As promised, here is a tutorial with photos on how I make my cardboard roofs, in which people have expressed quite a bit of interest. Start with collecting lots of cardboard, the type that comes on the back of writing paper pads is great, but the best cardboard, because of its size, is the back of flip charts. I no longer have access to this, as neither of us are working in offices any more, so I zealously save any and all writing paper backs.
Two of the pieces are thicker than the others, so I have decided not to use those on this project; it is best to use card of equal thickness. You will also need knives, blades, a long ruler and a pencil. Cutting through cardboard is hard on Xacto-type knives. If you have access to an office paper cutter or stationery guillotine, that cuts the work down tremendously. (Metric measurements are as close as I can approximate; you can work out your own proportions if you don't have access to a British Imperial ruler in inches.)
Start by slicing your cardboard into 3/4" (approx. 2 cm) strips. You will need lots, each strip will only be covering 1/2" (approx. 1.5 cm) of your roof surface. Mark a line 1/4" (.7cm) along one long edge; this is where the strips will eventually be glued. Divide your marked strips in two piles, and mark the other long edge at 1/2" - 1.5 cm intervals, on one pile. Start the other pile of your strips with a 1/4" - .7 cm line, then continue on with 1/2"-1.5 cm as before, as the slates will overlap like a brick course.
While you can cut the half slates as you glue the strips to the roof, I find I get a more accurate fit if I start half of my strips with a half slate, and I also don't have to repaint that cut edge later.
One side of these card strips is white, this is the wrong side as it will not accept paint the same way as the raw cardboard side. Below the two white strips along which I have started cutting, you can see two completely cut gray strips showing the eventual overlap. Now cut all your slate strips as marked, taking off a tiny sliver at each cut, stopping at the marked line at the top, and once in a while cutting off a corner to suggest the odd damaged slate. For this cutting I use scissors, but if you are good with a knife, you could use that.
As this will take quite a bit of time, and your fingers will likely get sore, this is where I will leave it for today. Part 2 will be the painting of the slates.