I wrote a blog post earlier this summer about the importance of field priming the back sides of exterior lumber (back-priming). This is a relatively new practice – you certainly won’t find the original trim on a century old house back-primed, yet it may be in pretty good condition. Part of the reason for this is the quality of the lumber itself. The photo above is of 2 studs – one pulled from a house constructed around the turn of the last century and one installed this past year. Present day lumber indicated as ‘stud-grade’ is only suitable for vertical load applications. It has a very low allowable bending stress and modulus of elasticity (the criteria for determining the allowable loads and spans of horizontal elements like floor joists and roof rafters).
Immediately evident is the difference in the growth ring pattern of these two samples. All trees grow radially and add a ring of growth each year. Lighter colored ‘earlywood’ is added in the spring and summer while the darker areas of ‘latewood’ develop toward the end of the growing season. This latewood is inherently denser and therefore stronger and more resistant to decay. The 100 yr old piece of wood exhibits over 60 very tightly spaced growth rings, while the modern counterpart has only 6. More growth rings means a greater amount of latewood resulting in a denser, stronger, more decay resistant piece of lumber. The large expanses of earlywood in the modern piece are inviting to decay penetration.
Additionally, one can see that the curvature of the modern rings is very pronounced – this is from a very small diameter, relatively young tree. Wood moves, in response to changes in moisture content and humidity, and it moves in different amounts relative to the orientation to the growth rings. This modern piece of wood is highly susceptible to movement, especially to cupping or warping given the radial nature of the rings.
We can lament the loss of this vintage material and even if there were old-growth forests around I would not be advocating harvesting the lumber for commercial purposes. Instead, I think the fact that modern dimensional lumber cannot compete with its vintage relative is a reason to use engineered alternatives.
Over subsequent blog posts, I will explore alternatives to conventional framing and trim lumber that we have specified and will be specifying in the coming year. Stay tuned.