A 70mm dia vent has a 2000mm2 vent capacity, therefore, I would need 63 of these installed at 80mm centres. leaving only 20mm between each vent. Looking here I would need 15 at 300 centres, which is an option, thanks for the link.
You are correct if the requirement is for a 10mm gap

Do you know what the 'average' ridge vent is equivilent to. Regs say a 5mm cont gap is needed in the ridge ie 25,000mm2. I have (only) 2 installed.


What I'm really trying to establish is whether or not I can reasonably expect my contractor to come back and install the correct (required) amount of venting, including taking down the ceiling ( ) to put a 50mm x 300mm 'slot' in each nogging in the roof.

 

[Mantamad]

oops, I didn't read your post properly

If your roof ventilation was noted or detailed on the plans or specification that your contractor used, or formed plart of his written quotation then he should really come back and install it, if its not detailed at all on the above, then most likely he will use that excuse not to rectify it.

Should be done though, to avoid problems from condensation.

 

[Mr Winston]

did you have approved plans or was it a building notice?

 

[Breesey]

oops, I didn't read your post properly

That's all right, it was quite late

If your roof ventilation was noted or detailed on the plans or specification that your contractor used, or formed plart of his written quotation then he should really come back and install it, if its not detailed at all on the above, then most likely he will use that excuse not to rectify it.

Should be done though, to avoid problems from condensation.

Yes, it is on the plans. But as a 'regulation', even if it wasn't the fact that it hasn't been done, when it should have, amounts to the same thing, doesn't it?

 

[Breesey]

did you have approved plans or was it a building notice?

Do you mean did I need building permission? No I didn't.

The plans were 'approved' by the building inspector's office, but I only needed to pay for the building notice. The inspector visited 3 or 4 times, but did not pick up the serious lack of ventilation.
IMHO he has made a serious error here.