If you want to use wood, Kappler is good. Personally, I'd use styrene: Evergreen offers HO scale 1" thick strips in widths from 2" to 12", and likewise for 2" stock. They also have 4"x4", 6"x6", and possibly some larger sizes. For HO scale sizes not available, they have an even more extensive range of sizes measured in thousandths-of-an-inch: 1 inch in HO scale is approximately .011". For corrugated siding, you can use basswood sheets from NorthEastern, or styrene sheets from Evergreen. If you're looking for something a bit more 3-dimensional (most likely if you're doing board-by-board), I'd recommend the aluminum sheets from Campbell. The strips (several in a package - the amount depends on the length) come in a variety of lengths from 4' to 16' HO, I believe. These can be applied over a built-up framed roof, or atop a plain styrene sub-roof. In the photo below, the sheets are applied over a sub-roof of .060" styrene sheet.
This structure has a sub-roof of built-up styrene trusses, strapped with styrene strips. The sheets are applied directly to the framework.
Finally, on this structure, the walls are Evergreen styrene corrugated siding, while the roof is individual Campbell sheets, on a .060" sheet styrene sub-roof.
The Campbell sheets are also useful for making fences:
In all examples, the Campbell sheets are joined to the sub-roof or framework with contact cement. To ease the tedium of applying the cement to each individual sheet, apply the cement to the uncut strip of roofing material, then allow it to dry to the touch, as per the manufacturer's instructions. I use a NorthWest Short Line "Chopper" to cut the individual sheets: set the stop to the width you need, then chop away. Make sure that the pieces don't come in contact with one another: I spread them, face-up, on a spread-out sheet of newspaper. Most contact cements will give you at least an hour of "working time", so you don't have to rush. When you've got the sheets cut, use a brush to apply more contact cement to the framework or sub-roof, and again, let it dry to the touch. For a sub-roof of sheet styrene, I find that it helps to use a suitably wide brush to apply a coat of lacquer thinner first, as a "prep" for the contact cement. By the way, if you've got a lot of roofing to do, or some other projects around the home that require contact cement, splurge on a can of gelled contact cement - a lot less strings and a lot neater. I've found it only in 1 litre or bigger cans, though. Anyway, when all of the contact cement is dry to the touch, start roofing, as you would on the prototype, from one side and from the bottom up. Overlap the sheets by a corrugation or two at the sides, and by at least a couple of scale inches over the course below. The sheets will stick as soon as they touch the cement on the sub-roof, so be careful as you position them. They won't stick where they overlap another sheet, though. This is not normally a problem, but if the situation calls for it, use a small, stiff brush to slip a bit of contact cement beneath the overlapping sheet, wait a few seconds (or minutes) then press it into place. I use an airbrush to paint or dullcote the shiny sheets after installation, as required, then add rust or other weathering with an airbrush, drybrush or chalks.
Wayne
