[TimT]

Funny I see gaps between every board...

My house was built in 1929? the roof construction looks similar,

Also I found receipts that a new roof was put on my house in 1974..... I re-roofed in 2009.. I had a roof leak so I suspected I would be replacing a lot of the substrate..

I was home when the existing asphalt shingles were stripped and took the opportunity to check out the wood... the area where I thought the leak was, was fine... In all the wood looked fine... I did have to replace some T&G along the gutter edge...

I think when the existing shingle are stripped you will find the wood planks are in pretty good shape...you will probably have to replace some... but most is still ok to place a new roof on...

Shoot a picture of the sagging.... it may be that some of the supports you see in the pics you posted rest on bearing walls... and other areas of the roof system are less supported..