The structural elements of your home provide the solidity and strength needed to safely support all the other parts.
What immediately comes to mind when thinking about design is the size, shape, layout, texture, color etc. that make up the general look and feel of the home. These are the elements that appeal to human senses and needs. But the structural parts of the home’s design are even more important because they give it posture, and durability.
The critical structural elements of your home include soil, grading, foundation, rainwater control, framework, roofing and passive ventilation.
Design for trouble
Your home must be strong enough to safely survive where it is located for many years. This can be a tall order.
Every location is different so studies must be done and variations taken into account. Each variable will require a specific structural design. The design of the foundations and framing systems are greatly influenced by the characteristics of the building site.
The world can throw brutal heat, cold, wind, rain, hail, blowing sand, earthquakes, fires, floods, mudslides, sea spray and Tsunamis at your home. Sometimes it seems that all of the above get hurled at the same time. Good structures must be able to withstand some serious punches.
Hug an engineer, they deserve it
The job of the structural engineer is not as easy and straightforward as it would seem. Everyone wants their home to be durable and safe, but no one wants to live in a cave again. Paraphrasing Frank Lloyd Wright, we have become rather attached to the niceties of our age. Big open spaces, graceful aesthetics, fresh air, sunlit rooms, beautiful views and all the other comforts of home need to be accounted for. Good structures and the engineers that design them, have to support all these things as well.
Buildings have historically been constructed using local materials that are readily and affordably available. Structural engineers have developed many unique and clever methods to make use of these materials. Fortunately, material availability is robust enough that virtually anything is available so long as you can afford it. Material availability no longer dictates design. In a litigious world, the safety of the homeowner is todays primary structural design factor. The good news is that so long as the safety requirements are met, architects can be fairly free to design any way they want.
Some of the materials commonly used for structural applications are reinforced concrete, steel, wood, masonry, adobe and rammed earth. Of these, wood is by far, the most commonly used structural material for building a home in the US.
The US is blessed with an abundance of forests, so wood is by far the favorite material for framing homes. Wood for framing is made from sawn lengths of tree logs and it is renewable and organic.
Wood is exceptionally desirable for construction for a number of reasons: 1) it is extremely strong and resistive to both tensile and compressive forces, 2) it has exceptional length versus its cross dimensions, 3) it is somewhat flexible and 4) it is easy to work by hand. These advantages far overshadow wood’s drawbacks of being vulnerable to rot and fire.
How its made
When harvested after growing for about 100 years, trees are cut into rough lengths (logs) in the field and then transported to mills. At the mill, the bark is removed, the log examined and then carefully sawed into specific standard dimensions that are appropriate for use as boards or other structural members.
You are probably familiar with the terms two-by-four, or four-by-six. These describe the approximate thickness and width of each board. A two-by-four is approximately 2” by 4”. The boards sold in lengths of 2’ increments with 8’ being the minimum up to about 24’ in length. Longer lengths or larger dimension can be special ordered at extra cost. Huge dimensions are rare and expensive since any trees big enough are mostly gone now.
Kiln dried lumber
Freshly cut lumber retains high levels of moisture and is thus too wet for use as framing material. When wet lumber dries it shrinks and changes shape slightly. Also if wet lumber is trapped in an unvented wall cavity. it will quickly develop mold and rot. Neither changing shape nor rot would be acceptable. To remedy those problems, framing lumber is methodically dried down to below 14% in special drying kilns at the mill. When the lumber is dry, all sides are planed smooth to finished dimensions before being shipped for sale.
Planing and grading
The drying and planing process removes approximately 0.1875” per side from all sides of the boards. So the final dimensions of say a 2×4 will actually end up at around 1-5/8” x 3-5/8” give or take 1/4″. This variation factor drives good carpenters crazy as they have to allow for up to ¼ inch or more of unknown error. Other quirks of lumber, like bowing, cupping, twisting and knots require hand selection before use. This makes the carpenters job harder and any kind of precision with regard to framing extremely difficult. These error factors have led to lumber being replaced with steel framing on jobs where extreme tolerances and precision are needed.
An ancient and highly effective method of preserving and preventing decay is to char the exterior of the dried lumber. Charring means to burn and blacken the outer layer of the lumber with flame. Preservative treatments are valuable and badly needed. These treatments help prevent the lumber from rotting when exposed to moisture, or where it may come into direct contact with concrete or soil. Some examples are mud-sill materials, posts and retaining walls.
You never hear about charring anymore because it is the industry practice to treat lumber with poisonous chemical preservatives instead. Charred piles and posts used for Greek and Roman bridges and foundations have been found in almost perfect condition after more than 2000 years. I would say that’s a pretty effective record without chemicals.
An argument could be made that charring might be a possible lumber option to the sensitive green building market in lieu of chemically treated lumber. I would think it would be fairly easy to adapt the existing drying kilns and other machinery to perform this process.
The structural elements are the unseen hero’s of your home. Oh, and say thank you to a structural engineer and a carpenter when you get a chance!