DIY Doctor UK DIY Forums

Hello, a total “newbie” here.
My new (110 years old) house had a serious leak in the flat roof over an extension with bathroom on first floor and kitchen downstairs. The damage was so excessive that the whole roof had to be replaced. Now I’m having the old plasterboard ceiling removed as it was all mouldy and ruined by prolonged exposure to rainwater. Soon I’ll have would a visible from underneath joist structure supporting chipboard. What would be the best way to finish it? A damp surveyor who carried out an inspection suggested insulation boards (Kingspan or similar) to attach directly to the bottom of the joists. Is this the best option? If yes, do I need to use plasterboards to finish this? Or should I use something else? It’s bathroom so there will be high condensation etc.

Hi aktinidia.
Fit insulation between the joist leaving a ventilated air gap above it, also insulate below the joists with more insulation with taped joints. You will then need to overboard with plasterboard.
Regards S

Thank you Stoneboy.
It looks like there is no ventilation under the roof. Brick walls all around and no air movement even on a windy day like today. (Please see attached photo)
Can I use insulation (rock wool?) in-between the joists and water-resistant plasterboards attached to the underside? Do I need to cover the plasterboard vapour barrier?
I already had two tradespeople suggesting very different approach. One suggested rock wool between joists and plasterboard underneath, the other nothing in-between joists and only insulation boards (Kingspan) under the joists.
Which approach is better?

Hi acktinidia.
From your picture I would assume you have a fall away from the main house roof. I suggest you fit insulation between the joists leaving say a 50mm gap above and a further layer of insulation with foil face with taped joints overboarded with plasterboard. Also fit vents in the facia end to give some air movement - this is important because it is a bathroom and you won’t want condensation in your new roof.
Regards S