The window and door come next. The door is easy - a piece of scribed plasticard to represent the planked door, and some thin strip for the door surround. Etched brass windows are quite useful, if you can get them to match the size you need. An alternative is to cut them from paper. Drawing a window in your drawing software is simple enough, and doesn't take long to print and cut out. I use the same grade photo paper as for the brickwork.
For the brewery warehouse, I needed 57 identical windows, plus another eight for the sack hoists. Drawing one and copying it 56 times is easy - cutting out 57 windows, each of which has 15 panes of glass requires a bit of patience!
For the buildings at the back of the layout I have used Wills plastic roofing, but I find this is rather too thick and heavy in appearance for buildings nearer the front of the layout. For the mess hut I have resorted to strips of paper, scored to represent slates 6 by 4mm (18" by 12" - slates can also be 24" by 18" - 8 by 6mm). Slates and plain tiles have to be doubly overlapped (otherwise they leak) so that only one third of the height of the tile is visible.
The photo shows a sheet of paper with lines printed at 2mm intervals. Starting at the eaves, I have stuck down 6mm strips, each overlapping the one beneath, and the one beneath that. This gives the appropriate overlap.
Proprietary guttering in white metal is available, but for the mess hut I have used a jig to make half-round guttering out of nickel silver sheet. I have cut the n/s into 3mm strips, and heated it to red heat to soften it. Pressing the strips into an appropriate size piece of brass tube (with the top half filed away) produces short lengths of guttering which can be cut and soldered together. The down-pipe is a piece of copper wire, stretched to straighten and harden it, before bending to shape, and soldering to the guttering.
Finally, I have made the chimney separately, and fixed to the end wall of the hut. The whole building has then been lightly weathered, with a bit of dry brushed white on the guttering and roof to give a better impression of depth.