A rathe large flat roof... what's the worst case scenario?

[doogyscoot]

Hi all, I hope I can gain your collective insight on this...
We are looking at a property which is part of an old manor house now split into eight separate dwellings. When it was converted in 1986 a large extension with a flat roof was added to the rear. Part of this covers the ground floor of the property we would be buying and the rest covers 8 small single garages belonging to the other residents. I'd estimate the surface area of this flat roof to be between 150 and 200 m2.
If we buy this place we would be planning on replacing the roof covering, probably with epdm ( currently felt, not sure how old). I'm not sure how the current roof was constructed, I'm assuming cold roof if it was mid to late 80s?
However my concern is that it hasn't been well ventilated, if so should I expect the rafters to be shot by now? I'd worked out it should cost around 8k for materials to reboard, add insulation (not sure what building regulations were like in 1986 so no idea how it might already be insulated) and cover with epdm. I'm guessing it's a job two roofers could do in a week, say a day rate of about £200, that should come out about 10k. Would you say I'm in the right ball park?
Anyway, my main point was how much would it be likely to cost extra if the rafters are rotten? If a joiner came in would it necessitate removal of all the interior ceilings or is there a clever way to replace the rafters without ripping out all the ceilings?
Sorry, it's a bit of a long winded one but I'm trying to get as clear an idea as I can of how this could pan out.
Thanks in advance for your thoughts.

 

[palaceray]

I would say your 10k figure is way out. I would imagine that day work in Scotland is about £120 per man but on top of this the company would charge for their overheads and profit. Plus whatever scaffold and health safety measures need to be put in place.
The price could be closer to £20k plus any timberwork needed

 

[doogyscoot]

Thanks for the input, fair enough, though there shouldn't be any scaffold required, it's only a single storey and not particularly high. I'm not sure what the mark up would be on materials but I figured what I can buy it for without trade discount would be pretty close to what the company would bill me. I'm getting a ball park quote from a local firm so will let you know how that comes out.
I'd like to do the roof myself, having watched the firestone training videos it doesn't look particularly complex. I saw one of their suppliers do a training course for about £100, maybe I should go down that route. The roof isn't especially complex, just big so I guess I'd need a few other sets of hands to manage the membrane.
Any ideas on the least destructive way to check the condition of the rafters before pulling everything up?

 

[Alastairreid]

You can buy a boroscope for under a hundred quid or hire one.

 

[cotswoldbuilders]

Id get a firm in to do it, a roof that size would need on site joining of the rubber, you would never lift or manage a sheet that size.

You would need to also fit Russ Strips, which seems to be another requirement for rubber roofs.

 

[doogyscoot]

I'd thought that if I was going to attempt it I'd do it in three sections to make it more manageable. More joins, which isn't ideal but probably more manageable. Thanks for the suggestion on a borescope. I could probably pop out the roof halogens and poke it up through there to save making holes in the plaster.