Flat Roofs and Mushrooms

[Izard]

I am having a new extension built with a flat roof measuring 3m x 5m including a sky lantern 1.5m x 3m, so very little roof surrounding the lantern. It is a cold flat roof, therefore the void which is 60mm deep needs ventilation. We have been recommended to use 4 x Oft1f 166 mushrooms by the company advisor. Not only are these enormous and will be obstructive to access if needed, I think it is somewhat overkill. What alternatives do I have to convince Building Control, will be grateful for any suggestions.

 

[chappers]

does the roof have an abutment at the upslope end, if so you can get a flat roof abutment ventilator, or build one using timber and an over fascia ventilator covered with a weatherproof flashing.

 

[^woody^]

Completely fill the roof with insualtion to the required L1 standard, then it wont need ventilating.

 

[securespark]

I understood on a cold roof, there has to be a gap between the deck and the insulation (as per diagram here: http://www.buildingregs4plans.co.uk/guidance_flat_roof_types.php. )

Is that not the case?

 

[^woody^]

I understood on a cold roof, there has to be a gap between the deck and the insulation (as per diagram here: http://www.buildingregs4plans.co.uk/guidance_flat_roof_types.php. )

Is that not the case?

The issue with any roof is to prevent condensation, which can then lead to mould.

Whether you deal with the risk by ventilation or removing the chance of condensation, is up to you.

 

[securespark]

Cheers, woody

I have not seen that method mentioned, that's all.

 

[^woody^]

Cheers, woody

I have not seen that method mentioned, that's all.

I invented it.

 

[John D v2.0]

If you can guarantee no air flow through the layers then it might work, but as soon as any air comes through you'll get condensation in the layers.