How would you mould this?

[Zaheer]

My inclination is to make a two piece mold where the tiles face down, and the gap between the halves of the mold runs along (and just under) the eaves. The pour hole would go into the 'base'. Can anyone see any problems with this? It's the first two-piecer of any size that I've attempted, and am having a bit of trouble working out the best way to go about it.

[snitcythedog]

How you described the split between the two halves is how I would do it.  Instead of pouring from the bottom of the roof I would do my pour hole from one of the roof corners.  Looking at the mold box from the bottom of the roof it would look like a diamond inside the square mold box.  This would allow you to add vent holes from each of the side corners.  I hope that my rambling makes sense. 
Snitchy sends.

[Zaheer]

Are you suggesting titlting the roof relative the the base of the mould, so that each corner of the roof is at a different elevation/height relative the base of the mould? I assume you would still be pouring into the bottom of one of the eaves (the milliput green bits showing) with vents from the other three corners so as not to obliterate any detail. I may do a little drawing in the morning! Thanks for your input, it's been bending my brain a bit today, I think I may have to just get my hands dirty.

[snitcythedog]

My suggestion would look something like this.

View it as a one half of the mold (top or bottom in this case front and back).  Your pour hole would be the top corner.  Your vent holes would come up from the two side corners and the bottom would naturally fill up in the pour.  Hope that gets the point across.  I am sure that others with more commercial and hobby experience can give you either other ideas or more input.
Snitchy sends.

[Zaheer]

Ah I'm with you now. Should be possible to have the pour hole and vents enter under the eaves too with a little dog leg on each of them. My only concern is the weight of the big lump of rubber that will form the cavity inside the roof. I suppose I could carve it out or create a box inside the mould box to make that hollow but am still worried that it will cause that particular half of the mould to sag and deform the final cast. Thanks snitchy!

[SBRPearce]

I would make an exterior "plug" to fill the space beneath the roof in the mold. In the molding process, fill that space with a plasticine or clay plug shaped to leave 6-10mm between the underside of the roof and the plug. Make sure there are no/minimal undercuts. Your mold on that side will be 6-10mm thick, and then you remove the plasticine and pour cheap plaster into the space. The plug is slotted in place when you assemble the molds, to keep it in shape, and is removed when you're opening it to de-mold your finished piece.

This is an offshoot of the technique we used in casting life-masks for art & theatre work: the flexible mold compound on the subject's face & body was usually only 10-15mm thick, and to support it from the outside we built keyed-together supporting rigid jacket pieces. In this case, you only need one.

[snitcythedog]

My only concern is the weight of the big lump of rubber that will form the cavity inside the roof.

The bottom half of the mold should hold its structure as long as you add keys into the surface during the mold making process

but am still worried that it will cause that particular half of the mould to sag and deform the final cast

Again the keys will hold the two pieces together and keep them aligned.  As long as two halves are thick enough to maintain their shape you should be good.  Getting a harder silicone will help too.
Just in case you have not seen it before, here is a good primer for mold making including two part molds. 
http://www.hirstarts.com/moldmake/moldmaking.html
The folks on their forum have a wealth of information and readily answer questions about casting and molding in their "questions and modeling tips" section.  I have run ideas by these folks in the past and on occasion they have come up with solutions that were completely outside the box. 
Hope that helps.
Snitchy sends.

[dampfpanzerwagon]

I would be interested in a flat pantile roof option as I can see this being of use to scratch-builders.

Tony

[Zaheer]

Thanks for the advice. I'm going to firm up the silicon using recycled shredded old molds which I've used with some success in the past. I hope this will make the halves stiff enough to hold thier shape. Without any undercuts I think that flexibility is less of a concern.

I would be interested in a flat pantile roof option as I can see this being of use to scratch-builders.

Tony

I can do you some flat pantile sections, and will probably make them generally available.

[Geudens]

Simple: roof (tiles) in 2 parts, sides too. Saves on rubber for starters.  Works fine, did it this way already 30 years ago. 

[dampfpanzerwagon]

Thanks for the advice. I'm going to firm up the silicon using recycled shredded old molds which I've used with some success in the past. I hope this will make the halves stiff enough to hold thier shape. Without any undercuts I think that flexibility is less of a concern.

I can do you some flat pantile sections, and will probably make them generally available.

Great.

You can contact me via my Blog or via this site once you have some moulded up.

I look forward to checking them out.

Tony