Replacing felt upstands on cement sheet garage roof

MasterplanC

Hi everyone,
We are buying a house where the garage roof needs some fixing as advised by our survey, where some previous water penetration could be seen. 
The roof is covered with corrugated cement sheets (likely including asbestos fibres) that are in fair condition. However, the bitumen felt flashing around the edges of the corrugated sheeting is in poor condition and requires repair works. Also the timber joist above the garage gate/door might have rotted a bit. The garage is attached to the neighbours garage, I attached some pictures to make it more clear. I have two questions:
- What is the right way to replace the felt upstands on the brickwork? (Googling suggested that the old felt needs to be cut away, whatever is below needs to be treated in order to attach a new felt and add some sealing.)
- In case the timber joist needs replacing, how would this be done without replacing the roof?
Thanks in advance!

Section62

MasterplanC said:

Hi everyone,
We are buying a house where the garage roof needs some fixing as advised by our survey, where some previous water penetration could be seen. 
The roof is covered with corrugated cement sheets (likely including asbestos fibres) that are in fair condition. However, the bitumen felt flashing around the edges of the corrugated sheeting is in poor condition and requires repair works. Also the timber joist above the garage gate/door might have rotted a bit. The garage is attached to the neighbours garage, I attached some pictures to make it more clear. I have two questions:
- What is the right way to replace the felt upstands on the brickwork? (Googling suggested that the old felt needs to be cut away, whatever is below needs to be treated in order to attach a new felt and add some sealing.)
- In case the timber joist needs replacing, how would this be done without replacing the roof?

If you want to retain the garage then I'd work on the basis of replacing the whole roof. Fibre cement roofing isn't the best material, with or without the complication of asbestos being present.

If you do opt for just replacing the felt then you need to bear two things in mind -

i) You don't want the contractor walking on the roof.  Fibre cement is brittle and the load from a person (even if using crawling boards) can be enough to crack it.  Cracks will allow leakage and more rapid deterioration.

ii) There is no separation between this roof and the neighbour's, they are in contact with each other.  This means you need to make sure your contractor doesn't do anything which damages the neighbour's roof, as they are likely to hold you liable if it starts leaking.

Both of which means you need to find a very professional roofer... which won't be cheap.

It might also be worth talking to the neighbour to see if they are interested in getting a re-roof done at the same time. Theirs looks like it has had work done to it relatively recently - but if it is still leaking they may be interested in getting something better.