Split purlins

[chappers]

So the loft was converted 14-15 years ago and was only signed off by the council recently, I presume as a regularisation as there wasn't a formal sign off when it was built. To be honest I'm surprised they didn't turn round and say that it's out side of enforcement now. You can be sure they wouldn't have been as thorough as inspecting during construction. I would find out what they have in the file and what they inspected recently.
What lead your SE to believe that the purlins were cosmetic and more so why does he believe the roof has been supported with steelwork within the roof, as this wouldn't be the normal way to carry out a loft conversion. how far up the roof are the bottoms of the roof lights from the eaves and how high off the finished floor are they. Any signs of sagging on the roof outside.

 

[bernardgreen]

I believe the loft was converted 14-15 years ago

This was requested by our solicitors prior to purchase and completed back in March of last year.

Getting a certificate "completed" 14 years after the work was carried out is suspicious. Are you sure this is a Building Control certificate and not a Retrospective Planning Permission for the loft conversion. Planning will not have made any inspection as to the building control ( safety ) aspects of the conversion.

Structural engineer, believes there is a possibility the purlins may just be decorational and therefore

Very unlikely they are decorative. And if they are decorative then there has been ( and may be still is ) enough movement in the roof structure to cause them to bend and split.

The position of them relative to the window structure suggests they are in direct contact with the rafters, hence structural and not decorative.