Flat roof to a-frame garage conversion

Hi all,

I would like to find out how practical it might be to convert our garage's flat roof to an a-frame. I would like to do the conversion for two reasons:
1. It currently leaks.
2. I would like to store things in the trusses.

Here's a photo of the interior, Figure 1. The top of the photo is the garage door in the open position.

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Figure 1.

There are five frame portals with a centre-to-centre spacing of approximately 1820 mm. These portals are comprised of a pair of 38 mm SHS uprights with a 125x65 mm c-purlin sitting on top, Figure 2.

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Figure 2.

There is a small angle bracket that is bolted to the upright and welded to the purlin. There is also a 400x25x5 mm stiffener bolted to the upright and to the purlin. The uprights appear to be just sunk into the slab, Figure 3.

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Figure 3.

The roof sheeting is tek screwed into the purlins, Figure 4.

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Figure 4.

The portals are joined together by 70x45 mm timbers on the floor, middle and near the ceiling, Figure 5.

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Would this be a case of finding pre-fabricated trusses, attaching them to the top of the existing purlins, and then adding longitudinal purlins on top of those for the roof sheeting? Or is there some other way it should be done?

Thanks,
Clueless