I'm still new enough to this form of communication to be getting a big kick out of it.
As with the linseed oil issue, I'm glad to see all the usual suspects have added their 2 penny's worth, and very illuminating it is too.

Instead of rushing straight off home to deal with howling babies smelly nappies (diapers?)etc. I can visit here and pick up some stimulus for the ride home. Thank you.

I do however hope that there are more than we happy few visiting this site, and would ask that all visitors feel free to add to the debate, and/or simply say 'hello'.

As to the big truss debate:

All our big areas to date have been aisled barns, these plainly have the clear spaces interrupted by posts coming to ground. I have seen French markets where there are huge open sided covered spaces, fantastic mediaeval timberframes. Its hard to describe, but it seems that sometimes, they only bring to ground every other post , the intermediate posts, being picked up on beams spanning the ones that do come to ground.

Plainly there are mediaeval roof trusses that have large spans, (like the 50 foot span of our Scottish Hammerbeam roof at Stirling castle, but as we all know these need a considerable mass of masonry wall to buttress them

There are aircraft / balloon hangers, with monumental spans, (hundreds of feet), Mills too, and mostly in North America?

Moving closer to Timberframe, the Late Georgians, & Victorians were past masters of building large spans, using some metalwork (mainly straps). I had always thought (in this country anyway), that their designs were driven by a need to conserve / reduce the timber. This would also reflect the reducing cost of metal, (and the fabrication there off) as against the cost of big timber. We, in this overcrowded small island have always imported softwood, and the change to tension joinery, based on the Kingpost Truss, with slender lower chord, held up by a Kingpost in tension, was matched by a move towards building in cheaper softwood.

When these trusses became very wide with lots of queen posts as well, you could start to consider them as girder trusses.

Once when dealing with an Enquirer for a sports hall I designed a clearspanning truss 60 feet wide, all Oak - No metal, the project didn't get of the ground, so we didn't build it, but I did get a provisional nod from our Engineer, who thought it could be made to work. Although the span was 60 feet there were no timbers longer than 26 feet, and all the joints were proposed as wedged / pegged.

As the price per cube, (we buy by[?] the cubic foot not the board foot 12 Bd. Ft = 1 CuFt), rises exponentially as the length increases, it is economic to keep maximum lengths in the low twenties.

Is there the means to post visuals into this forum, and could someone as techo-challenged as I be trained to do this?

buy by for now, talk later