prefab garage new roof

[dilly]

Hi mother inlaw has a prefab garage which is in good nick,it has cementous boards on roof which she wants to change.Looking at it i noticed it is held together by some metal runners which these sheets sit on.Is it possible for me to put in some joist hangers and joists and add more strength to it,thus allowing me to put some 8x4 sheets and new covering.Or is there some other way
Thanks

 

[dilly]

ps also realised its only about 2 inches thick so am i going to be able to use joist hangers

 

[Deluks]

Removing and replacing the roof structure (metal angle trusses by any chance?) might seem simple enough by they are probably holding the walls up, so anything you put in their place will need to brace several tons of concrete wanting to fall over.

The cementious type roof boards (careful they could be asbestos if they are old) are quite heavy and the roof supports would have been designed to take this weight plus snow plus a bit more.
So you could probably just screw down some ply to the runners (purlins?) and felt over that. Use self tapping or countersunk tec screws.

You might get away with using 12mm wbp ply( lighter) if the spacings aren't too wide between supports.

Also consider plasticoated metal sheets, and or putting a layer (50mmish) of insulation over/under the roof deck. Will reduce condensation in colder months.