I'll second Wayne's suggestion to buy 4'x8' styrene sheets. The last time I bought .040" styrene they were $23 each. Just Google "plastic suppliers" or "plastic supplies" for store locations in your area. You can scratch-build a lot of structures from one 4'x8' sheet. I have also made background buildings by laminating building photo or software printouts to building cores made from sheet styrene. If the printouts are well sealed with a matte clear overcoat, they should remain fairly weather/water resistant.
Hornblower
Try WonderBoard (aka HardieBacker) it's a cement fiberboard, it comes pretty thin but is sturdy enough.
Steve
If everything seems under control, you're not going fast enough!
Take a look at Gator Board:
http://www.artsupply.com/foamcore/gatorboard.htm
A vendor at one of the local shows always had a piece sitting in a bowl of water to show its durability.
Another option is Sintra: http://tri-dee.com/sintra_plastic_pvc.htm
Look for "Plastic Suppliers"....you can pick up 4'x8' sheets at fairly reasonable prices and it's available in various thicknesses. I've gone through at least four or five .060" sheets, as it's useful for all sorts of modelling applications.It's also easy to transport, as the seller rolls it up and tapes it....will fit in your car's back seat or trunk.
Wayne
Find a local sign maker. They deal in plastic sheet material and probably have sheets thicker than for sale signs. Or, you could take a few For Sale signs and laminate them together for more thickness.
It takes an iron man to play with a toy iron horse.
I was thinking of using the plastic for sale signs but thought them too thin here- (I've used them lots of times before in other stuff) matter of fact being over there is what had me looking at the corregated stuff. The thickness of that was about the same as that of the foamcore. I thought CA or plastic model cement- just wondered if anyone had used it and one worked better than the other. (or had an adverse reaction) Foamcore is not an option here even with bracing as water will warp it. The extra structure of heavier material would help too- one of the structures is a Magneson hotel- kinda front heavy and City Classics and these would be narrow flats further throwing the balance off.
The area these are going in has been subject to occasional roof leaks. It is not a constant problem but with heavy rains it happens occasionally. (generally after we think we are safe and do any work there.) It is a city owned building and they are redoing the roof but we still do not trust it. At one point we were looking at adding an awning over this part of the layout before they started work on the roof. Jim
CA glue is great for dissimilar materials. I use foam core for roof material and interior walls and floors. I keep it all stable by gluing balsa strips in the corners.
Have you thought of using styrene? Go to the hardware store and pick up some plastic For Sale signs, low priced styrene sheet you can spray paint for unseen backings.
I am building some building flats for our club layout. Normally I use foamcore for back walls that are not seen. In this case there are some occasional water problems that have happened in this area (It's a long story- it's in a city owned building- they are fixing the roof but we still don't trust it) Foamcore of course will warp if exposed to water. I was trying to find a material to use in the back and I was looking at using the corregated plastic material that is used in yardsigns (coreplast I think is the technical name) Has anybody had experience using this in model building- will it glue to plastic kit walls?