Roof Joist Sizing

[PeterW]

OK...

Informal chat with building control company has resulted in two things.

We were going to use the LJ Joists for roof timbers but talking it through the BCO reckons it is creating a lot of extra work and cost. Notwithstanding that the beams are about 10% more than solid timber, he advised that due to the slope we would need a pretty comprehensive steels arrangement that would add to the cost. He's also said that even with an SE doing the calcs that the roof joists "could" take the loading at a 50 degree pitch he would be cautious as a warranty company may not agree and we would be caught in the middle of an expensive argument.

I've agreed to go away and look at a standard cut roof but I'm slightly lost as to where to start with this due to a couple of non-standard elements that aren't covered in the Trada tables (50 degree pitch for one) and one section where there are no floor binders so there is no lateral restraint other than the collar binder.

To explain further, we have a maximum span of 4.6m, with a roof pitch of 50 degrees giving an unsupported span of 3.62m. Working from a nominal 225 x 38 ridge board, then we have no need for any purlins as the roof load of 0.86Kn/m2 doesn't create any issues if we were below 45 degrees as the Trada Tables here show the following at 600 centres :

15-22 deg slope      195 x 47      3.68m
22-30 deg slope      195 x 47      3.87m
30-45 deg slope      195 x 47      3.99m

Logic would assume, and engineering would follow, that at 50 degrees we would be at around 4.03m, well in excess of the required span. This would be birdsmouth onto the 100 x 75 wallplate.

For most of the build there is no issue as we have a binder at floor level, but for a 3.6m section we have nothing, just a 2.6m wall then rising to the ceiling. The plan was to put a binder in at around 600 down to allow for services, but I can't work out if this is enough or whether I'm going to need to put additional steel collar ties at the mid point. I'm not even sure how these are designed !! 

I've also got another issue that the ridge in this part is 7.6m between supports so would need to be a minimum of 7.8m. Given its not a structural member, is there any mileage in using a pair of I-beams with an OSB splice / infil as this would probably be the lightest and most cost effective idea !

Anyone have any ideas..?