Wood sheathing, reasons for differential weathering

Andy Carlson

There has been some disscusion on weathering wood on single sheathed box cars and I felt that quite a few folks were not too knowing of the effect of wood to the weathering differences. This is what i wrote:

My interest in lumber has led me to examine photos of single sheathed freight cars in a critical way. Unlike double sheathed cars, single sheathed cars usually did not have tongue & Groove edges while double sheathed not only had the T&G edges, they were commonly milled with a 'V' along the board's edge giving the characteristic finished look so attractive on both freight cars and passenger cars.

Logs milled into lumber start out as cylindrical and are sawn into flat panels where individual boards are sliced from. Depending on the location of this sawing relative to the circular ring pattern of the log, it will create different styles of finished lumber. Where the rings, as viewed from the board's edge, stand vertical that is called 'vertical grain' or quarter sawn. Boards with the ring grains running horizontally from side to side is called 'flat sawn'.

Understanding the reasons for the existence of tree rings in lumber is helpful in seeing the weathering differences found in ageing milled lumber. Each ring zone in a tree is from a single season of growth, particularly in temperate forests. The broader area is from spring growth and is the most rapid growth the trees will have in a season. This portion of wood growth is called "spring wood'. After this fast growth, the the slower growth of summer is much denser but also much slower in growth producing the dark ring which is called 'summer growth'. Summer growth is both denser and narrower and the denseness makes it the strongest portion of a log. The much faster growth of the spring wood is less dense and is therefore of less strength. The favorite lumber to mill into T&G was from old growth trees, where the darker forest from all of the neighboring trees blocking much of the light made for a lot of the year's tree growth to be somewhat minimal. This slower growth made for a much higher percentage of summer wood and any old carpenter will tell you that kind of lumber is the best, especially when quarter sawn into vertical grain wood.

Returning to the differences of flat and vertical grain sawed lumber; if the boards are cut from the flat grain area of a slab at the mill the face of the boards will have the characteristic look of ovals and wavy lines, which is what is seen of the rings as they are exposed to the cut lumber. Remember that the spring wood is less strong and that the face of the flat grain on a board will have that board exposed to the weather with the higher % of weaker wood. I like to call attention to wood fence posts. Commonly beveled at the top to reduce the pooling of standing water, accelerating rot, the tops of these boards clearly show the ring structure in the horizontal and circular view. After a few years, you will see that the areas within the rings will retreat downwards due to the softer spring wood's less resistance to rot. This same principle is at work on milled freight car siding. Since the commonly milling methods produce a mix of flat grain and vertical grain boards it was not uncommon for sheathing boards to also be a mix of mill cuts. Nowadays, many of the more premium boards are pulled out of the green chain to be sold at a premium due to their recognized better quality.

So a freshly sheathed SS car will usually have a mix of these boards and after a few years, the boards with faces of predominantly flat grain (spring wood) will decay noticeably faster. I have a sample photo of an older single sheathed car, a WP 40' box car in the 15001-16000 series.
Inline image

The difference between the boards with the paint intact vs. the boards with silvering weathering where the paint has flaked off is striking. The boards which are in between flat grain and vertical grain have ring patterns varying from within the two extremes and will have a variable amount of face spring wood and will show slower rotting of the flat grain wood but faster rot than the vertical grain boards.

Notice that these boards are truly aging, but the tightness of the boards to each other remains pretty good. No leaks from sand or wheat which will pour out.

In the history in this hobby of recreating freight cars in the form of models has shown the steady reduction of the very deep and wide grooves manufacturers used to delineate the individual boards. Some experiments have making the boards of different thicknesses, such as Tichy cars with boards that stand both outwards and inwards relative to each other. As shown in this closeup picture these boards do not show this kind of board differential at all.

As for how do we emulate the style visible in the photo for our miniatures? I am being more convinced that subtle groove lines coupled with differential painting is the way to more closely achieve this look. If I weren't so slow (lazy) I would be doing some painting experiments.

You all please stay safe and do well,
-Andy Carlson
Ojai CA