Hi NH,

I think that you may well be correct about sawing of long purlins on site. Our own little 2 bay granary has a pair of 23 foot long single piece oak purlins that have first been hewn out square and then halved by sawing. If this was done on a sawmill then the bed track would need to be going on 50 feet long to allow passage of the whole log length through a fixed blade so for long logs it probably makes much more sense to keep the log position fixed and then move the saw down through the length of the log.

Regards

Ken Hume