America is one of the few countries in the world that builds predominantly with wood. This is due to the rich natural resources in North America. Basically we have lots of trees and not a lot of people. By contrast, Europe started running out of significant forestlands in the 1200s. Engineering knowledge has advanced over the centuries, but the fundamental ideas for how to build a wood framed house are still the same. You start with some kind of foundation or structure to build on top of. Then you create a network of wooden supports crossing your foundation walls and supports. Add some walls and repeat. Add some more walls and repeat one more time, cap it off with a roof, and you have a house. This network of cross linked wooden supports consists of beams. Depending on the size, style, and design of your house, you may have one or more central support beams. If you crawl under your house or go down to your basement, this is the big structure that spans your foundation to which all the other boards are attached in some way. Attached to that beam, will be boards running perpendicular called joists. This are pieces of lumber set on end (the skinny side is turned to the sky) which make up the bones of the floor.
In the olden days, the central support beam in a house would be a single sewn log. Depending on how far it had to span and how big the house was going to be, this thing might have been huge. Imagine more than 18″x18″ and 40 feet long (most weren’t this big). As more forest were cut down, it was more and more expensive to harvest wood like that. Technology progressed and crafted support beams began to replace single log beams. Essentially the idea is that you take some number of boards, usually the same size as the joists you’ll be using, and sandwich them together using fasteners and glue. When you take two boards and nail and glue them together like a sandwich, the result is a stiffer, stronger piece of wood. Importantly, even if you can build up a beam the same size as the old, single tree design, it won’t be as strong as the single log. Two boards are always weaker than one big board*. Still, this method is very useful. For example, if you have sagging joists (usually you’ll notice your floor sinking a little bit), it’s often a good solution to glue and nail another board parallel to it in order to strength the original.
Crafting a beam in this style is used all over in modern framing. When you install a large opening in a wall for a doorway, window, or pass through, you’ll usually construct a beam by sandwiching several boards (and sometimes plywood to make it flush with your framing) together. This beam, called a header, is used to carry the load above the opening to the posts on either end and down to the foundation. As part of our kitchen redesign, we built a header using 3 2x8s and some 1/2″ OSB. The final beam was about eight and a half feet long and will span the opening over what will one day be our pass through bar seating.
*I feel like I should point out that this isn’t true anymore, exactly. In the past 20 years or so, technology has continued to evolve and we now have engineered lumber products. The idea is that you can take wood pulp, lay it out mechanically, stiffen it with glue, and knit it together. The resulting product is a board with the same dimensions as sewn lumber (i.e. 2×6, 2×8, 2×10, etc etc etc), but many times stronger. As a result, today’s building designers can create structural frameworks that are longer, larger, and support more weight over a greater span than anything that could be done in wood framing even 75 years ago.