Advice Needed, Raising low collar for better loft access

[Vanny]

I live in an end of terrace house build in 1965. The roof construction looks like this;

Untitled

• Vanny
• 9 Jun 2011
• 1

Width (length of purlin) = 5m
Depth (length of ceiling joist) ~ 7m

As can be seen there is just one collar between the purlins (and just one set of struts). Currently the collar sits on top of one purlin (towards the front of the house) and below the other purlin (towards the rear of the house). The collar sits about 4' from the loft floor which is a bit of a pain when traversing from end to end.

I want to raise this colar to say 6' high. The loft height is around 8' high at the ridge board. I want to add additional colars at every 3rd rafter.

In my mind this should be plenty of support to stop the rafters slipping off the wall plates and or purlins bowing, assuming my understanding of the purpose of the colar is correct.

Is my logic correct, or should i be getting a structural engineer in? This isn't a conversion job, the loft is only used for storage.

Thanks