my roof is sagging, front and back?

If a pitched back and front roof then across the rafters in the loft should be a 'purlin' a horizontal usually 6 x 3 beam with support struts 3x 2 coming down to the centre wall against a wall plate nailed to ceiling joists. Same for the back roof there should be on a large span at least two to each purlin and each end of the purlins should be on corbeled bricks or again struts. It may be some bird brain before you took out the struts and purlins.... Only a guess as I cannot see the structure. You could get your local council building control structural engineer to call but there may be a charge for this check before the roof gets worse.

Bob
November 2010