Flat roof timber rot. How often does it happen?

[Albumen]

Small flat roof (2.4ms x 1.5ms). It's a kitchen extension. There's some evidence of leaks. Our roofer, who I trust, has had a look. The boards are sound, the joists are sound (only about 15 years old). He's just going to felt over.

The ceiling's down, I've put 75mm BallyTherm in between the joists. The joists are 100mm and theres a couple of inches of firrings. I'll fill gaps with expanding foam. I'll put DPM on underside of joists and use foil backed insulated plasterboard.

But... there is no ventilation to the space between the joists. Do I really need to worry? Though it's a kitchen I'm putting in a powerful cooker hood. All cooking is electric, including induction hob. Big opening window next to hob. So I'm doing all I can to remove moisture at source, and to prevent it getting into the ceiling void.

Roofers - do you often go out to jobs where poorly ventilated flat roofs have a rot problem caused by lack of ventilation?

 

[dann09]

When you say that the "boards" are sound, i presume that you are talking about ply or OSB sheeting?

You wont know the true condition of the boards until the felt has been stripped. The old felt should be removed.

Have the terms cold roof, and warm roof been used? Perhaps, you should google pics and diagrams of them.

The "firrings" will probably be tapered firring pieces, they reduce to almost nothing at the discharge end of the roof - dont rely on them for an air gap.

 

[Alastairreid]

If you have no ventilation between the deck and insulation you should pack the insulation hard to the deck, In other words full fill then your vapour check.