What do I need re: loft changes

[ccam108]

I'm looking to make some changes to my loft space to make it more accessible for storage.

I have a cut roof construction, with horizontal beams 1240mm off the loft floor which I'd like to remove. Here's a diagram with the beams to be removed in green:

The trusses are all identical and at 450mm centres (I'll need to double check this - may only be 400mm).

The previous owner had already cut most of these out over 10 years ago, but had never got a building warrant, so they had to be re-instated before they could sell the house. I want to do it properly, notifying the council, so that we're not stuck with the same situation when we come to sell.

I also want to add a velux window which will involve cutting one of the rafters.

I want to do the work myself so I won't be employing a builder who can liaise with the council, so who do I need to employ to apply for a building warrant? Do I need an architect to do plans, or a structural engineer, or someone else?

EDIT: I forgot to include some photos of the roof construction. Here they are:



EDIT 2: I'm in Scotland, I know the rules are different here from England

 

[tony1851]

Don't waste time with an architect, you would need a SE for that to do the necessary number-crunching.
The ties will have been added to prevent the roof from sagging, in the absence of heavy timber purlins.
Depending on the span wall-to-wall, a steel purlin each side may be the way to go, but they would need to be designed and approved by your b/c people.

 

[CambridgeBrummie]

As a structural engineer, those horizontal members will be working quite hard. As tony1851 states steel purlins at mid-span would work. Depending on the span between your gables, you might get an engineering timber purlin to work. You will also need something to pick up the ceiling joists as well as the roof rafters.

Whatever you do, don't cut out the horizontal (green) memebrs and expect the roof to remain standing!

 

[ccam108]

As a structural engineer, those horizontal members will be working quite hard. As tony1851 states steel purlins at mid-span would work. Depending on the span between your gables, you might get an engineering timber purlin to work. You will also need something to pick up the ceiling joists as well as the roof rafters.

Are they in tension or compression? I'd be surprised if they are in compression, considering they're only 20mm thick, so have a lot of horizontal movement.

Whatever you do, don't cut out the horizontal (green) memebrs and expect the roof to remain standing!

Heh, it worked for the previous owners for a decade maybe they just got lucky.