Elliptical Roofs

[IrishPeter]

I think this was a case of the thought process getting ahead of any actual thinking through as to how I am do this, but having a spare "37 foot" underframe I have built a carriage body that requires an elliptical roof.  As anyone had a go at this sort of roof?  I am a bit at a loss where to begin other than with it being 'colonial' stock it would probably have had a double roof anyway, so an arc "inner" with a varnished laminated card "outer" braced by plywood formers?

Any thoughts, please?

Peter in AZ

[GTB]

As anyone had a go at this sort of roof?  I am a bit at a loss where to begin other than with it being 'colonial' stock it would probably have had a double roof anyway, so an arc "inner" with a varnished laminated card "outer" braced by plywood formers?

To be pedantic, they are semi-elliptic roofs, as the cross section is half an ellipse bisected along the long axis. Don't know about other railways, but the VR drawing office cheated and the cross section was actually just two radii, rather than a true ellipse.

Can't say I've ever seen them used with a classic colonial double roof, even in India. The ones I'm familiar with just had a false ceiling inside to conceal the roof structure from the paying passengers. I've only ever made pass cars with that type of roof in HO and all I did was shape it out of a solid block of suitable softwood.

Full size VR practice on wooden cars was to use narrow boards on the outside over profiled roof sticks, covered outside with dressed canvas for waterproofing. For the interior they fitted spacers between the roof sticks and fixed sheets of ply to form a false ceiling. VR wooden cars with semi-elliptic roofs also had domed ends in the American style, which requires the roof boards to be both tapered and bent to form the ends.......

If I was intending to build this sort of car in large scale, I'd probably make a wooden former and lay up the roof out of bits of shellac soaked paper. Much the same as making it out of ordinary papier-mache, but shellac is waterproof. Fibreglass is another option, but more work making the plug and then the mould.

You could of course do it board by board over profiled roof sticks, like the prototype. I've built model ship hulls that way, but it wouldn't be my method of choice for pass cars.

Regards,
Graeme

[IrishPeter]

The profile of the carriage is more like early GSR/CIE laminate stock, with a bit of WAGR 42' suburban stock thrown in for good measure, so I don't have to deal with the 'domed' ends, just the semi-ellipse.  I like the idea of forming the roof out of shellac and paper over a former.  Does it need any framing, or does it form a hard enough shell that the sides and ends of the carriage and the internal partitions provide sufficient support?

I thought about the frame and plank method, but decided that it fell into the 'stuff that for a game of darts' category. My other thought was using aluminium flashing, which also sounded as though it might be a pain, as I have to rely on broom handle bending!

Thanks for the tip.

Peter in AZ

[GTB]

I like the idea of forming the roof out of shellac and paper over a former.  Does it need any framing, or does it form a hard enough shell that the sides and ends of the carriage and the internal partitions provide sufficient support?

Depends on how thick you make it I guess. If you build the layers up to at least 30 thou. thick, it should be self supporting between partitions. It will be at least as rigid as curved polystyrene sheet.......

I've used Al sheet in small scale models, but imagine it would be a pain to form a long roof out of it using a rod and a V-block and my bending rolls aren't long enough for a bogie vehicle. The local variety of Al flashing wouldn't work. It is as soft and malleable as lead, as it replaced lead as the flashing material for coated steel roofs.

Graeme

[IrishPeter]

We used to get some fairly heavy aluminium flashing around here associated with heating ducts, etc., but I have not seen it recently, so I assume they have found a better/cheaper way of doing it.  The roofing stuff available here now is fairly lightweight and soft. There is also the dreaded Galvy flashing which is too stiff for high arc and semi-eliptical carriage roofs and has nasty edges, though I have given it a go in the past on low arc roofs when I have been out of plywood.  So it looks like I am going to have to get shellacking!

Cheers,
Peter in AZ