Jon,
talking to
@Pencarrow - Chris’ approach is to laminate or at least substantially thicken his walls and floors to prevent warping.
your building is quite large, and I wonder if the same approach might not be a good precaution?
best
Simon
Morning, Simon, and many thanks for your interest and wisdom as usual
Can’t disagree with any of that which is why the front part weighs a metaphorical ton…….
It’s not apparent at this early stage, but I’ll be employing the same here, but adapting the approach as I go. I’ll try n explain…..
So, taking the sides for instance, I’ve already started the laminate approach as can be (hopefully) seen. ‘But that’s just two-ply!’ (Reader’s voice

). As I say, just the start. To make it three ply, I’ll be adding a final section of Ashlar stone cladding to match the front/sides of the original building which will also be extended to blend.
With the front, the plan was to make it three ply with the some thinner card forming a sandwich with the two thicker ousters all purchased in quantities for the purpose (the price of this stuff, as usual the price increase blamed by the supplier on Brexit …………).
However, when I came to cut the window apertures, it proved a battle as this stuff is so incredibly tough. Doing it with two more sheets saw me driven almost to the realms of despair. It would have been a case of away with the knife and out with the drill and piercing saw. However, as there’s an intended change in the ‘frontage’ of this one (I’m not using the same stone appearance here) I hedged my bets on keeping it to a single skin as this stuff is as tough as ol’ boots as mentioned and thicker than it looks (closer to two milli than one and a half), thus the suck it n see approach I’m using this time.
Presently, I’ve been at pains to ensure that the ends of the floors are tightly bonded to the front and sides which will also be the case with the rear.
What you haven’t seen are the umpteen bits of off cuts which I’m going to add here n there inside to shore up the floors and increase the bond with the front and rear when it’s attached. However at present, the intention is to keep access to the interior to a maximum as I’ll still require leeway for my pinkies when fitting the windows eventually plus lighting - which is why the roof will be removable - which will be fixed with uhu to further reduce the risk created by the (excessive) use of polystyrene cements. Also, additional decorations will be attached in a manner which ensures three ply approach. In the main, the only addition to the surface will be primer and mostly acrylic paint which is kinder to plastic, so they say
All in all, this new ‘approach’ should prove fruitful whilst preventing me from ‘kickin’ it to the kerb’ due to the amount of effort required just to cut it.
Sorry to waffle on but hope this goes some way to explaining my off-pat approach this time.
Jon