Hi Mike, the wood I'm using is pressure treated "tile batten"as we say in Scotland. Kinda wood strips used here for fixing tiles on roofs. They're about 1.25"X0.5"profile..... As for warping ......doesn't happen. The timbers are laid in wet cement .To stop springing or warping....before laying the strips I hammer nails half way in on the underside of them leaving the nail heads exposed. A few nails protruding at various angles ensures that when the cement sets round them it takes a grip of the nails...result.. the wood is going nowhere. As for cold and frost ground-heave,again,doesn't happen because with the rubble underbase everything is well drained....nowhere for water to collect so no frost heave. Only 6.45 AM as I write this so too early to go hammering in the garden just yet....don't think my neighbours would appreciate it...lol
. but I'll be laying timbers again today so I'll post method-photos later on for you. 
