Galvanised steel trusses or rafters. I've never come across them.
Sorry to disagree with VHG, but they are quite kosher. With yours, You'd have to see the engineering calcs to decide on possible load etc.
The roof is the same age as the house I believe, I think the house was buillt around 1940's ish, that is it finished, the contractors were laying insulation, the roof tiles on the outside are in poor condition and have a lot of holes in them, they were confused to see the roof the way it was inside
It looks like the metal members were designed for industrial/ agricultural buildings, I don't think it is a one-off because there appear to special cleats and brackets at the joints. It would certainly have been erected by a local builder, it would be interesting to see if there were any other contemporary barns of factory roofs like it in the area.
The war-time furniture was called "utility" (now collected) and made with the most minimal amount of wood for the sake of economy. It seems you might have a "utility" roof, and it would be of architectural interest. Like the furniture, It has stood up well to the test of time.
The house is ex-local Authority, It's on a busy road with lots of similar looking houses, there are roof tiles on the outside of the slates, a lot of the roof tiles are in bad condition /loose or missing.