Inverted roof - possible/advisable?

[Kaymo]

Hi,
Below is a pic of the roof of my extension, built I think around 1990. The roof is built on to wooden joists, coming out from the back of the house, which was built around 1820. The high edges are due to a short slope of slate at the front, which can be seen in the next picture, presumably done to blend with the slate roof of the old house. There is very little in the way of insulation in the small space between the joists, and the ceiling inside is quite low restricting internal insulation.

My thinking was whether this very deep space above the roof might be ideal for an inverted roof. I understand that there can be a weight issue that might be too much for a wooden deck, but my understanding is that this is mainly due to the ballast. Not sure if much ballast would be needed in this case as the insulation could be butted up against the wall surrounding the roof, as I presume it is the edges where the wind lift is most likely to occur. Obviously the skylights would need to be raised, but they need replaced anyway as there are clouded, but beyond this would this plan be fee-sible and any recommendations on how to proceed?

 

[Makie]

Weight may be an issue depending on how much ballast is used. Not really anyway you could know 100% unless you knew what the wood was like under it.

You could put some insulation on top of your current roof and then put down some EPDM rubber or a 3 layer felt system on top of the insulation.

 

[Kaymo]

Ok, so I think what you are suggesting is a conventional 'warm roof'. I hadn't quite got my head around why inverted roofs are being promoted heavily or what the advantages are, but guess I may be getting sucked in to thinking that is the preferred option.

There are a number of potential issues I will have to deal with in either case, for one the skylights will probably need to be replaced, and there is also a window that is probably too low and the insulation will be above it, but bridges to cross!

 

[Makie]

It would depend on how much insulation you want on the roof really. If you went along the lines of say 100mm you could probably keep the existing windows.

The problem with the inverted roof would be you have an old roof under it and if there is any wear and tear you then need to lift everything up again.

I'm in the central belt in Scotland and have been working as a roofer for the last 12 years and I have never seen an inverted roof done up here.

 

[Kaymo]

Thanks, but I don't think I would get away with even 100mm without changing the window. The window is not exactly new so it wouldn't be a great loss, but I now have a double glazed secondary window fitted on the inside of this bay so I wouldn't have any reason for replacing it if I wasn't doing the roof. Same is true of the skylights. Starting to wonder if this is going to be worth it!

 

[Makie]

If it's not leaking I wouldn't bother

 

[catlad]

Why not buy a couple of 100mm sheets of celotex and put some bags of sand on them to stop them blowing away.
You can then see if it makes a difference to that area.

 

[Kaymo]

Why not buy a couple of 100mm sheets of celotex and put some bags of sand on them to stop them blowing away.
You can then see if it makes a difference to that area.

Thanks Catlad, its not a bad idea, and I had thought of doing something similar with some EPS (on a dry day) that I have handy, but its currently a cold roof, so it will/should be ventilated so any heat coming through the ceiling will be lost to the ventilation in the roof space. If I was converting to a warm roof I would also have to open up the ceiling space and make sure it was sealed to the outside. There are no obvious vents, but I suspect it is open at the eaves and also at the wall on the old house side, via the suspended floors.

 

[catlad]

You need to investigate further to find out what is going on with regard to ventilation and insulation.