Help with warping Evergreen sheet styrene walls

Think: If this were a real building, how would it be constructed? Frame yours accordingly.

Zardoz02

I'm building a simple trackside hut in HO scale. What I'm modelling has walls made of weatherboard with an upper strip of rendered timber so I'm using 1mm clapboard and 1mm sheet styrene glued edge to edge. Like so:

Holes for windows and doors are then cut. It seemed straightforward but I'm surprised by the amount of curvature that has developed in the two longer walls that contain the window and door. 

I'm looking for a simple way to reinforce the walls without adding too much extra complexity to the markup and cut-out process. 1mm is clearly a bit thin for this job but the clapboard only comes in 1mm thickness. Some further internal reinforcement perhaps? 

Suggestions please?

 

 

A mm is not to thin.  It just needs to have a bit of structure behind it.  I'd recommend a couple of slall angle sections lenthwise a third and 2/3 way down running lengthwise (but not where windows and doors are).

HI!

     When I make buildings I always use .040 Square strip at the bottom of the eaves inside and around the perimeter both inside and out of site.This also stabilises the building and prevents twisting of the entire building after Painting,