Hi Mutch,
A 40 foot clearspan is a significant undertaking but not impossible.
Westminster hall roof is 67 foot clearspan using hammer beam trusses and I have seen another 60 foot clearspan with low pitch roof used as an inside horse dressage area.
I produced a flat roofed 32 foot clear span truss design for a customer sometime back in Massachusetts photos of this can be seen on our website under the Librett project
http://www.kfhume.freeserve.co.uk/pages/featureprojectspages/librettpages/librettframe.htm Whatever design you choose to adopt you will need to recognise that the shallower the pitch the more that deflection and distortion problems are likely to arise and hence it is essential to run your design on a structural modelling package to check out loads, stresses, moments and deflections. See :-
http://www.kfhume.freeserve.co.uk/pages/engineeringpages/engineeringframe.htm It goes without saying that a clear span design of 40 feet really lies in the lap of professional engineers who understand designing with big timbers.
Regards
Ken Hume
Visit
http://www.kfhume.freeserve.co.uk
